ASNT NDT Handbook Volume 1_ Leak Testing

membrane accumulation chamber is the (1) Q = ∆ PV
very long probe hose (up to 60 m or ∆t
200 ft) that may be used with little or no
loss in response time and no loss in test If Q = leakage rate in mass flow units
sensitivity. This allows the operator to (Pa·m3·s–1 or std cm3·s–1), V = volume of
place the helium mass spectrometer at the test system (cubic foot), ∆P = change
one location near an entry hole and scan in pressure (in. Hg) and ∆t = change in
the welds of the entire bottom from that time (hour), then 3.8 = conversion factor
one location. The probe hose is so small and Q = ∆PV/3.8∆t.
as to be virtually weightless, whereas a
halogen diode or electron capture As an example, if a tank were 30.5 m
instrument that weighs 5 to 10 kg (several (100 ft) in diameter and the grating space
pounds) must be carried with the probe. between the inner and outer bottoms
were 13 mm (0.5 in.) and the grating
A disadvantage is the added technical occupied 20 percent of that space, the
training and experience needed to volume of that space would be about V =
perform helium mass spectrometer (50)2π (0.4)/12 ≅ 7.4 m3 (262 ft3).
detector probe leak testing versus what is
needed to perform halogen diode or If this volume were evacuated to a
electron capture detector probe leak negative gage pressure of 98 kPa
testing. (736 torr), then held for 8 h and the
vacuum gage reading had increased
Another disadvantage of the helium 333 Pa (2.5 torr) in that time, then one of
mass spectrometer is the greater cost. the following would be indicated.
Depending on the degree of
sophistication of the model purchased, 1. Real leakage Q = (0.1)(262)/(3.38)(8) =
the cost of the helium mass spectrometer 0.86 std cm3·s–1.
and associated equipment may cost
anywhere from three to six times more 2. Or system volume changes normally
than the best halogen diode or electron because of the tendency of large flat
capture instruments. membranes to change shape with very
slight temperature changes.
If a quantitative leak test of a double
bottom is required, then the welds outside 3. Or nothing significant is indicated
the tank shell between the inner and because the pressure change is within
outer bottoms on double bottom designs the gage’s listed accuracy of
must be leak location tested before the 0.33 percent of full scale. This is about
quantitative test. The fastest and most the listed accuracy for such gages.
economical test technique is usually a
bubble pressure test. This is the first element of uncertainty
when performing a quantitative pressure
Comparison of rise measurement test of an aboveground
Quantitative Leak Testing storage tank with a double bottom. That is,
Techniques does a small pressure increase during such
a test reveal actual leakage or a false
Pressure rise measurement is one of the indication of leakage? When this occurs,
quantitative leak test techniques that has the test can be repeated or continued for a
been appearing quite frequently in many longer time period in order to average
aboveground storage tank double bottom down any errors. This may or may not
design specifications. After completion of produce a more conclusive result, but in
required preliminary leak testing, the any event it will cost more time and
specifications normally require that the money.
space between the double bottoms be
partially evacuated to some pressure If the conclusion is that this is real
below atmosphere and held at that leakage, the second element of
pressure for a defined period of time uncertainty becomes apparent — namely,
without any increase in the pressure (loss where is the leakage? Is it in the outer
of vacuum). A typical requirement is to bottom, in the inner bottom or in the
evacuate to a negative pressure (vacuum) perimeter welds between the bottoms
of 98 kPa (14.2 lbf·in.–2 or 735 torr) below outside the tank shell?
atmosphere and hold for 8 h without any
loss (degradation) of the vacuum. Another To reach a conclusion, the only option
typical requirement is to evacuate to is to retest the inner bottom and the
68 kPa (9.8 lbf·in.–2 or 508 torr) below perimeter welds between the bottoms
atmosphere and hold for 24 h without outside the tank shell by a leak location
any loss of the vacuum. technique. If leaks are detected in one or
both of these areas, the leaks can be
The basic pressure rise relationship is: repaired, the hold test can be rerun and
the results will most likely be satisfactory.
If no leaks are detected in either of these
areas, it would have to be assumed that
the leakage was from the outer bottom.

Pressure loss measurement is a
quantitative leak test technique that can
be specified for a double bottom system.

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This technique should not be performed
with the tank empty. This is because of
the very small positive pressure
differential that a large flat membrane
bottom can tolerate before it balloons;
i.e., 600 to 750 Pa (63 to 76 mm H2O or
2.5 to 3 in. H2O) water pressure. Even a
slight variation in temperature will
change the pressure and make a sizable
change in the volume due to the
movement of the inner bottom. This will
produce test results impossible to
interpret.

When this technique is specified, it
should include the requirement that a
head of water must be in the tank during
the test. If a tank contains 3.7 m (12 ft) of
water, assuming an adequate foundation
under the outer bottom, the space
between the bottoms could then be
pressurized to 34 kPa (5 lbf·in.–2), without
any ballooning of the inner bottom. This
would also virtually eliminate temperature
as a variable for that space because the
water would be a large heat sink that
would keep the temperature of that space
stable. On completion of the test, the
drain or sampling pipes between the
bottoms can be checked for signs of
moisture. If the hold test fails, an
indication or lack of an indication of
moisture between bottoms would indicate
the source of the leakage.

Mass flow measurement is a
quantitative test technique that is not
usually specified by owners but that has
several decided advantages. First, it can be
performed at a pressure either above or
below atmospheric pressure; second, it
can be performed in a minute or two.
Limiting the time duration of the
measurement can eliminate temperature
as a test variable. If conducting this test
technique at a pressure above atmospheric
pressure, it is suggested that it be done
with water in the tank so that a pressure
of about 20 kPa (a few pounds per square
inch) can be used rather than only about
1 kPa (3 or 4 in. H2O). A disadvantage of
this test technique is that the mass flow
meter must be purchased for the specific
pressure level and anticipated potential
leakage rate range that would have to be
measured.

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PART 3. Determining Leakage Rate in
Petrochemical Structures

Sealed Volume Flow Meter pressure or with gage pressures of about
Leak Testing of Large 300 Pa (1.25 in. water) in air or gas above
Pipelines and Systems the liquid petroleum product contained in
the structure. These storage tanks are
The sealed volume flow meter leak testing normally built and tested in accordance
technique can be used for leak testing of with API 650 standards.5,16-18
pipelines and other large volume systems.
The pipeline segment to be leak tested is Tanks such as those sketched in Fig. 12
plugged off at its ends. The isolated are used to store crude oil, various grades
section is connected to a source of of refined oils, jet engine fuels, kerosene,
pressurizing gas or air and equipped with gasolines and other petroleum products.
a pressure regulator and a bubbler. When constructed of corrosion resistant
Pressurized gas or air is injected through materials, they are also used to store
the bubble to fill the pipeline segment or acidic and caustic products. As single wall
large system under test. If the line or low pressure structures with external
vessel is leak tight, the bubbling will insulation, or as double wall structures
eventually cease when the pressure within with the same configurations as cryogenic
the system is equal to the pressure applied structures, they are used to store liquid
externally. If leakage exists, there will be propane, liquid butane, anhydrous
no cessation of bubbling while the ammonia and other low temperature
pressurizing source continues to inject gas liquid products. They are normally built
or air to replace that lost by leakage in the and tested in accordance with API 620,
system under test. Design and Construction of Large, Welded,
Low·Pressure Storage Tanks.6
Flow Meter Leakage
Testing of Petrochemical To reduce vapor loss from fixed roof
Structures tanks containing volatile products such as
gasoline, several of these tanks containing
The flow measurement leak test technique the same product are sometimes
finds applications in leak testing of large manifolded to a variable volume structure
petrochemical structures. Flat bottomed such as a lifter roof tank, vapor tank or
storage tanks are designed with column vapor sphere such as those sketched in
supported or self-supporting fixed roofs, Fig. 13. When properly sized, these
various types of floating roofs and constant gage pressure structures maintain
combinations of fixed and floating roofs, the pressure in the manifolded fixed roof
as sketched in Fig. 12. These structures tanks at levels below the settings of the
normally operate with atmospheric pressure relief vents and above the
settings of the vacuum relief vents. Thus,
they not only reduce loss of product
vapor but also prevent the intake of
oxygen laden atmospheric air, which can
create explosive atmospheres above
flammable products.

FIGURE 12. Petrochemical storage vessels with various roof designs.

Column Self-supporting
supported fixed roof
fixed roof
Fixed roof
Floating
roof Seal Seal Floating
roof

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Leakage Requirements for performance data on a seal design or
Petrochemical Structures when the specifications for a floating roof
tank require that the roof-to-shell seal
Functional and code leakage requirements leakage rate be determined, it is usually
for petrochemical structures such as those accomplished by the flow measurement
sketched in Fig. 12 and 13 are intended to leakage test technique. When a customer
ensure that finished product tanks have specification requires the determination
no liquid product leakage. For tanks that of the total leakage rate of a variable
are to store volatile products that are volume structure with a movable
flammable, toxic, acidic or caustic, no diaphragm, the leak test is performed by a
vapor leakage is desirable. But for tanks volume change leak testing technique.
such as those with floating roofs with
moving flexible seals, this is not a realistic Techniques Used in Leak
functional requirement. For these types of Testing of Typical
tanks the product vapor leakage must be Petrochemical Structures
sufficiently limited so that it does not
create a toxic or flammable hazard in the Preliminary leak testing of small critical
area around the structure and so that it areas of weldments or assemblies in
minimizes product evaporation losses. petrochemical structures uses bubble tests
These same requirements apply also to performed by direct pressurization or
variable volume vapor storage tanks. The vacuum box techniques. As an alternative
leak testing requirements of the API code to bubble testing, leak testing with liquid
are normally adequate for these types of tracers is performed either directly (using
structures. Any additional leak testing capillarity of penetrants rather than
performed is normally done because of pressure differentials) or with a pressure
customer specification or manufacturer differential obtained with a vacuum box.
requirements.
A pressure drop orifice flow
Selection of Leak Testing measurement test technique is adequate
Technique for for the accuracy required to determine the
Petrochemical Structures leakage rate of a floating roof-to-shell seal.
The volume change technique, auxiliary
The leak testing performed on chamber displacement technique or
petrochemical structures such as those auxiliary chamber accumulation flow
sketched in Fig. 12 and 13 is normally meter technique may be used to
limited to the code required bubble determine the total leakage rate of a
testing or solution film testing technique variable volume petrochemical structure
and the through penetrant leak testing with a movable diaphragm. These
technique. Past experience has shown techniques are described next.
that the sensitivity of these two
techniques, when they are performed
properly, is adequate. When the
manufacturer of a floating roof storage
tank desires new or additional

FIGURE 13. Various designs of variable volume petrochemical storage vessels, including some
with diaphragms.

Lifter roof Vapor tank Vapor sphere

Seal
Diaphragm
Diaphragm

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Pressure Drop Orifice Test Leakage Rate Test of
for Leakage Rate of Variable Volume Vessel
Floating Roof Seal with Moving Diaphragm

Figure 14 shows schematically the Figure 15 shows schematically the
technique for connecting a water connections of a water manometer and a
manometer to the seal space between the pressurizing line to a tank in preparation
tank shell and the outer rim plate of a for a leakage rate test for determining the
floating roof of a petrochemical storage total leakage rate of a variable volume
structure. Then the leakage rate of the structure with a movable diaphragm.
floating roof seal can be determined by Vessels of this type operate at a constant
the pressure drop orifice leak test gage pressure except at the upper and
technique, after installing an orifice with lower limits of diaphragm displacement.
a pressure regulator in a line to the seal However, due to the very low operating
space. The pressure regulator is then pressure of the diaphragm system,
adjusted until the orifice flow maintains changes in barometric pressure cannot be
the seal space at the test pressure ignored during the leakage test. Surface
specified. thermometers are placed at various
locations on the shell and the barometer
The flow rate for the system orifice for is placed in a sheltered area near the tank.
the pressure shown on the pressure
regulator is found by reference to the flow With a roof fitting open to the
chart relating flow rate to pressure drop atmosphere, air is injected into the tank
for the specific orifice being used. (The below the diaphragm until the diaphragm
downstream pressure at the outlet of the has been raised to a level near the middle
orifice can be ignored because it is of its travel range. The water manometer
insignificant compared to the orifice inlet measures this air pressure, relative to the
pressure.) The flow rate that is through atmosphere (249 Pa = 1 in. H2O; 9.8 Pa =
the orifice and that just maintains the seal 1 mm H2O). However, it is necessary to
space pressure constant is a direct measure obtain forecasted trends in ambient
of the total leakage rate of the seal. The temperature for the test duration. If the
preceding steps can be repeated using temperature trend is upward, start the
different sizes of orifice (to vary the air leak test with the diaphragm in the lower
flow rate), if needed to increase the half of its travel range. If the temperature
accuracy of the leakage rate test is expected to fall, start the test with the
measurements. diaphragm above the middle of its range.

Other leak test techniques could be At the start of the leak testing period,
used to determine the total leakage rate of record the average, surface temperature
the seal of the floating roof in this T1, the barometric pressure Pb1, and the
example. These include use of a vessel gage pressure Pv1. Also, measure the
combination flow meter and orifice or use height of the diaphragm at its center (see
of a low pressure flow meter and regulator Fig. 15). Determine the initial measured
to control inflow to match the leakage of volume Vm1. From these data, determine
the roof seal. the quantity P1V1 of gas (mass at standard
temperature conditions of 20 °C or 70 °F)
from the relation:

FIGURE 14. Arrangement of manometer and pressure drop orifice connections for leakage rate
test of the seal of a floating roof petrochemical storage structure.

Flexible seal Manometer Regulator

Air

Orifice

Shell Floating roof

Seal space

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( )(2) 293
PV = Vm Pv + Pb T + 273 Auxiliary Chamber Leakage
Rate Test of Movable
where PV is quantity of gas (Pa·m3); Vm is Diaphragm in Variable
measured volume estimated from tank Volume Petrochemical
Structure
capacity chart (cubic meter); Pv is pressure
within vessel (pascal); Pb is measured Figure 16 and 17 show schematically the
barometric pressure (pascal); and T is leak testing equipment arrangement used
for measuring leakage rates of a movable
average temperature (degree celsius). diaphragm in a variable volume storage
vessel, such as might be used in a
At the end of the required leak testing petrochemical industrial facility. In
preparation for this leak test, first remove
period, record the same parameters as Pv2, the pressure vacuum vent and blank this
Pb2, T2 and Vm2 and again determine the fitting (shown at top of sphere in Fig. 16).
final mass P2V2 of gas, corrected to the Then an automatic or manual relieving
standard temperature (see Eg. 2). The mechanism is connected to the tank, as
shown at the lower right of Fig. 16. A
leakage rate in SI units is now given by: water manometer and a pressurizing line
are next connected to the tank (as shown
(3) Q = P1V1 − P2V2 at bottom left and bottom right of
t2 − t1 Fig. 16). Then, open a roof fitting to the
atmosphere to let air above the movable
where t is time (second). diaphragm escape as pressure is applied
In applications where English units and beneath the diaphragm.

fahrenheit temperatures are used and time Pressurization and
is measured in hours, the quantity of gas Preliminary Leak Tests of
at standard conditions is computed as: Vessel Test Connections

(4) PV = Vm Pv + Pb 530 The tank volume below the movable
14.7 T + 460 diaphragm is then pressurized as follows.
With the roof fitting still open to the
where PV is quantity of gas (standard atmosphere, add air to the tank until the
cubic foot), Vm is measured volume movable diaphragm rises to the top.
estimated from tank capacity chart (cubic Continue to add air until the tank is at
foot), Pv is pressure within vessel the required test pressure. Add air to the
(lbf·in.–2 gage), Pb is barometric pressure tank or release air from the tank as
(lbf·in.–2 absolute), T is average necessary to maintain this pressure at a
temperature (degree fahrenheit) and t is fixed level during the leakage rate test.
time (hour).
After pressurization is completed, fill
Again, it is necessary to determine the the previously tested displacement
initial quantity of gas V1 at the beginning chamber with water through the inlet
of the leak test and the final quantity valve at the top of this chamber (see
P2V2 of gas, at the end of the test by use Fig. 17). Then close this inlet valve and
of Eq. 4. The leakage rate in mixed install hose or tubing from this valve on
English units is then given by: the displacement chamber to a roof
connection on the large tank. Next,
(5) Q = P1V1 − P2V2 pressurize the space above the tank
t2 − t1 diaphragm through a valve connection on
the topmost access hole cover to provide a
where t is time (hour). slight air pressure. Use bubble testing
solution to leak test the connections from
FIGURE 15. Arrangement for air injection and water the tank roof through the hose or tubing
manometer connections for leakage rate test of variable connecting the roof to the displacement
volume petrochemical vessel with moving diaphragm. chamber, to the blanked vent and to the
access hole valve. Any leaks indicated
Open must be repaired and the connections
retested before proceeding further. Finally,
Diaphragm open the access hole valve to release the
pressure above the diaphragm, then close
Air the access valve and install a plug in it.

Manometer Air

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FIGURE 16. Arrangement for auxiliary chamber displacement leakage rate test of the movable
diaphragm in a variable volume petrochemical storage tank. Overall view of spherical tank
and leak test equipment.

Close

Blank Diaphragm
in full
position Hose or tubing

Pressure/
vacuum

vent

Diaphragm Air
at beginning Displacement chamber

of test

H2O Air line Air supply
Manometer H2O

Relieving device

FIGURE 17. Detailed drawing of design of displacement chamber shown at lower right in
Fig. 16.

Locate to suit

Valve

Sightglass
Scale

Overflow

Locate
to suit

To sightglass tube (side view)
(front view) To overflow tube

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Procedure for Auxiliary Chamber where Q is leakage (Pa·m3·s–1 at 20 °C), Pb
Displacement Leakage Rate Test and Pm are pressure (pascal), V is volume
of Tank (cubic meter), T is temperature (degree

Following these preparations, the leakage celsius) and t is time (second).
rate test for the tank system of Fig. 17
may be started. A surface thermometer is If mixed English units are used, the
installed on top of the displacement
chamber and this chamber is shaded from leakage rate Q is given at a standard
direct sunlight for the duration of the pressure of 101 kPa (14.7 lbf·in.–2), a
leakage test. A barometer is placed near standard temperature of 20 °C (70 °F) and
the displacement chamber to measure
changes in atmospheric pressure. The top volume K1 (cubic centimeter) per unit of
end of the overflow tube is then adjusted height H of water in the sight glass of the
to the same height as the water in the
displacement chamber sight glass. Then displacement chamber:
the valve on the top of the displacement
chamber is opened. Start the leakage rate (8) Q = K1  Pb2 530
test just as the water begins to move. H2 14.7 T2 + 460

At the start of the test, record time t1,
surface temperature T1 of the − H1 Pb1 530 
displacement chamber, barometric 14.7 
pressure Pb and displacement chamber T1 + 460 
sight glass water height H1. During the
leakage rate test, maintain the top end of ( )÷ t 2 − t1
the overflow tube at the same height as
the height of the water by using the If the initial water height H1 is equal to
pulley cable attached to the overflow zero, this reduces to the form of:
tube. This is done in the displacement
chamber sight glass to prevent any K1 H 2 Pb2 530
pressure buildup in the displacement 14.7 T2 + 460
chamber. At the end of the required leak (9) Q=
testing time, record the time t2, the t2 − t1
surface temperature T2 of the
displacement chamber, the barometric Auxiliary Chamber
pressure Pb2 and the displacement Accumulation Flow Meter
chamber sight glass water height H2. Note Leak Test of Variable
the elapsed time t2 – t1 of test duration. Volume Structure with
Movable Diaphragm
Calculating Total Leakage Rate for
Leak Test Using Auxiliary Chamber Figure 18 shows schematically the leak
testing equipment arrangement used for
For the auxiliary chamber leakage rate test measuring leakage rates of a variable
of the variable volume tank of Fig. 17, the volume petrochemical storage vessel with
following relations are used: a movable diaphragm, using the auxiliary
chamber accumulation flow meter
(6) Q =  KH 2 Pb2 T2 293 technique. Preliminary steps include
 + 273 removing the pressure vacuum vent and
connecting a water manometer to this top
− KH1 Pb1 293  fitting, connecting an automatic or
T1 + 273  manual pressure relieving mechanism to
the tank and connecting a water
÷ (t2 − t1) manometer and a pressurizing line, all as
shown in Fig. 18. Then, with a roof fitting
In Eq. 6, the SI units are those listed open to the atmosphere, air is added to
previously under Eqs. 2 and 3 with the the tank until the movable diaphragm is
addition of the height H of the water and at the top. Additional air is injected into
K equal to the volume (cubic meter) per the tank until its pressure is equal to the
unit of height of water in the sight glass required test pressure. Add air to the tank
of the displacement chamber. or release air from the tank as necessary to
maintain this pressure throughout the
If the initial water height H1 is equal to leakage rate test.
zero, the last term of Eq. 6 disappears and
the leakage rate is given by: Flow Meter Accumulation Leakage
Test Instrumentation
K H2 Pb2 T2 293
+ 273 Instrumentation required for the auxiliary
(7) Q= chamber flow meter technique of leakage
t2 − t1 testing of the system shown in Fig. 18

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includes a cube shaped bladder, a very low Leakage Rate Testing of Tank by
pressure differential integrating flow Auxiliary Chamber Accumulation
meter, a manometer, a barometer and a Flow Meter Technique
thermometer. After tank pressurization is
complete, the previously tested cube After completion of preliminary
shaped bladder (shown to the right of the instrumentation system tests, the access
tank in Fig. 18) is connected to the roof hole valve is opened and the cube shaped
fitting at the top of the tank. The bladder completely deflated. Then the
differential integrating flow meter is access hole valve is closed and a plug
installed with a valved connection to this installed in the valve outlet. The valve
bladder. Both the manometer and the between the bladder and the flow meter is
thermometer are included in the line to closed. The leakage rate test is started and
the flow meter. The barometer is placed the starting time t1 is recorded. At the end
near the bladder container. of the required leak testing period, the
time t2 is recorded and the valve is closed
For preliminary leak testing of the between the bladder and the tank. At this
instrumentation and its connections, the time, the bladder must be at least just
flow meter outlet valve is closed. The slightly less than completely inflated or
valve in the line from the roof connection the leak test will be invalid.
to the bladder is opened, as is the valve
between the bladder and the flow meter. The bladder container (a cube shaped
Then, through a valved connection on box with one side transparent) is now
the tank top access hole cover, the pressurized to the very low differential
volume above the tank diaphragm is pressure specified for the flow meter. To
pressurized to a slight gage pressure measure the leakage accumulated within
sufficient to inflate the bladder. This the flexible cube shaped bladder, the valve
pressure is shown by the manometer on between the bladder and the flow meter is
top of the storage tank. Then bubble leak opened, as is the outlet valve from the
tests are made by applying solution film flow meter. The pressure in the box
to a bladder and flow meter connections, containing the bladder is adjusted as
the top manometer connections and the necessary to maintain a specified water
access hole valve and its connections. Any column height in the manometer
leaks indicated by the bubble test solution connected into the line between the
must be repaired and retested to show no bladder and the flow meter. Record the
leakage indications. manometer pressure m and the
temperature T in the line to the flow
meter. Also, record the barometric

FIGURE 18. Arrangement for auxiliary bladder accumulation flow meter technique for leakage
rate test of variable volume petrochemical storage vessel with movable diaphragm within the
tank.

Roof fitting

Close

Manometer Hose or tubing
Diaphragm

Air
Container (one side clear)
Bladder

H2O Air line Air supply
Manometer Flow meter
H2O

Relieving device

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pressure Pb. As soon as the bladder is fully
deflated, close both the outlet and inlet

valves to the flow meter. Record the

accumulated flow meter reading as the

volume V.

Calculation of Leakage
Rate for Accumulation
Flow Meter Leak Test

The total leakage rate Q for the vessel of
Fig. 18 is then calculated in SI units by
use of the relation:

( )(10) Q 293
V Pb + Pm T + 273

= t2 − t1

where Q is leakage rate (Pa·m3·s–1 at

20 °C), Pb and Pm are pressure (pascal), V
is volume (cubic meter), T is temperature

(degree celsius) and t is time (second).

In mixed English units, the leakage rate

is given by:

V Pb + Pm 530

(11) Q = 14.7 T + 460

t2 − t1

In Eq. 11, Q is leakage rate (std ft3·h–1 at

70 °F), T is temperature (degree

fahrenheit), Pb is barometric pressure
(lbf·in.–2 absolute), Pm is manometer
pressure (lbf·in.–2 gage, converted from
in. H2O) and t is time (hour).

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References

1. Guide to EPA Materials on Underground 12. API Publication 307-92, Engineering
Storage Tanks. EPA-510-B-94-007. Assessment of Acoustic Methods of Leak
Cincinnati, OH: Environmental Detection in Aboveground Storage Tanks.
Protection Agency (February 1993). Washington, DC: American Petroleum
Institute (1992).
2. Beall, C., L. McConnell, A. Nugent and
J. Parsons. Detecting Leaks: Successful 13. API Publication 322-94, Engineering
Methods Step-by-Step. Evaluation of Acoustic Methods of Leak
EPA/530/UST-89/012. Cincinnati, OH: Detection in Aboveground Storage Tanks.
Environmental Protection Agency Washington, DC: American Petroleum
(November 1989). Institute (1994).

3. Straight Talk on Tanks: Leak Detection 14. Cole, P.T. “Acoustic Methods of
Methods for Petroleum Underground Evaluating Tank Integrity and Floor
Storage Tanks and Piping. Condition,” First International
EPA 510-K-95-003. Cincinnati, OH: Conference on the Environmental
Environmental Protection Agency Management and Maintenance of
(July 1995). Hydrocarbon Storage Tanks [London,
United Kingdom]. East Sussex, United
4. Sherlock, C.N. “A Catch-22: Leak Kingdom: Business Seminars
Testing of Aboveground Storage Tanks International Limited (November
with Double Bottoms.” Materials 1992).
Evaluation. Vol. 53, No. 7. Columbus,
OH: American Society for 15. Miller, R.K. “Tank-Bottom Leak
Nondestructive Testing (July 1994): Detection in Above-Ground Storage
p 827-832. Tanks by Using Acoustic Emission,”
Materials Evaluation. Vol. 48, No. 6.
5. API Standard 650-93, Welded Steel Columbus, OH: American Society for
Tanks for Oil Storage, ninth edition. Nondestructive Testing (June 1980):
Washington, DC: American Petroleum p 822-824, 826-828.
Institute (1995).
16. API Publication 327-94, Aboveground
6. API Standard 620-96, Design and Storage Tanks: A Tutorial. Washington,
Construction of Large, Welded, DC: American Petroleum Institute
Low-Pressure Storage Tanks, ninth (1994).
edition. Washington, DC: American
Petroleum Institute (1996). 17. API Publication 334-96, Guide to Leak
Detection for Aboveground Storage Tanks,
7. API Standard 653-95, Tank Inspection, first edition. Washington, DC:
Repair, Alteration, and Reconstruction. American Petroleum Institute (1996).
Washington, DC: American Petroleum
Institute (1995). 18. API Recommended Practice 574-90,
Inspection of Piping, Tubing, Valves, and
8. ASME B96.1-93, Welded Fittings, first edition [replaces Guide for
Aluminum-Alloy Storage Tanks. New Inspection of Refinery Equipment,
York, NY: American Society of Section 9]. Washington, DC: American
Mechanical Engineers (1993). Petroleum Institute (1995).

9. API Recommended Practice 651-91,
Cathodic Protection of Aboveground
Petroleum Storage Tanks, first edition.
Washington, DC: American Petroleum
Institute (1991).

10. API Recommended Practice 652-91,
Lining of Aboveground Petroleum Storage
Tank Bottoms, first edition.
Washington, DC: American Petroleum
Institute (1991).

11. API Recommended Practice 575-95,
Inspection of Atmospheric and
Low-Pressure Storage Tanks, first edition.
Washington, DC: American Petroleum
Institute (1995).

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14

CHAPTER

Leak Testing of Hermetic
Seals

George R. Neff, Isovac Engineering, Incorporated,
Glendale, California
Jimmie K. Neff, Isovac Engineering, Incorporated,
Glendale, California
Donald J. Quirk, Fisher Controls International, North
Stonington, Connecticut

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PART 1. Characteristics of Gasketed Mechanical
Hermetic Seals

Functions and Limitations one side of the seal while the other side of
of Hermetic Seals the seal is subject to atmospheric pressure.
However, if it is decided to draw a
By strict definition, a hermetically sealed vacuum on one side of a seal for test
device or system is one in which the gas purposes, the leakage rate test data would
or gases contained in the internal free obviously also be applicable to hermetic
volume of the sealed system cannot seals used for vacuum sealing as well. The
escape or be exchanged with any gas, alternative to drawing a vacuum on one
vapor or liquid contained in the side of the seal would be to pressurize one
environment external to the sealed side of the seal to 200 kPa (2 atm) and
system. allow leakage to occur through the
pressure boundary of the sealed device to
In reality, such hermetic seals do not air at 100 kPa (1 atm).
exist: given enough time, any gas should
be able to permeate or diffuse through Classification of Levels of
any known material. For this reason, either Molecular Sealing by
maximum rates of leakage or degrees of Vacuums
hermeticity are usually specified for
particular operating environments and Leakage from atmospheric pressure to
applications. If a device passes the vacuum is typically molecular leakage
specified leak testing requirements for the because different gases leak at different
hermetic seal, this device can then be rates in a given seal. In addition, liquid
stated as being hermetically sealed to the solutions will be selectively separated as
degree specified. This does not imply, they go through a seal having a leakage
however, that the same part or device rate of the order of magnitude of 1 × 10–7
could not leak later because of its Pa·m3·s–1 (1 × 10–6 std cm3·s–1) or less.
deterioration, mishandling, more Selective permeability and diffusion of
stringent service, environment or other gases and liquids through solid sealing
causes. materials also depend on molecular
structure. Thus, molecular sealing is
Typical Pressure needed for prevention of leakage into
Differentials Applied to vacuum. Three commonly defined levels
Hermetic Seals during Leak of molecular sealing are (1) commercial
Testing vacuum sealing, (2) hermetic sealing and
(3) hard vacuum sealing. Vacuum levels
The purpose of a hermetic seal is to are defined by the American Vacuum
prevent any transfer of gases or vapors Society (see Table 1).1
from one area to another. It is commonly
thought that a hermetically sealed device Requirements for Sealing of
is used to preserve the contents of a Commercial Vacuum
container in some steady state. In the
strict definition of the word hermetic, a Commercial vacuum systems typically
pressure differential across the seal is not operate at pressure levels between 10 and
a requirement. However, many types of
hermetic seals are used in service under a TABLE 1. American Vacuum Society levels of vacuum.1
pressure differential of only 100 kPa
(1 atm or 760 torr) or less. Thus, many Vacuum ____________________P_r_e_s_s_u_r_e__________________
leak testing specifications call for
application of a testing condition that Level SI Unit (torr)
uses 100 kPa (1 atm) differential pressure
across the seal for purposes of testing only Low 1.01 × 105 to 3.3 × 103 (7.6 × 102 to 25)
the leakage rate of the seal. Medium
High 3.3 × 103 to 1.33 × 10–1 (25 to 1 × 10–3)
The easiest way to provide a 100 kPa Very high
(1 atm) pressure differential across the Ultra high 1 × 10–1 to 1.33 × 10–4 (1 × 10–3 to 1 × 10–6)
pressure boundary of a hermetically sealed
device or system is to draw a vacuum on 1 × 10–4 to 1.33 × 10–7 (1 × 10–6 to 1 × 10–9)

≤ 1 × 10–7 (≤ 1 × 10–9)

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0.01 Pa (100 and 0.1 mtorr). Very seldom damage to components and their
will these vacuum pressures be lower than performance.
10 mPa (0.1 mtorr) — that is, of the order
of 7 mPa (1 × 10–6 lbf·in.–2). They are Permeability Requirements
simply gross vacuums used in vacuum for Hermetic Seals Having
distillation of chemicals, vacuum melting Very Small Leakage Rates
furnaces and vacuum processes used to
reduce the amount of water in food For hermetic seals to have very low
products, for example. However, effective leakage rates, it is essential that the seals
vacuum seals are required for such and envelopes be fabricated from
systems to decrease the energy needed to materials having very low permeabilities
evacuate the system to working pressures to passage of vapors, gases and liquids.
and to maintain the system’s vacuum Fluids travel directly through permeable
when the pumps are shut off for one materials by a process somewhat like
reason or another. It should be osmosis. The main difference is that
emphasized that, at this vacuum level, the permeability refers to transmission of
main concern is to minimize leakage gases, whereas osmosis refers to
across the seal. transmission of liquids. Like solvents,
different gases have different rates of
Requirements for Sealing against travel through any specific permeable
Outer Space and Hard Vacuums material. Conversely, a single type of gas
travels through different permeable
Hard vacuums are those with pressure materials at different rates. For example,
below the level of 100 µPa (1 µtorr). These Table 2 indicates the air permeabilities of
low pressures cause problems in sealing different types of rubber materials.2 These
not encountered when sealing for higher permeabilities are expressed in terms of
vacuum pressures. The low pressures of gas at a specific temperature passing per
hard vacuum cause volatilization of unit time through a specified area of a
materials such as plasticizers in elastomer material with a certain thickness (1 m2 of
sealing materials. There can also be area with 1 m thickness), under a
outgassing or sublimation of rubber differential pressure of 100 kPa (1 atm).
materials into the vacuum. It has been The SI unit of permeability is taken as
suggested that most elastomers have (Pa·m3·s–1)·(m2·m–1)–1 or
vapor pressures between 10 mPa and
100 nPa (100 and 0.001 µtorr), so that TABLE 2. Air permeabilities of elastomers. Permeability is
they will vaporize selectively as pressures expressed in Pa⋅m3⋅s–1⋅(m2⋅m–1)–1 at 22 °C (72 °F), which
are reduced in a hard vacuum system. would permeate through 1 m2 of elastomer 1 m (40 in.)

Alternatively, it may be possible that thick at a differential pressure of 100 kPa (1 atm) at a
vaporization of such polymers is preceded
by depolymerization, whose products are temperature of 80 °C (176 °F). The permeabilities given
subject to sublimation. The loss due to
vaporization also depends on seal or are for typical reinforced compounds: special
device geometry, for only those
monomers at the surface exposed to compounding techniques can substantially change these
vacuum are removed. Certain rubbers do rates.2
sublime at a rather rapid rate in the
pressure range from 10 to 1 µPa (100 to Elastomer Permeability
10 ntorr). This sublimation is a surface
phenomenon and can erode exposed 10–12 Pa⋅m3⋅s–1⋅(m2⋅m–1)–1 or
surfaces of sealing materials and gaskets. 10–7 std cm3⋅s–1⋅(cm2⋅cm–1)–1
However, other polymers may be virtually
unaffected at vacuum pressures as low as Butyl 0.32
100 nPa (1 ntorr) at room temperatures. Thiokol™ 0.37
High acrilo nitrile 0.41
The most critical uses of seals for hard Hypalon S-2™ 0.7
vacuums have been in the aerospace Kel-F™ 0.80
industry. Hard vacuum pressure Low acrilo nitrile 0.8
conditions are attained at very high Viton A™ 0.88
altitudes in the earth’s atmosphere and in Polyurethane 0.97
outer space. In these applications, it is Chloroprene 0.98
essential that space capsules for human Acrylon EA-5™ 1.5
occupancy not lose the air needed to Hycar 4021™ 1.8
support life from their pressurized interior GR-S™ 2.9
to the near vacuum of outer space. In Natural rubber 4.4
addition, sealed devices that contain air to Fluoro-rubber 1F4 9.6
conduct heat to their external surfaces Fluoro-silicone 12.8
could lose their heat dissipation capability Silicone 45.0
if this air were lost by leakage to the
vacuum of outer space, with resultant

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(1 std cm3·s–1)·(cm2·cm–1) –1 at 80 °C Requirements for Seals to
(176 °F) and at 100 kPa (1 atm) Maintain Internal
differential pressure. Because the Atmospheres in Outer
quantities are very small, it is expressed as Space Systems
(µPa·m3·s–1)·(m2·m–1)–1. This unit reduces
to µPa·m2·s–1 but is unrecognizable in this There are two critical requirements for
form. In Table 2, butyl has a permeability seals and gaskets that are of importance in
of 3.2 × 10–13 (Pa·m3·s–1)·(m2·m–1)–1 the hard vacuum and other operating
whereas silicone has a permeability of conditions of outer space environments
4.5 × 10–11 (Pa·m3·s–1)·(m2·m–1)–1 or more (see Table 3).3 One is the effect of hard
than 100 times greater. vacuum on the physical characteristics of
flexible sealing materials; the second is
Requirements for Seals to the effectiveness of all seals in
Maintain Internal maintaining the earth’s atmosphere
Atmospheres in Near within vehicles or individual components.
Space Systems Many space vehicle components (as well
as astronauts) can function only in a
Hermetic seals are used to contain preserved earth’s surface environment.
breathable air atmospheres in aircraft that Consequently, seals that can cope with
fly in near space vacuum conditions in molecular leakage are required to
the earth’s atmosphere at altitudes up to maintain pressure in these components
30 km (100 000 ft). Leak testing for this for long time periods. Surveillance and
type of vacuum imposes one more communication satellites, as well as deep
condition in addition to those required
for hermetic sealing of devices whose seals TABLE 3. Characteristics of outer space environment.
are not subject to vacuum pressure
differentials across their pressure Low pressure
boundaries. This unique condition is the 10–4 to 10–12 Pa (10–6 to 10–14 torr) dependent on altitude
slight movement of the seal gasket in
response to the 100 kPa (14.7 lbf·in.–2) Low density
pressure differential repeatedly impressed 10–9 to 10–17 kg⋅m–3 (1.7 × 10–9 to 1.7 × 10–17 lbm⋅yd–3)
across it. Such movement is aided by a
small shrinkage of rubbery materials used Chemical composition
for flexible seals. In aircraft, movement of Dissociated molecules
seals used in access doors and wing panels Ions
was overcome by seals that were molded
in place. Many vacuum seals use the same Thermal radiation, influencing vehicle temperature
closely controlled sealing principles to Infrared solar radiation
attain completely reliable and reusable Earth’s albedo
performance. Infinity radiation sink (0 K)
Early space probes indicate thermal equilibrium at 70 °C (160 °F)
The movement of a gasket or seal
caused by low pressure and shrinkage of Other solar radiation
the rubbery element of mechanical seals Visible radiation
in a vacuum environment acts to displace Ultraviolet radiation
the flexible sealing element just enough X-radiation
to break the original line of sealing
contact. Not enough force is generated by Cosmic radiation
the system pressure to reestablish a seal Electromagnetic (gamma rays, X-rays)
on the interfaces of the sealing surfaces if Primary particles (protons, atomic nuclei)
contact is once broken (see Fig. 1). In Secondary particles (electrons, positrons, mesons, neutrons)
some seal designs, the ratio of rubber to Van Allen belt radiation (protons or electrons)
void space within the seal is controlled to
eliminate this problem of low pressure Meteoric particles
rollout. This prevents slight movements of Penetration
seal. It also creates high contact loading at Abrasion
the sealing line that increases the flow of
the flexible gasket material into the Force fields
microirregularities of the sealing interface. Electromagnetic
This reduces or eliminates the problem of Gravitational
shrinkage of the rubbery sealing material
when seals are subjected to near space Conditions of Human Origin
vacuum levels of 100 to 1 µPa (1.0 to Propulsion products
0.01 µtorr). Vehicle outgassing
Acceleration
Vibration
Space debris
Hostile action

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FIGURE 1. Rollover of loose gasket that breaks hermetic seal at original contact surface, and of
molded-in-place seal that resists loss of sealing contact as the pressure across seal varies
during service: (a) O-ring in improper gland for low pressure sealing; (b) molded-in-place seal
maintains same configuration regardless of pressure level.

(a)

Seal with no pressure applied Seal with low pressure applied Same seal with high pressure
(some portions roll as shown,
(b)
other portions maintain
original configuration)

Before assembly After assembly at any

TABLE 4. Leakage rates for elastomer at vacuum conditions.3

_____________A_i_r _L_e_a_k_a_g__e_R_a__te____________

_______T_e_s_t_P_r_e_s_s_u_r_e_________ Leak Testing µPa⋅m3 per mm (std cm3 per inch
Pa (torr) Method
of Seal per Annum of Seal per Year)

1 × 105 (7.6 × 102) Radioactive gas 3 (0.0008)
6.7 × 10–4 (5 × 10–6) Helium leak detector 600 (0.1500)
2.9 × 10–5 (2.2 × 10–7) Pressure decay (0.0011)
4.4
2.9 × 10–6 (2.2 × 10–8) (ion pump) (1.2200)
Pressure decay 4 900

(ion pump)

space probes, that are designed to operate A weight loss test of this material made
in deep space for many years are in by exposing a microtome of material to a
special need of adequate molecular sealing vacuum of 3 µPa (20 ntorr) showed a
to prevent loss of internal atmospheres. weight loss of about 31 percent whereas at
a pressure of 13 µPa (0.1 µtorr) the weight
One way to provide long term sealing loss was only 2.0 percent. The typical
against outer space vacuum environments leakage rate for this material at various
is to select envelope and sealing materials pressures (by using appropriate leak
that have low vapor pressures. For testing techniques for each pressure
example, hard vacuums are known to range) were reported (Table 4). The
erode metals such as cadmium and leakage rate obtained when using the
magnesium. Elastomeric materials that pressure decay test technique and air as
have low vapor pressures (see Table 2) the medium were lower than those found
must be used for flexible sealing elements. by adjusting the helium leakage rates for
A seal made of silicone rubber would molecular weight ratios. This suggests that
allow the pressurized air within an outer the viscous flow rate assumption may not
space vehicle component to leak into the apply if the flow of air through a leak
void of space. The effect of progressively path is restricted by forces of molecular
harder vacuum levels rapidly increases the attraction in a rubber polymer, for
out-leakage in outer space. For example, example.
the leakage rates measured at 3 µPa
(22 ntorr) were ×103 greater with the same
material than when measured at 30 µPa
(220 ntorr).

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PART 2. Characteristics of Hermetically Sealed
Packages

Some Functional (Some electrooptical devices are
Requirements for controlled or switched on or off by
Hermetically Sealed light signal inputs.)
Packages 7. The package must isolate the internal
electronic package from external
In general, packages for hermetic devices electromagnetic radiation. Also, in
are designed to meet numerous specific some instances, the package must act
requirements based on anticipated service as an electromagnetic shield to
and environmental conditions. Some prevent internal circuit radiation from
common requirements include the permeating the enclosure and
following. adversely affecting the operation of
other external circuitry.
1. The package must provide a 8. The package must provide a physical
mechanically stable device enclosure shield for mechanical contacts and
so that reliability screening tests do moving parts in relays and similar
not affect the operational devices. It must physically protect
characteristics of the device. Typical explosive powders and charges in
screening tests for high reliability ordnance devices.
service applications involve subjecting
the devices to vibration, drop shock, Conditions Causing Leaks
centrifuge, thermal shock test in Glass-to-Metal and
conditions and leak testing. Glass-to-Ceramic Seals.

2. The package must provide electrical The reliability of many devices depends
feed-through connections from the on the degree of package hermeticity. For
internal electronic device to the many military or high quality devices, the
package externals electrically insulated critical manufacturing process is the
from the package that supplies power formation of the glass-to-metal or
to the device. glass-to-ceramic seals. The quality of the
final seal determines the degree of
3. Some packages must provide means package hermeticity.
for heat transfer away from an
electronic device within the package Generally, this manufacturing process
to avoid damage by overheating. is well controlled. However, much more
Operating parameters can change critical processes are required to provide
significantly with variations in hermetic seals around a large number of
temperature. electrical lead wires when maintaining
electrical insulation between the various
4. The package must be resistant to parts of the package. Manufacturing
corrosion from external environments. conditions that can affect both the quality
of initial seals and the degradation of
5. The package must be hermetic (sealed glass-to-metal or glass-to-ceramic seals
against air or gas flow in or out) to when they are exposed to severe
prevent exposure of the internal environmental or operating conditions
device to ambient environments, such as high temperatures, thermal shock
particularly those with high relative and mechanical stressing include
humidity (high moisture content). The (1) differences in coefficients of thermal
electrical parameters of discrete expansion of metal and glass or ceramic
elements comprising an integrated materials forming the seals, (2) uniformity
circuit can change values or be of oxide layers grown on the metal
degraded when exposed to a moist surfaces to aid bonding to glass or
environment. The proper function of ceramics, (3) cleanliness of the metal
relay contacts can be degraded by surfaces before growth of these oxide
contaminates entering the device. Or, layers and (4) quality or volume of glass
the reliable performance of an surrounding the lead wires and metal or
explosive (such as an air bag firing ceramic surfaces.
system) can be impaired by moisture.

6. The package must provide a shield
against light from external sources
because semiconductor junction
device parameters are light sensitive.

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Functions of Oxide Layers base with (2) the lead frame resting on
in Glass-to-Metal and (3) a slurry layer of glass beads and
Glass-to-Ceramic Seals organic binder. During the heating, the
glass melts, the binder begins to burn off
A slight mismatch in the thermal and the frame sinks into the glass and
coefficients of expansion of metal and become partially bonded to the base.
glass, for example, is sufficient to cause a Devitrification of the glass begins during
separation of the metal from the glass at this process and continues during the
some temperature, thus permitting subsequent semiconductor die
possible leakage. To prevent such attachment. The process for attaching the
separation between metal and glass in semiconductor die involves a
hermetic seals, an oxide layer is grown on glass-to-metal eutectic bond made at
the metal surfaces before sealing them to temperatures as high as 425 oC (800 °F).
the glass. The fusing then results in a
continuous transition from (1) the metal After wire bonding and visual
to (2) metal oxides to (3) metal oxides in inspection, the ceramic cap is put in place
glass and finally to (4) the glass. The and the final sealing is performed at a
ceramics most commonly used for temperature in the range from 450 to
hermetic seals are themselves metal 525 oC (850 to 975 °F). If all elements of
oxides, so their sealing operation involves the manufacturing process are completed
formation of transitions (1) from ceramic successfully, the resultant dual inline
(2) to ceramic in glass and finally (3) to packages may be hermetically sealed. If the
glass. manufacturing process uses improper
materials or is not precisely controlled,
It is essential to remove oxides, oil or the resultant dual inline packages may
particle contaminations from metal become leakers.
surfaces before processing them to grow
the oxide transition layers used in Causes of Leaks in Dual
glass-to-metal seals. The cleanliness of Inline Circuit Packages
these metal surfaces before oxide growth
and careful control of the oxidation Manufacturing Conditions
process that forms the oxide layer are
critical factors in control of the thickness Seal failures in solder-and-glass dual inline
of the oxide layer. In general, any surface packages may be caused by inadequate
contamination of the materials used control over the manufacturing processes
during the sealing process will produce a (described just previously) or from
nonuniform seal. Uniformity of oxide improper handling. Inadequate control of
thickness is required for high quality the heating process and the quality and
hermetic seals. mixture ratio of the glass and binder can
result in a leaking seal. If the glass is
Process of Manufacturing exposed to excessively high temperature
of Solder Glass Dual Inline during sealing, the organic binders will
Circuit Packages not burn off before the glass devitrifies (is
made hard and opaque, as by prolonged
Dual inline packages (DIPs) are presently heating). This can result in gases frozen
used for more than 50 percent of all into the seal in small, bubblelike cavities.
hermetic integrated circuits, of which tens Structurally weak seals, which may be
of millions or more are manufactured porous, can also result. (Such excessive
each year. In fabricating dual inline temperatures could occur if, for example,
packages, the glass used to seal the metal a furnace belt were stopped by a power
lead frame (for external connections) with failure, if a package base were left too long
the ceramic base and cap is introduced as on a heater block in preparation for die
a slurry mixture of vitreous glass beads attachment or if the die attachment
suspended in an organic binder. In process were too slow.)
general, the sealing process involves the
heating of this mixture in such a way as Cavities and porous seals can also
to allow the organic binder to evaporate occur if the temperature at which the
while the glass is transformed from its frame is attached to the base is too low. In
initial vitreous or liquid state to a this case, the glass under the frame does
devitrified or crystalline state that forms not outgas sufficiently. Blow holes can
the seal. then result during the final sealing
operation due to the buildup of gas
During manufacture in a particular pressure. (Such insufficiently high
process, the devitrification process begins temperatures might be caused, for
with an initial heating in a belt furnace of example, by increasing the speed of the
an assembly consisting of (1) the ceramic furnace belt to shorten the processing
time.)

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Some electro tin plating processes used Leaks Caused by Improper
to provide a bright finish on the external Handling of Other
lead frames of the sealed packages can Hermetic Packages
also cause leaks. Acids used in this plating
process can deplete the glass, particularly The improper handling of mechanical
adjacent to the metallic lead frame. Such seals in the manufacturing process, the
leaks are most prevalent in parts that have testing process and the customer
been exposed to plating solutions for application process are major causes of
extended periods of time. These leaks damage to hermetic seals. Many
could also result in devices that have been glass-to-metal feed-throughs frequently
replated, as in attempts to strip and depend on a meniscus seal at the surface
replace tin plated leads that oxidized of the pin-to-glass interface. The seal is
during burn-in test. easily damaged or broken by handling.
Such devices as relays use up to a dozen of
Sealants these feed-throughs, all of which are
vulnerable to this type of damage. A
Care must be exercised in controlling the commonly applied technique for this is
seal materials used in manufacture of dual an adjunct sealant in the form of an epoxy
inline integrated circuit packages. Too film applied over the glass-to-metal seals.
high a concentration of the binder (in the It is allowed and defined in the military
glass bead mixture) can result in failure to standards for relays. Adjunct sealants are
burn off all of the binder during the known to be only temporary under most
sealing process. The glass mixture used for conditions. Relay socket insertion is
sealing must also be evaluated to ensure endless and causes damage to pins due to
that it does not exhibit any abnormal misalignment in and out of the sockets,
devitrification behavior. For example, often the fault of the technician.
some lots of glasses have exhibited a
partial devitrification at an abnormally Hybrid electronic devices, radio
low temperature that has prevented the frequency devices and many of the very
complete removal of the binder during large electronic, electromechanical devices
the sealing process. Another potential are subjected to extreme thermal cycling
problem with the incoming glass is that that induces severe stresses on hermetic
additives designed to adjust the thermal seals and makes them additionally
coefficient of the expansion may not be vulnerable to subsequent handling.
in the proper balance. Integrated circuit
packages made with such glasses may fail The ordnance devices are frequently
in later temperature cycling tests. Such installed in physically abusive
undesirable types of glasses can be environments. Their handling is often
eliminated in incoming inspection by damaging to seals due to lead wire
making thermal expansion tests on each soldering or welding and potting.
lot of glass materials received.
Manufacturing Conditions
Improper Handling Causing False Indications
of Leakage in Packages
Inadequate control of the mechanical
handling of integrated circuit (IC) So-called false indications of leakage can
packages can result in the degradation of result from causes such as surface porosity
well made package seals. The application of the seal material used in manufacture
of excessive physical stress on the package of dual inline packages for integrated
leads can be transmitted to the brittle circuits or electronic devices. Surface
glass seals, causing fracture and cracking porosity could result from inadequate
that permit leakage to occur. Examples of control of the seal materials and the
handling procedures that could apply sealing processes. Such porosity can trap
excessive stress to dual inline integrated the leak testing tracer gas or liquid so as
circuit packages include (1) clipping to produce an erroneous indication of a
excess length from leads, (2) excessive or hermeticity failure. False cavities with
repeated flexing of leads, (3) dropping of volumes nearly as large as 10 mm3 can
integrated circuit packages, possibly in sometimes be found in the seal materials
automatic handling and loading near the end of the integrated circuit
operations, and (4) excessive heat when package, totally separated from the
soldering integrated circuits to printed internal cavity of the package.
circuit boards.
Perhaps the amount of seal material
has been selected only on the basis of
providing a desired thickness of seal in
the area of the lead frame. If too little seal
material is used, it may happen that the
surface tension of the liquid glass will pull

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the seal material toward the frame to higher temperatures. Such leaks are
leave a false cavity at either end of the apparently the result of weak seals that
integrated circuit package during the fail under the stress resulting from
sealing process. differences in the thermal coefficients of
expansion of the constituent parts of the
The glass in this area may also be seal. This behavior is not restricted to one
weaker than that near the frame and type of package but rather has been found
package cavity. In this case, cracks or a in a wide variety of integrated circuit
porous seal may develop to provide a leak packages.
to the outside of the package. Such a false
cavity would give a leakage indication Analyses have also revealed that high
that could not be distinguished from a temperature burn-in tests and baking
true leak to the device cavity by using before gas analysis alter the types and
either the helium or radioactive tracer gas concentrations of gases of the ambient
leak testing techniques. In some cases, a atmosphere within the integrated circuit
false leak can be verified by using a liquid packages, especially with fine leakers. For
dye penetrant test. In all cases it should example, such tests have resulted in
be noted that specifications state that the effects such as (1) increases in the
package shall not leak. Therefore, it is concentration of hydrogen, (2) decreases
valid to reject these leakers. in the concentration of water and
(3) increases or decreases in the
Handling Conditions concentration of carbon dioxide.
Causing False Indications
of Leakage from Hermetic The outgassing of package materials
Packages during a reliability stress testing at
elevated temperatures is a possible source
False indications of leakage, usually seen of contamination of the internal
only during helium leak testing, can be atmosphere that could be detrimental or
caused by careless handling that leaves degrading to the operational
fingerprints, pencil marks or dirt on the characteristics of the hermetically sealed
surfaces of hermetically sealed packages. A electronic device. In other cases, the
single fingerprint has been shown thermal shock tests (specified in military
experimentally to increase an initial qualification procedures) had fractured
leakage rate reading of 10–8 Pa·m3·s–1 the glass-to-metal seal in the lead frame of
(10–7 std cm3·s–1) to an erroneously high integrated circuit packages. These
indication level of 5 × 10–7 Pa·m3·s–1 fractures, that allowed water vapor to
(10–6 std cm3·s–1). This would correspond enter into the package interiors during the
to an increase of 50 times the true leakage subsequent water quench, went
rate before the fingerprint was made. undetected in the hermeticity leak tests
Treating electronic packages with a that followed.
solvent rinse, followed by a high pressure
air blast, after helium pressurization, can Sources of Water
reduce this problem significantly. Some Contamination during
users have found that a 10 min bake at Manufacture of Sealed
100 oC (210 oF), following the helium Circuit Packages
pressurization, improves the repeatability
of the helium leak testing operation. The contaminant of greatest concern in
Surface gas rejections are almost the internal ambient atmospheres of
nonexistent with the radioactive gas test sealed integrated circuit and electronic
technique. component packages is water (both
adsorbed and in the form of water vapor).
Elevated Temperature It is generally believed that there is a
Testing Conditions Causing relationship between the relative
Leaks in Hermetic Seals concentrations of oxygen and water that
will penetrate a leaking package, with the
In addition to standard hermeticity tests ratio of oxygen to water content near
performed at room temperatures, it has seven to one. However, there are no
often been very useful to perform similar general data to indicate at what
tests at elevated temperatures in the range concentration moisture has an adverse
from 55 to 85 oC (131 to 185 °F). This effect on the sealed electronic devices.
range includes the typical operating The ideal situation would be that in
temperatures of many sealed devices. It which the internal gas atmosphere in the
has been found that some devices that sealed integrated circuit package was pure
pass fine leak tests performed at room nitrogen. The presence of appreciable
temperature will fail when tested at concentrations of moisture makes the
measurement of package leakage rates
more difficult.

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PART 3. Techniques for Gross Leak Testing of
Hermetically Sealed Devices

Definitions of Gross and reliability uses, hermetically sealed devices
Fine Leak Testing of with leakage rates greater than some
Hermetic Seals prescribed value are rejected. This limit
may be as small as 1 × 10–9 Pa·m3·s–1
The gross end of the range of leakage rates (1 × 10–8 std cm3·s–1). Leak test screening
of hermetic seals is defined as those has been necessary for achievement of
devices with leakage rates grater than high reliability systems, not just for
10–7 Pa·m3·s–1 (10–6 std cm3·s–1). In spite of military uses but for many other uses.
the fact that these leakage rates are called Typically, these leak tests may cause
gross, the leaks are those that would occur rejection of about two percent of the high
with circular holes with diameters in the quality devices in production tests, but in
range of micrometers. Figure 2 shows the some cases rejection rates as high as
theoretical relationship between 30 percent are found in critical military
equivalent circular hole size and leakage tests. Failures of hermetically sealed
rates. Actually, the leaks in hermetic seals semiconductor packages have lead to
are not circular holes, but rather consist of about 13 percent of the operational
cracks or pores of various sizes. Thus, the failures that occur in high reliability
task of leak testing such devices is more integrated circuit electronic systems.
difficult than would normally be implied
by the word gross. In hermetically sealed Although hermetic seal testing is both
packages, overall leakage rates greater necessary and extensive in industry, there
than 10–8 Pa·m3·s–1 are typically found to is still a lack of a sound technical basis for
be harmful. clear cut specifications for maximum
allowable leakage rates.
The fine end of the range of hermetic
seal leakage rates extends downward from It has been established by experiment
10–7 Pa·m3·s–1 to an indefinite limit, and specified in the military standards,
perhaps as low as 10–12 Pa.m3·s–1. For high that all wet or liquid immersion gross leak
test procedures interfere with the
reliability of the dry gas leak testing of
hermetic seals.

FIGURE 2. Theoretical relation of leakage rate to ideal circular hole leak diameter, assumed for
hermetic seals. Empirical test data obtained with capillaries ranging from 8 to 30 mm (0.3 to
1.2 in.) in length.

10.0 (40)

Leak diameter, µm (10–5 in.) 1.0 (4.0)

0.1 (0.4)

0.01

10–9 10–8 10–7 10–6 10–5 10–4
(10–8) (10–7) (10–6) (10–5 ) (10–4) (10–3)

Leakage rate, Pa·m3·s–1 (std cm3·s–1)

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Major Leak Testing surfaces. The clear glass devices are then
Techniques Used for Gross examined visually for dye indications
Leak Testing of Hermetic trapped within the cavities of the
Devices transparent device packages.

Gross leak testing techniques used for The use of dye penetrant for leak
testing hermetically sealed electronic testing of opaque packages for electronic
devices in the gross leakage rate range devices has not been found to be reliable.
from 10–7 to 10–3 Pa·m3·s–1 (10–6 to For example, 100 devices that indicated
10–2 std cm3·s–1) have included (1) liquid large leaks during a fine leak test were
dye penetrant tests, (2) elevated subsequently tested for gross leaks with
temperature bubble tests, (3) back pressure the dye penetrant leak testing. A
bubble tests, (4) weight gain leak tests, commercial dye penetrant was used and
(5) holographic leak tests and the devices were bombed during
(6) krypton-85 pressurization techniques. immersion in this liquid penetrant for 3 h
at a pressure of 600 kPa (90 lbf·in.–2 gage).
Each of the preceding leak tests will After their exterior surfaces were cleaned,
detect leaking devices. However, the these devices were placed in a vacuum
choice of which test to use depends on chamber to produce a higher differential
the type of package used to enclose the pressure. They were carefully observed for
device, as well as on the choice of evidences of the dye leaking back out of
additional fine leak tests to be used on the the packages. After having been sorted
same device packages. Cleanliness is into good and bad groups, these packages
absolutely essential for accurate leakage were repressurized and opened at
testing with hermetically sealed device atmospheric pressure to determine if there
packages. Dye penetrant leak tests are was any evidence of dye within the
used for glass enclosed devices (where the cavities. In these tests, 19 percent of the
dye can be seen through the device “good” devices (that had shown no
enclosure in case of leakage) and for evidence of leakage in the dye penetrant
failure analysis of devices whose tests) were found to contain dye within
enclosures can be opened to reveal the sealed cavities. Of the rejected “bad”
leakage paths during destructive failure devices that showed prior indications of
analyses. Elevated temperature bubble leakage in the dye penetrant tests,
testing is used for testing the gross leaks 3 percent were found to have no dye
into the 10–5 Pa·m3·s–1 (10–4 std cm3·s–1) within the cavities. After duplications of
range. Back pressure leak testing is used the dye penetrant tests on other devices
for leak testing into the 10–6 Pa·m3·s–1 produced similar unreliable test
(10–5 std cm3·s–1) leakage range. Weight indications, it was concluded that the dye
gain leak tests are used for leak testing to penetrant gross leakage test was not useful
the limit of gross leakage ranges of for leak testing of devices sealed in
10–6 Pa·m3·s–1 (10–5 std cm3·s–1). The opaque packages.
radioactive krypton-85 back pressurization
is being used for gross leak testing of both Limitations and Advantages of
devices with cavities and for zero cavity Dye Penetrant Tests of Electronic
devices. The application with cavities uses Devices
an extension of the fine leak applications.
The zero cavity technique uses the The primary limitations of dye penetrant
adsorptive gettering of krypton-85 gas by leak testing of hermetically sealed
activated carbon that has been enclosed electronic devices are (1) interference
within the device. caused by particulate matter in the dye
penetrant liquids that plug up holes and
Dye Penetrant Gross Leak (2) improper cleaning of the exterior
Testing of Hermetically surface of the devices under test, following
Sealed Electronic Devices pressurization, to remove the excess
surface penetrant. Inadequate cleaning can
A visible dye liquid penetrant is result in rejection of good devices.
commonly used for gross leak testing of Overcleaning can wash the dye out of the
clear glass hermetic packages for leakage paths in device packages, in the
electronic devices. The technique used is largest part of the gross leakage range.
to apply external pressures of 300 to
600 kPa for bombing times of the order of However, the dye penetrant test
3 to 10 h to the device packages when technique is useful as a failure analysis
submerged in fluorescent dye or in a technique that can be used not only to
commercial dye penetrant liquid. The confirm the existence of the leak, but
package exterior surfaces are then washed sometimes also to identify the leakage
to remove excess penetrant from the outer path through the device packages. If the
cavity within the device package has not
filled completely with the dye penetrant,
the dye indications usually make the
leakage path definable.

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Elevated Temperature immersion baths. Identification tags and
Bubble Leak Testing of coating are the most common causes of
Hermetic Devices contaminants on the surfaces of devices
under test. Lint is the major contaminant
Elevated temperature bubble testing is in the immersion bath itself.
performed by immersing the hermetically
sealed device package under test in a hot Surface contamination of microcircuit
fluid and observing any indications of packages can be eliminated by careful
bubble emissions caused by gas escaping selection of the time and location during
through leaks as a result of the increase in production processing of devices under
internal temperature and contained gas test. Alternatively, the devices under test
pressure. For microcavities, the normal could be subjected to precleaning to
technique is to immerse the devices in remove surface contaminants and
125 °C (257 °F) perfluorocarbon liquid for adsorbed gases before immersion in the
a period of 30 s, observing any bubble bubble test bath. Cleanliness of the
emissions and rejecting the sealed package immersion bath fluid is best controlled by
for any leakage indicated by a stream of a circulated filtration system and by
bubbles. A stream of bubbles is defined as prerinsing of the test specimens. The
a visual indication of more than two problem of emission from heater elements
bubbles originating from the same point can be eliminated by the immersion types
on the package and typically rising of heaters or by sand placed so that
through the immersion fluid. external heaters are not in direct contact
with the glass container used for the
In immersion bubble testing, most of immersion fluid.
the hermetically sealed devices that are
rejectable will be characterized by a very Back Pressure Bubble
small stream of bubbles that will start to Emission Leak Testing of
be visible after about 15 s of immersion in Hermetic Devices
the heated liquid bath and last until test
object removal after 30 s. Because these Back pressure bubble testing is one of the
bubble streams are sometimes very fine more commonly used gross leak tests.
and hairlike in appearance, it is necessary Many persons start to use this test when
to illuminate the immersion bath they have the first flat tire on their
inspection zone with an intense light bicycle, immersing the pressurized tire in
(such as the lamp used with a microscope water to see where bubbles indicate leak
or a projection lamp). The background locations. Generally, the back pressure test
against which the bubble emission stream is conducted by immersing a test
is to be observed should be dull and specimen that has an internal pressure
nonreflective to provide high contrast for greater than 100 kPa (1 atm) into a liquid
the illuminated bubbles. The observer bath and by watching for streams of
should use an optical magnifier of at least bubbles coming from it. The higher this
3× magnifying power that provides a field internal pressure, the greater the chance
of observation large enough that all of the of leak detection.
visible surface of the device being tested
can be seen at one time. The angle of The normal technique used to provide
view should be at least within 30 degrees back pressurization of sealed microcircuit
of the perpendicular to the area under packages is first to evacuate each device to
observation. Inspectors should have a vacuum of 700 Pa (5 torr absolute) for a
adequate vision acuity and patience to period of 1 h. Then the devices are
persist in observation for the entire pressurized when immersed in a
immersion period. perfluorocarbon liquid with a low boiling
point, that is, 50 oC to 57 oC (122 oF to
Control of Interfering Effects in 135 oF). Pressurization occurs either (1) at
Elevated Temperature Bubble Leak an absolute pressure of 620 kPa
Testing (75 lbf·in.–2 gage) for 2 h or (2) at a gage
pressure of 200 kPa (30 lbf·in.–2 gage) for
Effects that interfere with the ease and 10 h. The test devices are then air dried
reliability of elevated temperature bubble for a period of about 2 to 4 min before
testing of sealed devices include leak test processing by the elevated
(1) contamination on the surfaces of temperature bubble testing technique.
devices under test that causes false
bubbling or bubble emission from In back pressure testing, the time
adsorbed gases or sources other than between removal of the test devices from
leaks, (2) particulate matter in the the pressurizing fluid and their immersion
perfluorocarbon immersion bath that may into the hot bath of bubble testing fluid
look like a bubble stream and (3) bubbles must be carefully controlled. If the device
generated from the heaters used in the is immersed too quickly, it will be covered
with escaping bubbles (and look like
carbonated water). This makes it very

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difficult to decide whether bubbles this fluid occurs during a bombing time of
indicate true leaks. If the test devices are from 2 to 10 h, the pressure rise will be
dried for an excessive time period, the quite significant when the liquid is
fluid that had been forced in during brought to a boil at 50 oC (122 °F). This
pressurization will have time to escape extends the range of leakage
and evaporate. The best range of waiting measurements to leakage sensitivities
time between removal from the considerably better than those attainable
pressurization bath and immersion into with elevated temperature bubble tests
the bubble testing bath is in the range of made with no pressurization.
2 to 5 min. However, the size of the sealed
electronic package and its material also Weight Gain Leak Testing
have to be considered to select the of Hermetically Sealed
optimum wait time. Electronic Devices

Characteristics of The measurement of weight gain of
Perfluorocarbon Test hermetically sealed electronic devices is
Liquid Used in Weight not a widely used technique for their
Gain and Back Pressure gross leak testing. The technique can be
Bubble Leak Testing quantitative and the leakage rate can be
determined if the bombing time and
Figure 3 shows that the rate of flow of a pressure are known because the rates of
perfluorocarbon test liquid is about flow of the perfluorocarbon test fluid are
0.1 mg·min–1 under a pressure of 700 kPa known.
(100 lbf·in.–2 gage) through a leakage path
that would have a standard leakage rate The test procedures used in weight gain
for air of 5 × 10–7 Pa·m3·s–1 (5 × 10–6 std leak tests include the following steps:
cm3·s–1). The perfluorocarbon liquid (1) weigh each specimen to be tested,
shown in this example has a density of (2) pressurize the specimen in a
1.7 g·cm–3 at 25 oC (77 °F). This liquid perfluorocarbon liquid for a specific time
boils at 50 oC (122 °F). period at a specified pressure, (3) reweigh
the specimen to determine if liquid
Thus, it is apparent that, if a sealed in-leakage has occurred and (4) compare
electronic device leaks and in-leakage of the pretest and posttest weight values.

FIGURE 3. Leakage flow rates observed with liquid fluorocarbon test fluid under pressures of
200, 400 and 700 kPa (30, 60 and 100 lbf·in–2 gage), passing through leaks with known
atmospheric pressure air leakage rates. The gas leakage rates of the capillaries were measured

using one atmosphere of helium at one end of the capillaries and a mass spectrometer helium

leak detector at the other end. (These data were converted to equivalent rates of air leakage.)

The fluorocarbon liquid leakage rates were measured by connecting one side of each capillary

tube to a pressurized reservoir of fluorocarbon liquid, connecting the other end to an empty

reservoir and measuring the amount of fluid that had passed through the capillary in 24 h.

Five data points were obtained at each leakage rate and at each pressure.

Flow rate, g·s–1 (oz·min–1) 1 × 10–2 (2.1 × 10–2) 700 kPa
(100 lbf·in.–2 gage)
1 × 10–3 (2.1 × 10–3)
400 kPa
1 × 10–4 (2.1 × 10–4) (60 lbf·in.–2 gage)

1 × 10–5 (2.1 × 10–5) 200 kPa
(30 lbf·in.–2 gage)
1 × 10–6 (2.1 × 10–6)
10–6 10–5 10–4 10–3
1 × 10–7 (2.1 × 10–7) (10–5) (10–4) (10–3) (10–2)
10–7
(10–6)

Leakage rate, Pa·m3·s–1 (std cm3·s–1)

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Before pressurization, the electronic packages, generally bodies with metal lids,
devices to be leak tested are first weighed, by using laser light reflected off the lid of
usually to the nearest 0.1 mg the package being evacuated. The
(3.5 × 10–6 oz) or sorted by weight into a technique measures the leakage rate of all
cell that has a span of 0.5 mg packages on a tray simultaneously.
(1.8 × 10–5 oz). The devices are then Devices are placed in a chamber and the
normally subjected to a vacuum with a chamber is either pressurized or
pressure of 650 Pa absolute (5 torr) for a evacuated. The resultant change in
period of 1 h, before pressurization. The pressure on the lid of the package will
test devices are then immersed in a liquid cause the lid of a package to undergo
perfluorocarbon. The bombing time (or concave or convex deformation. Multiple
time of device exposure to the pressurized hybrid electronic packages mounted on
liquid) is typically in the range of 2 to circuit boards can be leak tested by this
10 h, with bombing pressures of 300 to method, which uses laser interferometry
600 kPa (45 to 90 lbf·in.–2 gage). After to measure microscopic deformations in
completion of this pressurization period, the lid of the package (Fig. 4a).
the test devices are air dried for periods of
2 to 5 min, then reweighed to the same The package lid theoretically will not
accuracy as that used during preweighing. deform if there is a severe, gross leak in
If an electronic device gains more than the seal, as the internal and external
1 mg (3.5 × 10–5 oz) of weight or if its pressures will remain the same. For fine
weight shifts by more than 0.5 mg leaks, when a package lid is deformed due
(1.8 × 10–5 oz) cell, it is rejected. Devices
that show no significant gain in weight FIGURE 4. Holographic leak testing system operation:
between initial and final weight (a) schematic of application to hybrid electronic packages
measurements are usually accepted for with metal lids; (b) holograhic image of two pharmaceutical
this gross leak test based on weight gain. blister packages with ten cavities

Radioactive Gas Gross Leak (a)
Testing
Laser Charge coupled device
Krypton-85 tracer gas is used to detect video camera
gross leakage in devices with cavities Beam
> 0.5 cm3 (> 0.03 in.3) by pressurization at expansion Phase sensitive optics
pressures from 150 to 600 kPa (20 to
90 lbf·in.–2 absolute) for 36 s. The devices lens Beam path of selected light
are measured for internally trapped tracer ray reflected from center of
gas within 10 min after bombing. Large Illuminating package lid
cavity devices such as relays are easily laser beam
rejected up to 1 h after bombing. Chamber window

Devices with cavities of 0.5 to 0.05 cm3 Inspection Bulging lid (not to scale)
(0.03 to 0.003 in.3) are normally tested by chamber Package under test
pressurization at 300 to 600 kPa (45 to
90 lbf·in.–2 gage), for 36 s, with the final (b)
evacuation of the pressurization chamber
to 2.66 Pa (0.02 torr), instead of 66 Pa
(0.5 torr). The devices should be measured
for krypton-85 tracer within 10 min.

Very small cavity devices or zero cavity
devices are tested for gross leaks through
the use of charcoal.4 The charcoal is added
to the device cavity in quantities of 1 to
1.5 mg (3.5 × 10–5 to 5.3 × 10–5 oz) and
acts as a gettering agent for the
krypton-85 gas. This technique ensures
the retention of the krypton-85 for over
30 min, allowing adequate time for
testing.

Holographic Leak Testing5

Holographic leak testing is a technique
developed in the 1980s and limited by
package type and leak rate range.
Holography can be used to leak test sealed
devices such as hybrid electronic

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to external pressure change, the lid will
slowly recover from deformation at a rate
depending on the leakage rate. The rate of
recovery of the lid deformation on a
specific package design can be determined
to reflect a specific leakage rate for that
package. The larger the leakage rate, the
more rapid the recovery. Holography is
able to detect a single wavelength of laser
light change in the surface deformation.

Gross leakage is quickly detected
whereas fine leakage requires several
minutes to detect a recovery (or change)
in the lid deformation of a few
micrometer (about 10–4 in.). Ideally, lid
materials should be consistent in
thickness and material. Application of the
method to other device configurations
can be more complicated. Very large
hybrids may require evacuation of the
chamber, if they have internal lid
supports, whereas smaller lids and
cylindrical cans are more difficult to read.
The practical leak rate range of
application appears to be from gross leaks
to the 10–7 Pa·m3·s–1 (10–6 std cm3·s–1)
leakage rate range.

The technique may be adapted for
rapid, full range leak testing to
10–8 Pa·m3·s–1 (10–7 std cm3·s–1) in some
production applications. Figure 4b shows
the method’s application to leak testing of
pharmaceutical blister packages. The
inspected package has laser milled holes
of 15, 35 and 60 µm (6 × 10–4, 1.4 × 10–3
and 2.4 × 10–3 in.) diameter. Figure 4b
shows cavities leaking at rates between
5 × 10–5 to 1 × 10–6 Pa·m3·s–1 (5 × 10–4 to
1 × 10–5 std cm3·s–1) in 0.1 s.

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PART 4. Fine Leak Testing of Hermetically
Sealed Devices with Krypton-85 Gas

Major Leak Testing bombing time are specified in advance for
Techniques Used for Fine specific types of components and package.
Leak Testing of Hermetic
Devices During the radioisotope leak testing,
the packages are placed within the
Basically, there are only two fine leak test bombing chamber. Before introduction of
techniques that are widely used in the krypton-85 mixture, the chamber is
industry for leak testing of hermetically evacuated to remove the air and prevent
sealed devices: (1) the radioisotope tracer dilution of the krypton gas mixture. The
gas leak testing technique and (2) the bombing chamber is then backfilled with
helium tracer gas leak testing technique. the radioactive tracer gas mixture to a
Wide variance has existed in both the test specified pressure and the devices are
conditions and the resultant leak test exposed to this pressurized tracer gas
sensitivity limits, depending on such mixture for a specified period of time.
factors as the pressure differentials or
bombing pressures, the low pressure The bombing time is chosen to permit
conditions into which leakage occurs and a sufficient quantity of krypton-85 tracer
the sensitivities of the leak testing gas to enter a leaking device to ensure
instruments (used in vacuum or in air at detectability. The quantity of tracer
atmospheric or other pressures). required is about 1011 molecules of
krypton-85. That concentration is
In the past, many procedures and sufficient to produce an easily measured
specifications used for fine leak testing radioactive emission through the walls of
specified a maximum leakage rate when the device. Typically, in small cavity
measured at a differential pressure of packages, the krypton-85 concentration
100 kPa (1 atm). The latter was not a represents about 1 part per hundred
realistic requirement for the helium test, million parts of air inside the part. The
because if the device were a leaker, the radioactive gas pressurization mixture is
pressure differential of 100 kPa (1 atm) then pumped out of the bombing
would not be maintained as time chamber into a permanent tracer gas
progressed after pressurization and during storage system.
the leak test exposure. More recently,
efforts have been initiated to develop Following completion of bombing and
more realistic and consistent standards for purging operations, the bombing chamber
helium and radioisotope tracer gas fine is opened and each package device is
leak tests of hermetically sealed devices. placed (either individually or in batches)
within the well of a scintillation counting
Krypton-85 Fine Leak Test radiation detector system. They are easily
Methods for Hermetically tested by carrying the devices on a
Sealed Devices conveyor belt through a tunnel type
scintillation crystal detector at rates of
In the radioisotope gas leak test one per second. The radiation emitted
technique, the tracer gas is usually the from external surface adsorbed gas is
radioactive (but chemically inert) typically beta radiation (electrons) that
krypton-85 gas, typically diluted in can be stopped by thin layers of
gaseous nitrogen. The sealed hermetic absorbers. The more energetic gamma
devices are bombed or subjected to the radiation produced by disintegrations of
radioactive gas and its carrier gas under krypton-85 gas molecules contained
pressure, nominally 300 to 800 kPa within the packages can then be detected
absolute (45 to 120 lbf·in.–2), for specified and counted with suitable scintillation
periods of time. The maximum external counters.
pressure the device package can withstand
must be determined in advance so that as The leakage rate is then calculated
high a bomb pressure as possible can be from the counting rate caused by
used to minimize the bombing time. In emissions proportional to the amount of
most cases, the bomb pressure and krypton-85 that has entered the device
during the bombing step and still remains
within the device package at the time of
the scintillation counter measurement.
This rate depends on the amount of
radioactive gas introduced during
bombing and on the loss of that gas
through the leak following the bombing

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operation. Thus, measurements are being Detection of Zero Cavity
made during transient conditions of tracer Device Leakage by Using
gas pressure within the test devices. The Charcoal Gettering of
transient conditions have little effect on Radioactive Tracer Gas
the krypton-85 test, because very small
partial pressures are needed for substantial A device without any internal cavity has
detectability. always presented a difficult and often
expensive task to verify hermeticity,
Radioactive Tracer Gas for usually through extensive environmental
Combination of Gross testing. Both gross and fine leakers are
Testing and Fine Testing easily detected in zero cavity devices
through the tracer quantities of coconut
One major advantage of the radioactive shell charcoal placed on the inside of the
tracer gas leak test is that large quantities device. The charcoal adsorbs large
of hermetically sealed electronic devices quantities of radioactive krypton-85 gas
can be bombed simultaneously and rate and easily retains the krypton-85 for
meter tests of gamma-ray emission can be sufficient time to ensure detection after
made on the individual components or bombing. The explosive industry uses
batches of components by automatic small quantities of coconut shell charcoal,
means. If time permits, essentially all of typically 1.5 mg (5.3 × 10–5 oz), in such
the radioactive tracer gas can escape devices as air bag squibs. The mixtures are
before the scintillation counter radiation compressed into cans at pressures as high
measurement, thus gross leakers could as 125 MPa (1.8 × 104 lbf·in.–2). A header
escape detection. is then compressed or soldered in place.
The powder and charcoal mixture can be
In devices of 0.05 cm3 (0.003 in.3) and bombed in radioactive gas (in the open
larger cavities, the technique is very can configuration) and easily detected as a
reliable for detection of gross or fine reject for 20 to 30 min after
leakers. The gross and fine leak test are pressurization.
easily combined into a gross/fine
combination test with a single Electronic devices such as integrated
pressurization. With typical radioactive circuits with less than 0.5 cm3 (0.03 in.3)
gas machine concentrations, a gross/fine cavities are tested for gross/fine
combination pressurization at 520 kPa sensitivities by adding coconut shell
(75 lbf·in.–2) requires 12 to 15 min, to charcoal to the cavity. The charcoal is
detect leaks from 10–2 to 10–9 Pa·m3·s–1 extremely light weight and bonds to the
(10–1 to 10–8 std cm3·s–1). The die bond material when fired. The
measurement time or readout time should radioactive gas gettering ensures detection
always be controlled to a maximum of 10 of circuit packages with large gross leaks.
min for devices with cavities less than
0.5 cm3 (0.03 in.3) to ensure detection of Leak Testing of Plastic
gross leakers. Devices with Radioactive
Tracer Gas
The radioisotope technique for leak
testing of hermetically sealed packages is Low gas solubility plastic hermetic
basically a more direct leakage test devices, both with and without cavities
technique than helium leak testing are tested for gross and fine leaks by using
because the radioactive gas is measured krypton-85 tracer gas. Krypton-85 gas
when it is inside the package. (In helium emits both beta particles and gamma-rays.
leak testing, the rate of escape of the The beta emission normally is only
tracer gas through the leak is being detected from krypton-85 gas that is
measured.) Smaller concentrations of absorbed onto the surface of a device. The
radioactive tracer gas are sufficient for monitoring of surface desorption of good
sensitive leak testing, so that the epoxies takes 3 to 8 min with krypton-85
minimum detectable leakage rate is lower pressurization for 1 × 10–8 Pa·m3·s–1
than that for the helium mass (1 × 10–7 std cm3·s–1) sensitivity tests.
spectrometer test, typically 10–12 Pa·m3·s–1 Following the desorption of the surface
(10–11 std cm3·s–1). Because detection is gas, the internally trapped krypton gas is
done with the devices at atmospheric measured to detect a leaker.
pressure, the sample handling rates can be
much greater than with helium testing, Many types of relays use an epoxy
where the package must be placed within coating over the glass feed-throughs to
an evacuated closed environment before seal broken glass-to-metal seals. Such
leakage measurements are made. However, devices are easily tested with radioactive
it is considered proper practice to krypton gas by monitoring the beta
complete the readout in a ten minute radiation as a wait time, after which the
time period.

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internally trapped krypton-85 is detected number that can be checked for
to show a leaking device. radioactivity at the counting station
within 30 min following the required
Zero cavity or extremely small cavity waiting time. However, it is always
plastic devices such as plastic air bag considered good practice to test
squibs can be tested for gross and fine batches of very small cavity parts
leaks through coconut shell charcoal within 10 min. At one part per
added internally in quantities of 1.5 mg second, that typically tests 600 devices
(5.3 × 10–5 oz). That allows wait time for per batch. Very small devices are
the surface desorption to occur and then tested in groups of three to five
the device is measured for krypton devices per analysis with the
radioactive tracer gas that was gettered scintillation crystal, that increases the
and held by the charcoal. throughput to 180 to 300 devices per
minute.
Special Considerations in 5. Sealed devices rejected during
Radioactive Tracer Gas radioactive tracer gas leak testing
Leak Tests of Sealed obviously contain radioactive
Devices krypton-85. Because it is logical to
control all radioactive materials,
The following precautions should be rejected or leaking devices should be
considered when selecting or using the destroyed, deactivated in a vacuum
radioactive tracer gas leak test procedure chamber or otherwise controlled. Most
for tests of sealed devices. rejected devices contain such small
quantities of radioactive tracer gas,
1. As with helium leak testing, the they emit no measurable radioactive
radioactive tracer gas test should be dosage to humans. Gross leakers will
performed before liquid immersion of release that gas quickly, whereas fine
test devices to prevent fluids from leakers will generally require many
clogging the leak holes or reducing the hours to vent the gas. Puncturing or
measured leak rate. smashing those devices out of doors or
in a well ventilated area will create no
2. Parts that are made of sorptive hazards to humans.
materials or that use adhesive labels 6. The possession of radioactive tracer
are often rejected as false leakers. This gas leak detection systems requires a
problem is easily avoided with radioactive materials license from the
radioactive krypton tracer gas, which regulatory body for that specific area.
emits beta particles as well as Each regulatory agency has a set of
gamma-rays. The beta radiation is only rules and regulations governing the
detected if the krypton-85 gas is on use of such isotopes. The manufacturer
the outside of the device. In such of radioactive tracer gas equipment
cases, a short waiting period will allow can provide information on the
surface gas to dissipate and after radiation technology and safe
verification with the beta detector that operation of such equipment.
the surface is clean, the device is
reliably measured for internally Equipment for Radioactive
trapped krypton with a scintillation Gas Leak Testing of Sealed
detector. Electronic Devices

3. In cases where a retest is required on In leak testing with radioactive tracer
sealed devices that have previously gases, special equipment is required for
been subjected to leak testing with the storage, transfer and handling of the
krypton-85 tracer gas, measurements radioactive tracer gases, including storage
should first be made for indications of vessels, pumps both for pressure and
gamma-ray emissions from residual vacuum, valves and pressure measuring
radioactive tracer gas trapped on or instrumentation. A typical krypton-85 gas
within the device package. If there is handling system is depicted in Fig. 5. It
any indication of krypton-85, the contains a pressurization tank where the
radioactivity reading must be recorded devices are bombed; a storage tank where
as a background radiation level for the krypton-nitrogen gas mixture is
that test part. Then a standard test is stored; two vacuum pumps, one to
applied. The background reading is evacuate the air from the bombing tank
subtracted from the final reading to and one to return the krypton-nitrogen
establish the new leak rate. mixture to the compressor after bombing;
and a compressor to transfer the tracer gas
4. Although the radioactive tracer gas mixture from storage to the bombing
leak testing technique may be applied chamber and then to compress the gas
on either a sampling basis or on
100 percent of all devices in a lot, the
quantity of small cavity devices
pressurized at one time during the
bombing step should be limited to the

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back into storage. These functional units integrated with a series of both functional
are coupled with solenoid valves, pressure and safety sensors.
and vacuum sensors. The operation of
such gas handling equipment is Measurement equipment is used to
controlled by a logic system that is detect the radiation from devices that
were nonhermetic and allowed radioactive
FIGURE 5. Example of krypton-85 gas handling system. gas to enter the device. Thallium activated
scintillation crystals, housed in a 50 mm
AT ST (2 in.) lead shield, are used to analyze the
radiation from devices. The
VV VVV instrumentation is usually a rate meter
that is measuring the signal from a
VV C photomultiplier tube coupled to the
V V scintillation crystal.

VP VP V VV Typical Radioactive Gas
Pressure Bombing Cycle
Legend
AT = activation tank The pressurization system is depicted in
C = compressor Fig. 6, showing only the major
ST = storage tank components of the system.
V = valve
VP = vacuum pump 1. A test cycle begins with loading a
batch of devices into the bombing
tank.

2. The tank is closed and evacuated to a
pressure of 65 Pa (0.5 torr), to prevent
dilution of the radioactive tracer gas
(Fig. 6a).

3. The tracer gas mixture is transferred
from the storage vessel to the
bombing tank. A compressor is used, if
required, to compress the tracer gas to
the preset bombing pressure (Fig. 6b).

FIGURE 6. Operation of krypton-85 gas system for leak testing: (a) evacuation; (b) bombing;
(c) return of tracer gas to storage.
(a)

Vacuum
pump

Bombing tank

(b)

Compressor Storage tank
Storage tank
Bombing tank

(c)

Vacuum
pump

Compressor

Bombing tank

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4. The devices then soak at the bombing Krypton-85 Radiation
pressure for the time necessary to Counting Station
achieve the desired sensitivity.
The counting station is where the
5. The tracer gas mixture is returned to krypton-85 tracer gas entrapped within a
the storage tank with a closed loop reject device is detected as a go/no-go
vacuum pump in tandem with a measurement for pass/fail only; or
compressor (Fig. 6c). measured quantitatively for an exact leak
rate. The measurement of leakers is
The vacuum level achieved at the end achieved by using a thallium activated
of the cycle is either 65 Pa (0.5 torr) for a sodium iodide crystal attached to a
fine leak test or 260 Pa (2.0 torr) for a photomultiplier tube. Gamma radiation
gross leak test. The higher venting pulses from the krypton-85 trapped within
pressure leaves a greater amount of gas a device produce quanta of light through
inside of the gross leakers and allows ionization of the crystal (Fig. 7). The
them to be reliably detected. The quanta of light are converted to current
bombing chamber is then vented to that in turn is read with a rate meter.
atmosphere and opened; the devices are
removed and taken to the detector station Typically a reject device will contain
and measured for any radiation from enough krypton-85 gas to produce a
internally trapped krypton-85 gas that minimum of 1000 counts per minute that
entered a leaker. in turn is about 5 × 1011 molecules of
krypton-85 gas within the part. (That
FIGURE 7. Radiation detection system. usually represents a krypton-85 partial
pressure of parts per million or parts per
Tube Lead cover billion). The crystals are mounted within
a 50 mm (2 in.) lead shield to minimize
Lead the atmospheric radiation and keep the
donut background reading as low as possible.
The time required to achieve a go/no-go
signal in a scintillation crystal is 20 to
40 ms. Each device configuration is first
measured with a counting efficiency in the
crystal. The efficiency is identified as a
K factor or the measurement of the
number of counts per minute per
microcurie of krypton-85 gas entrained
within that device.

γ γγ Sensitivity of Leak Testing
γγ with Radioactive
Krypton-85
γ
γ Lead The sensitivity of leak testing with
γ γγ γ shield
Scintillation crystal krypton-85 tracer gas covers the range
Quartz light pipe
from a visible hole, to as low as
Photomultiplier 10–14 Pa·m3·s–1 (10–13 std cm3·s–1).

Rate meter Typically production or mass testing

applications cover from a visible hole to
10–9 Pa·m3·s–1 (10–8 std cm3·s–1). High

sensitivities are achieved in some
production applications to 10–12 Pa·m3·s–1
(10–11 std cm3·s–1), by maintaining a high

krypton-85 concentration, bomb times of

a few hours and accurate readout.

Advantages of Leak
Testing with Radioactive
Tracer Gas

With krypton-85 tracer gas leak tests, it is
necessary only to bomb or inject the
pressurized tracer gas into the devices
under test to measure leakage. By

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contrast, other tracer gas leak testing Vacuum Decay
techniques require that the tracer gas first Confirmation of Leak Rates
be forced into the device and secondly
drawn out of the device to permit The radioactive tracer gas technique using
measurement of the leakage rate. The krypton-85 gas provides perhaps the most
technique of bombing and not drawing absolute technique of confirming a leak
the radioactive tracer back out of the rate. When a leaking device of known
device reduces the probability of missing internal volume is bombed in krypton-85
leaks because it reduces the chance that gas, the device will collect a measurable
the leak path will become blocked. Still quantity of krypton-85 gas. Quantitative
another advantage of this technique of measurement of the gamma reading
leak testing is that the rate of testing of provides an accurate measure of the
sealed devices is many times faster than number of krypton-85 molecules trapped
with other tracer gas techniques. in the device.

The beta (electron) emission from the Because the partial pressure of
krypton-85 can be used to determine krypton-85 is being measured, the exact
whether tracer gas is adsorbed on the leak rate may be verified by measuring the
external surface of the test object or is initial reading of krypton-85, placing the
contained wholly within the interior of device in vacuum for several hours,
the sealed test object. The radioactive reading the device and calculating the
emission from krypton-85 is more than percentage of gas loss by using the
99 percent beta (electron) emission, which equation Pt = P0 e–kt, where k is the
does not have sufficient particle energy to measured or calculated leak conductance
penetrate through most enclosure (cm3·h–1) divided by the internal volume
materials. A beta measurement can be (cubic centimeter). A typical plot of
made to avoid rejecting those devices vacuum decay curves is shown in Fig. 8.
with adsorbed external surface tracer gas
only. If beta emission is minimal and FIGURE 8. Vacuum decay for 20 mm3 (1.2 × 10–3 in.3)Partial pressure (percent)
gamma-ray emissions are observed, they package of krypton-85 for leaks A to L with range of
indicate the presence of radioactive conductances (cm3·h–1). One cm3·h–1 = 2.8 × 10–4 cm3·s–1 =
krypton-85 within the enclosure and thus 0.8 oz·day–1.
indicate true leakers. The ratio of beta
emissions to gamma emissions for 100
externally adsorbed krypton-85 is 90
typically of the order of 200 to 1. This 80
ratio drops to the order of 1 to 1 where
external surface adsorbed krypton-85 is 70
minimal and radiation comes from inside 60
the test object.
L
Beta Particle Counting to 50
Detect Parts with Surface
Contamination 40

Routine checking of rejected parts with a K
thin window Geiger-Müller tube 30
sometimes reveals surface contamination
of the test parts. Comparison of rejected 20 F J
parts with acceptable parts of the same A G
surface composition can be used to
determine the significance, if any, of the HI
surface contamination. Those parts with
significant contamination can often be 10 B CD E
decontaminated with a brief exposure to 0 50 100 150 200 250 300 350
heat. Frequently, such a heating cycle is
routinely incorporated into the testing Legend Time in vacuum (h)
procedure of certain parts having organic
coatings. It is quite reliable to test painted A = 3 × 10–7 G = 6 × 10–8
devices such as relays, by including an B = 2 × 10–7 H = 5 × 10–8
empty sample can, painted with the C = 1 × 10–7 I = 4 × 10–8
lot-of-relays but without any header or D = 9 × 10–8 J = 3 × 10–8
internal components. The can is measured E = 8 × 10–8 K = 2 × 10–8
after bombing in the radioactive tracer F = 7 × 10–8 L = 1 × 10–8
gas. When the radiation is gone from the
can, the rest of the lot may be measured
for internally trapped krypton-85.

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Accuracy of Radiation and K is the counting efficiency of the
Counting Techniques device being tested (that is, the efficiency
of measuring one microcurie of
With fixed conditions of bombing krypton-85 gas within the geometry of
pressure, exposure time and concentration the exact part being tested, that will
of radioactive krypton tracer, accuracy is provide a correction for the thickness of
limited by the efficiency of radiation the walls of the device as well as the
detection, assuming no external surface positioning of the device relative to the
adsorption of the gas. Small parts can be scintillation crystal detector used to
counted in a well or tunnel shaped measure the radioactive tracer gas trapped
thallium activated sodium iodide crystal in the device).
scintillation counter. A state-of-the-art
scintillation crystal counting station Functional Check on
operates with a background count of 1000 Krypton-85 Counting
to 1500 counts per minute from Station
atmospheric radiation.
The counting station used, for the
Detection of a device that has been detection of reject devices after bombing,
bombed in radioactive krypton gas will is checked for function at least once per
collect a quantity of krypton-85 that reads day. That check consists of placing a
1000 to 1500 counts per minute above krypton-85 glass reference vial into the
background. (Note that it is rare for scintillation detector and ensuring that
leakers to be found that are marginal). the amount of krypton-85 within that vial
Each increment of 1000 to 1500 counts is being detected by the crystal rate meter
per minute usually represent a leakage system. A tolerance of ±10 percent is
rate of one tenth of an order of acceptable for such a check. In a well type
magnitude crystal detector, the efficiency of detection
is about 14 000 counts per minute per
Such quantities of krypton-85 are quite microcurie of krypton-85.
accurately measured with state of the art
equipment. A device that leaks in the fine A becquerel (Bq) is one disintegration
leak range, <5 × 10–7 Pa·m3·s–1 per second whereas a curie (Ci) is a
(<5 × 10–6 atm cm3·s–1), can be measured quantity of disintegrations per second:
to accuracies of 0.1 of an order of 37 GBq = 1 Ci.
magnitude, plus the normal cumulative
errors associated with the process: i.e. the Counting Station Calibration
gas concentration, the pressure
measurement, the krypton-85 reference The counting stations are normally
sources etc. Thus a device may be calibrated monthly by using a traceable
measured for leakage with an absolute krypton-85 glass reference source
accuracy of about 0.4 of one order of corrected for half life decay. Because the
magnitude. beta detectors are classified as qualitative
instruments, they are normally checked
Equation Used for Radioactive Gas for function only by using a cesium-137
Leak Testing source (or other beta source) and verified
to be functional, because in the testing of
Equation 1 is used for the calculation of devices they are normally used as a
the testing parameters in performing a go/no-go indicator of surface gas
radioactive tracer gas leak test: contamination. The scintillation crystals,
however, require a determination of
(1) Q1 = R accurate detectability for krypton-85 and
S K Pd T assurance of stability. The rate meter used
to measure the signal from the
where Q1 is the leak rate sensitivity scintillation crystal can be calibrated
desired (Pa·m3·s–1); R is the reject point or electronically by using a pulse generator
to verify signal detection accuracy and
rate meter reading above background, at linearity, following manufacturer’s
procedures.
which a part is rejected (this net reading
The functional accuracy of the rate
should be at least equal to the normal meter is best determined as a crystal rate
meter system. The operational stability of
ambient background reading of the the crystal rate meter system must first be
established by placing a krypton-85
counting equipment, typically 1200 to reference vial into the crystals normal
1500 counts per minute net); Pd = Pe2 – Pi2 geometric reading position, as a device
(where Pe is the external absolute pressure would be placed into a well type crystal for
and Pi is the internal absolute pressure, in measurement.
pascal); S is the specific activity or

concentration of the radioactive gas

mixture (this concentration measurement

is normally measured at least monthly) in
Bq·Pa–1·m–3 (or µCi·atm cm–3); t is
pressurization or bombing time (second);

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Most crystals operate at a high voltage efficiencies of 13 000 to 14 400 counts per
of 1 to 1.5 kV. To establish the point of minute per microcurie.
stability, the voltage is reduced to 0 and
then increased in increments of about In large volume manufacturing
100 V, with the reading of the reference facilities, the devices in test are read
source being recorded at each 100 V. A automatically by using a conveyor belt
properly operating crystal will that carries the device through a tunnel in
demonstrate a stable range over perhaps a scintillation crystal. That allows the
several hundred volts. That point is radiation from a device to be read
considered the plateau of that crystal. circumferentially as the device passes
Such a plateau can be seen in Fig. 9. The through the detector. Because only 20 to
high voltage is then set to a point that is 40 ms are required to detect krypton-85
usually at the midpoint of the plateau. In within a device, the belt may carry
state-of-the-art counting system devices at speeds of 175 to 225 mm (7 to
equipment, the system should remain 9 in.·s–1). A calibration of such detection
stable at that voltage setting for years, systems requires the reference standard to
without any significant changes to the be measured with the tunnel detection
efficiency of the system. zone in a static state, the plateau to be
established, the efficiency to be
The resultant reading of the standard at determined (as with the well crystal case
that voltage, minus the background above) and then the measurement to be
reading taken without the reference read with the conveyor belt operating at
source in the well, will provide the actual full speed. Obviously, the dynamic
efficiency of the crystal to measure efficiency of the system will be less than
krypton-85 gas contained in a reject static. Tunnel systems achieve dynamic
device in that position within the crystal. efficiencies from 8000 to 11 000 counts
A good quality crystal will provide reading per minute per microcurie, for most
devices.

FIGURE 9. Typical operating plateau for scintillation counter instrument. Detector sensitivity
equals counts per second per nanobecquerel (counts per minute per microcurie).

62 (100)

56 (90)

Counts per s·kBq–1 (counts per min·nCi–1) 49 (80)

43 (70)

37 (60) Noise level
31 (50)
Set point

25 (40)

19 (30)

12 (20) Plateau
6 (10)

0
0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4
Adjustments (kV direct current)

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Determination of Counting pressure of 0.7 to 1.0 MPa (7 to 10 atm).
Efficiency for Hermetic The concentration required to perform a
Devices proper test, is not critical. The
concentration for testing plastic devices,
The efficiency of measurement of a and painted devices is usually maintained
quantity of krypton-85 gas within a quite low, < MBq·Pa–1·m–3 (< 0.1 mCi·std
hermetic device requires a representative cm–3). The concentration for testing
sample of the device to be tested, to be nonabsorbent devices, high reliability
filled with a known quantity of devices etc., are usually > 1 MBq·Pa–1·m–3
krypton-85 gas and then to be placed (> 0.1 mCi·std cm–3).
within the scintillation crystal in exactly
the same position as it will be measured The specific activity measurement is
when tested. The sample used for the normally performed once each month.
actual efficiency measurement must be of Two methods are commonly used with
the same geometry, material and internal radioactive tracer gas systems. In one
void as the actual parts to be tested with technique the gas mixture is sampled by
the radioactive gas. withdrawing a sample of the gas into a
glass vial, at a pressure of 250 Pa (2 torr).
Several techniques have been used to The volume of the glass vial must be
establish the counting efficiency of precisely known and is usually about
devices to be tested. A rather complicated 3 mL = 3 cm3 (0.18 in.–3). The sample of
technique involves the introduction of a gas is measured in a scintillation crystal
very accurately known quantity of and then compared to the measurement
krypton-85 gas, of accurately known of a similar geometry glass reference vial
concentration, through a tubulation that is of known quantity of krypton-85. The
then sealed off. The device is then placed concentration of krypton-85 in
within the crystal and measured for µCi·std cm–3 is calculated as the specific
detection efficiency, referred to as K factor. activity of the radioactive tracer gas
That is the measurement of the number of system. A number of errors are inherently
counts per minute per microcurie of associated with this technique: the
krypton-85 gas within that device. volume of the vial, the pressure of the
sample, the accuracy of the counting
A second technique of determination station, the accuracy of the reference vial
involves the introduction of a sample of and the human errors introduced by the
material that contains an accurately technician.
known quantity of krypton-85 gas. The
device is then assembled into its final The second technique is a closed loop
configuration and measured in the crystal. system built into the radioactive tracer gas
Both of these techniques are best handling system, which automatically
performed by the radioactive gas samples the gas in the system every cycle
equipment manufacturer. The equipment that the machine operates. The
manufacturer can usually provide the user concentration is electronically calculated
with the K factor for most devices that are and displayed on the operating control
manufactured, based on prior panel of the system. It involves the
experiments, and geometric approximation automatic sampling of the gas into a
of the device within a crystal. One rule is large, fixed volume chamber, at a pressure
that the nuclear physics of krypton-85 gas above atmosphere, and the continuous
only provides sufficient emission to measurement of that sample with an
achieve a maximum reading of 1.6 × 104 integral radiation detector. The error
counts per minute per microcurie of associated with this technique is usually
krypton-85. Any indications of greater < 0.001 of the hand sampling technique
efficiencies are ambiguous and indicate and is automatically performed every
errors in the technique or equipment used cycle of operation.
to determine the K factor.
Calculating Leak Rates of
Determining Specific Failed Devices
Activity of Radioactive
Krypton-85 Tracer Gas Following the bombing of the devices and
Mixture sorting of rejects, the actual quantitative
leak rates may be calculated for devices.
The concentration of the krypton-85 Leak rate values greater than 5 × 10–8
radioactive gas mixture used for Pa·m3·s–1 (5 × 10–7 std cm3·s–1), are
pressurization of the hermetic devices, considered to be viscous flow of the gases
must be accurately known to calculate the and the gases of concern in leak detection
proper bomb time and pressure to be systems are considered to have similar
used. The krypton-85 gas is normally flow rates through the same leak path.
mixed in nitrogen (or air) at a total Smaller leak rates are considered to be in
the molecular flow regime. The leak rates
are calculated based on the net radiation

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reading obtained on the device. Normally using standard half life equations or
devices are rejected on a go/no-go basis. corrections from decay tables. The normal
However, if the quantitative leakage value accuracy of such standards is ± 10 percent.
is desired on a device, the device should The reference vials should be corrected for
be read with the rate meter in the long, half life decay at least every six months by
time constant position. That is, the actual using the decay equation:
reading of the device is averaged over a
period from 1 to 10 s to ensure an (2) At = A0 e −0.693 t
accurate reading. Th

The ambient background of the where At = amount of krypton-85
equipment should also be read with equal remaining at time t; A0 = original amount
accuracy and subtracted from the gross at time 0; t = time passed from time 0 to
reading. The leak rate of the device is
calculated by dividing the net reading by time t (in unit of year); T1/2 = half life of
the reject value used in Eq. 1, times the krypton-85 = 10.76 yr. This half life decay
test sensitivity Q1. The leak rate value
obtained will be for krypton-85 gas. If it is calculation has been converted to a decay
desired to convert a leak rate less than
5 × 10–8 Pa·m3·s–1 (5 × 10–7 std cm3·s–1) to chart as shown in Table 5. From that chart
an equivalent leak rate for another gas
such as air or helium, the krypton-85 leak the quantity of krypton-85 within the vial
rate would be converted based on the
theory that the flow rate varies inversely can be calculated easily and accurately.
with the ratio of the square roots of the
molecular weights of the two gases of Pressurization System
concern. Thus, to convert the krypton-85 Calibration
leak rate to air, multiply by 1.712; to
convert it to helium, multiply by 4.7. The pressurization systems require
maintenance and calibration at specific
It should be remembered that the intervals to satisfy the metrology needs
actual leak rate of the device may be and to maintain a gas handling system
confirmed by applying techniques for required to reliably handle radioactive gas.
vacuum decay confirmation of leak rates, The maintenance cycle includes
described earlier. replacement of valve seals, vacuum pump
oil, compressor oil and normal wear
Krypton-85 Reference items. The calibration steps cover the
Source for Calibration vacuum gages, pressure transducers and
mechanical gages that control the actual
The krypton-85 reference calibration is test parameter accuracies. Most vacuum
conducted by using a reference standard and pressure gages are reliable for periods
in the form of a glass vial of about of one year. The vacuum gages are
3.5 cm3 (0.2 in.3) volume, with a quantity calibrated at the critical stepping points of
of krypton-85 gas sealed inside and 0.066, 0.266 and 100 kPa (0.5, 2.0 and
traceable to the United States National 760 torr). The pressure transducers are
Institute of Standards and Technology. calibrated against a traceable mechanical
The half life of krypton-85 is 10.76 years, gage for the normal operating range of
which allows for accurate correction by the pressurization system.

TABLE 5. Decay of krypton-85 with half life of 10.76 yr.

Years ______________________________________________M__o_n_t_h_s_____________________________________________
0 1 2 3 4 5 6 7 8 9 10 11

0 1.000 0.995 0.989 0.984 0.979 0.974 0.968 0.963 0.958 0.953 0.948 0.943
1 0.938 0.933 0.928 0.923 0.918 0.913 0.908 0.903 0.898 0.893 0.889 0.884
2 0.879 0.874 0.870 0.865 0.860 0.856 0.851 0.847 0.842 0.838 0.833 0.829
3 0.824 0.820 0.816 0.811 0.807 0.802 0.798 0.794 0.790 0.785 0.781 0.777
4 0.773 0.769 0.765 0.761 0.756 0.752 0.748 0.744 0.740 0.736 0.733 0.729
5 0.725 0.721 0.717 0.713 0.709 0.705 0.702 0.698 0.694 0.691 0.687 0.683
6 0.679 0.676 0.672 0.669 0.665 0.661 0.658 0.654 0.651 0.647 0.644 0.641
7 0.637 0.634 0.630 0.627 0.624 0.620 0.617 0.614 0.610 0.607 0.604 0.601
8 0.597 0.594 0.591 0.588 0.585 0.582 0.578 0.575 0.572 0.569 0.566 0.563
9 0.560 0.557 0.554 0.551 0.548 0.545 0.542 0.539 0.537 0.534 0.531 0.528
10 0.525 0.522 0.520 0.517 0.514 0.511 0.509 0.506 0.503 0.500 0.498 0.495
11 0.492 0.490 0.487 0.485 0.482 0.479 0.477 0.474 0.472 0.469 0.467 0.464

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PART 5. Fine Leak Testing of Hermetically
Sealed Devices with Helium Gas

Helium Fine Leak Test leakage rates in the range from about 10–7
Methods for Hermetically to about 10–11 Pa·m3·s–1 (about 10–6 to
Sealed Electronic Devices about 10–10 std·cm3·s–1). The free volume
of the evacuated test chamber containing
The helium tracer gas leak test technique the devices under test should be held to a
is the second of two widely used practical minimum level. Any empty
techniques for fine leak testing of space within the chamber has an adverse
hermetically sealed electronic devices. The effect on the limits of leakage sensitivity
helium mass spectrometer fine leak test is attainable with the helium mass
relatively simple and does not require spectrometer during leak tests of sealed
compliance with regulations governing devices with small internal helium filled
nuclear byproducts that produce ionizing volumes.
radiations (such as krypton-85 tracer gas).
Operational Limitations of Helium
Helium is the lightest inert gas. Its Back Pressure Leak Tests
molecules will penetrate through even the
smallest leaks but will not clog the leak. When the sealed test devices are placed in
Helium is not hazardous and is present in the evacuated chamber for leak testing,
the earth’s atmosphere only as a tracer any helium gas previously injected into
element present in a concentration of each sealed device may or may not have
about 5 µL·L–1 of air (about 1 part in escaped through leaks in their enclosure.
201 000 in air). In normal production leak This can affect the leakage rate indicated
testing, the mass spectrometer leak test by the mass spectrometer leak detector.
equipment provides capabilities for The number of sealed devices that are
detecting leakage rates in the range from removed in each lot from the (bombing)
about 10–6 to 10–10 Pa·m3·s–1 (10–5 to 10–9 chamber for leak testing should be limited
std cm3·s–1). to a quantity that can be helium leak
tested within a limited dwell time
With gross leakers it is possible that so (typically 30 to 60 min maximum).
much tracer gas may escape before mass
spectrometer leak tests of the devices that Devices that are gross leakers can lose
little or no tracer gas is left to escape essentially all of their contained helium
during the actual leak test period. Such tracer gas by leakage into air or vacuum
loss of contained tracer gas makes leak environments in very short time periods.
detection much less reliable. These could be missed entirely if the
helium leak test were used alone during
Principles of Helium Fine inspection. Thus, gross leakers should be
Leak Test Operation separated from each lot of sealed test
devices by gross leak tests (described
In leak tests, the helium tracer gas is above).
introduced into the devices under test
either (1) initially during device Lack of repeatability in helium fine
manufacture just before or during the leak testing of sealed devices can occur
hermetic sealing process or (2) by back because, in most cases, the actual quantity
pressure (bombing) techniques applied of helium contained within each
any time after the hermetic sealing of the component (assuming that it is a leaker) is
device has been completed. The tracer gas not known and cannot be calculated. (By
may vary from commercially pure comparison, with radioactive krypton-85
100 percent helium to a mixture of tracer gas of known concentration in
10 percent helium with 90 percent nitrogen, the quantity present within the
nitrogen (if pressuring up is to be used sealed test device is measured directly by
and the cost of helium is a deterrent to its scintillation or Geiger-Müller counter
use). During tests, the sealed devices are radiation measurements.) Thus, it is
placed in a test chamber that is evacuated possible to calculate the true leakage rate
and connected to a properly calibrated by helium leak tests only if an appropriate
helium mass spectrometer leak. With theoretical relationship is available to
proper calibration, the helium mass relate internal gas pressure to the
spectrometer leak detector can detect conditions of bombing and the delay
between device removal from bombing
pressure and its actual measurement of
helium leakage with the mass spectrometer.

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This correlation between measured shift to verify that the specified amount
leakage rate and prior bombing conditions of helium tracer gas is actually being
and subsequent time delays depends on sealed within each package. Within a
knowledge of the following critical maximum transfer time of 30 min after
factors: (1) the mechanism of helium gas completion of the package sealing
flow into the sealed test object during operation, the device under test is
pressurization or bombing; (2) the transferred into a chamber connected to
pressure differential applied across the an evacuating system and a helium mass
sealed boundary of the device enclosure spectrometer leak detector. Any tracer gas
during bombing; (3) the time of that leaks out is indicated by the leak
pressurization and external pressure of detector as the measured leakage rate R1.
helium used during bombing, as well as This measured leakage R1 is converted to
the degree of vacuum (or internal gas the equivalent standard leakage rate:
pressure) existing within the device when
bombing occurs; (4) the internal free (3) Q = R1 Pi
volume of the sealed device to be tested; PHe
(5) the unfilled free volume in the
evacuated test chamber while the test where Q is the equivalent standard
devices are contained in it; (6) the dwell
time or delay between completion of leakage rate defined as that quantity of
pressurization during bombing and the dry air at 25 °C (77 °F) in Pa·m3·s–1
time at which helium leakage
measurements are made (also significant flowing through one or multiple leak
is the external gas pressure during dwell
time, that is, the atmospheric air pressure paths when the high pressure side is at
or vacuum chamber pressure); (7) the
leakage flow mechanism (molecular, 100 kPa (1 atm) and the low pressure side
viscous etc.) during the dwell time delay
and the period of actual leakage is at a pressure not greater than 130 Pa
measurement; (8) the actual helium
concentration in nitrogen or air existing (1 torr absolute); where the standard
within the sealed test device at the
completion of the bombing period. leakage rate is expressed in units of
Pa·m3·s–1 (or optionally, in std cm3·s–1);
The concentration could vary if mixed
tracer gas (helium plus nitrogen) is where R1 is measured leakage rate defined
injected into an evacuated test device as the leakage rate of a given package
during bombing or if 100 percent helium
tracer gas is injected into sealed devices measured under specific conditions and
that already contain a significant air
pressure (if previously stored in air or not using a specified (tracer gas) test medium;
subjected to evacuation before helium
bombing). where Pi is total absolute internal gas
pressure within the sealed device in pascal
Most leak testing documents used as
helium leak testing standards or (or torr or bar); and where PHe is internal
specifications for back pressure leakage partial pressure of helium within test
testing are based on the assumption that
leakage occurs in the molecular flow device, in the same pressure units as
region and obeys simple exponential
relationships, as indicated in Eq. 4 below. selected for Pi.
In many specifications, electronic
Helium Fine Leak Testing
of Devices Filled with devices with an internal cavity volume of
Helium during 0.1 cm3 (0.006 in.3) or less are rejected if
Manufacture
the equivalent standard leakage rate Q
The fine leak testing technique described exceeds 5 × 10–8 Pa·m3·s–1 (5 × 10–7 std
here may be used for sealed electronic cm3·s–1). Devices with an internal cavity
device packages that have been enclosed volume greater than 0.1 cm3 (0.006 in.3)
in such a fashion as to ensure that the
internal cavity of the package contains a are rejected if the equivalent standard
helium tracer gas content that provides a leakage rate Q exceeds 5 × 10–7 Pa·m3·s–1
minimum of 20 kPa (0.2 atm) absolute (5 × 10–6 std cm3·s–1).
partial pressure of tracer gas (100 percent
helium) at a standard temperature of Fixed Helium Tracer Gas
25 °C (77 °F). A sampling inspection Fine Leak Test Technique
should be conducted on each 8 h work
In the fixed technique for fine leak testing
of sealed devices with helium tracer gas,
the sealed electronic devices are leak
tested under specific bombing time and
testing time conditions specified for each
size of internal device cavity (Table 6).

The bombing time is the time the
devices are exposed to 99.5 to 100.0
percent pure helium tracer gas under the
pressure of about 400 kPa for 2 h or a
pressure of about 200 kPa for about 4 h.
The maximum dwell time is the
maximum time allowed after release of
the bombing pressure, before the leakage
is measured from the sealed device under

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test. This fixed technique is not used if accordance with an inverse exponential
the maximum standard leakage rate limit relationship:
given in procurement documents is less
than the limits specified for the flexible  −Qt1 MA 
helium leak testing technique (described
next). (4) P = PE 1 − e V P0 M

Flexible Helium Tracer Gas
Fine Leak Test Technique During the dwell time following
for Hermetic Seals bombing, the rate of leakage of helium
tracer gas from leaking devices is assumed
In the flexible helium fine leak test, the to follow a typical exponential decay
sealed electronic devices to be tested are transient:
subjected to a bombing pressure whose
minimum level is 300 kPa (3 atm) of (5) R1 = Q PE MA
helium pressure. Values of bomb pressure, P0 M
bombing exposure time and dwell time
after bombing before leak testing are  −Qt1 MA 
chosen so that actual measured tracer gas × 1 − e V P0 M
leakage rates for anomalous devices will
be greater than the minimum detection 
sensitivity capability of the helium mass
spectrometer leak detector. If the dwell −Q t2 MA
time exceeds 60 min, graphs are plotted M
to determine an actual leakage rate R1 × e V P0
value that will ensure overlap of leakage
rates with those detectable with the gross where R1 is measured leakage rate of tracer
leak test technique selected for gas, helium, through the leak (Pa·m3·s–1);
subsequent tests of all devices.
Q is equivalent standard leakage rate in
For each combination of (1) sealed
package internal free volume V, the same units as R1; PE is pressure of
(2) bombing pressure PE to which devices exposure in pascal (or in a unit called
are exposed, (3) bombing time t1,
(4) dwell time t2 following bombing and atmosphere [atm] = 101.325 kPa); P0 is
before leak testing and (5) maximum atmospheric pressure in the same unit as
allowable equivalent standard rate Q
(specified in procurement documents), PE ; MA is molecular weight of air = 28.7
theoretical relationships are used to g·mol–1; M is molecular weight of the
calculate the rejection limit of the actual tracer gas, helium = 4 g·mol–1; t1 is time of
measured leakage rate R1 by Eq. 4 exposure to PE (second); t2 is the dwell
described next. time between release of pressure and leak

Exponential Equations detection; and V is internal volume of the
Assumed for Bombing and
Leakage Transients device package cavity (cubic meter).

During the bombing operation, the Criteria for Rejection of
pressure of helium tracer gas within the Leakage Devices in Flexible
hermetically sealed device enclosure (if it Helium Leak Test
is a leaker) is assumed to increase in
Unless otherwise specified, devices with
an internal cavity volume of 0.01 cm3
(6 × 10–4 in.3) or less are typically rejected

if the equivalent standard leakage rate Q
exceeds 5 × 10–9 Pa·m3·s–1 (5 × 10–8
std cm3·s–1). Devices with an internal
cavity volume greater than 0.01 cm3
(6 × 10–4 in.3) and equal to or less than

TABLE 6. Fixed technique for leak testing of sealed devices by using helium tracer gas.

Internal Cavity Package Volume, cm3 (in.3) ________B__o_m__b_in__g__C_o_n_d__it_io__n________ Dwell _______R__e_je_c_t__L_im__i_t_______
Time (h) Pa⋅m3⋅s–1 (std cm3⋅s–1)
_______P_r_e_s_s_u_r_e_______ Bombing
kPa (lbf⋅in.–2) Time (h)

≤ 0.01 (≤ 6.1 × 10–4) 515 ± 15 (60 ± 2) 2.0 to 2.2 1 7 × 10–10 (7 × 10–9)
0.01 to 0.40 (6.1 × 10–4 to 2.44 × 10–2) 515 ± 15 (60 ± 2) 2.0 to 2.2 1 2 × 10–10 (2 × 10–9)
515 ± 15 (60 ± 2) 2.0 to 2.2 1 2 × 10–9 (2 × 10–8)
≥ 0.40 (≥ 2.44 × 10–2) 310 ± 15 (30 ± 2) 2.0 to 2.2 1 1 × 10–9 (1 × 10–8)
≥ 0.40 (≥ 2.44 × 10–2)

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0.4 cm3 (0.024 in.3)are typically rejected if

the equivalent standard leakage rate Q
exceeds 1 × 10–8 Pa·m3·s–1 (1 × 10–7
std cm3·s–1).

Devices with an internal volume
greater than 0.4 cm3 (0.024 in.3) may be

rejected if the leakage rate Q exceeds
1 × 10–7 Pa·m3·s–1 (1 × 10–6 std cm3·s–1).

Simplified Equation for
Calculation of Leakage Rates after
Helium Bombing

The quantity √(MA·M–1) = √(28.7/4) = 2.7,
so Eq. 5 can be simplified as Eq. 6:

(6) R = 2.7 Q PE  −2.7 Q t1 
1− e V 
P0  

−2.7 Q t 2

×e V

In a similar way, the actual leakage rate
for a device that has been filled with
helium to the pressure P during
manufacture can be found from the
simplified Eq. 7:

(7) R1 = 2.7 Q PE −2.7 Q t 2
P0
eV

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References

1. American Vacuum Society Glossary of
Terms Used in Vacuum Technology. New
York, NY: Pergamon Press (1958):
p 6.34, 6.39.

2. Hayes, R.A., F.M. Smith, W.A. Smith
and L.J. Kitchen. Development of High
Temperature Resistant Rubber
Compounds. Wright Air Development
Center Technical Report 56-331.
Ft. Belvoir, VA: Defense Technical
Information Center (February 1958).

3. Roth, A. Vacuum Sealing Techniques.
New York, NY: Pergamon Press (1966).

4. Neff, G.R. Hermetically Sealed Devices
for Leak Detection. United States Patent
5 452 661 (September 1995).

5. Tyson, J. “Optical Leak Testing: A New
Method for Hermetic Seal Inspection.”
1991 ASNT Spring Conference:
Nondestructive Characterization for
Advanced Technologies [Oakland, CA].
Columbus, OH: American Society for
Nondestructive Testing (March 1991):
p 182-186.

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15

CHAPTER

Leak Testing Techniques
for Special Applications

Charles N. Sherlock, Willis, Texas
John F. Beech, GeoSyntec Consultants, Atlanta,
Georgia (Part 3)
Glenn T. Darilek, Leak Location Services, Incorporated,
San Antonio, Texas (Part 3)
James P. Glover, Graftel, Incorporated, Chicago, Illinois
(Part 2)
Daren L. Laine, Leak Location Services, Incorporated,
San Antonio, Texas (Part 3)

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PART 1. Techniques with Visible Indications of
Leak Locations1

Purposes of Leak Testing 1. Leak location techniques independent
to Locate Individual Leaks of any characteristic properties of the
tracer gas include those that use, for
Leak testing for the purpose of locating example, candles, liquid and chemical
individual leaks is required when it is penetrants, bubble testing and sonic
necessary to detect, locate and evaluate or ultrasonic leak tests.
each leak. Unacceptable leaks then can be
repaired and total leakage from a vessel or 2. Leak location techniques involving use
system brought within acceptable limits. of tracer gases with easily detectable
Methods for detecting and locating physical or chemical properties
individual leaks are generally quantitative include gases with thermal
only in the sense that the lower limit of conductivities or chemical properties
detectable leak size is determined by the differing from those of the
sensitivity of the leak detecting indicators pressurizing gas, for example, gaseous
and test methods used. Thus, only rather halogen compounds and gases having
crude overall leakage rate information characteristic radiation absorption
could be approximated by adding the bands in the ultraviolet or infrared
leakage rates measured for the detectable spectral ranges.
leaks.
3. Leak location techniques involving
Many different leak detecting, locating tracer gases with atomic or nuclear
and measuring techniques and devices are properties providing easily detectable
available. The selection of test equipment, leak signals include helium and other
tracer gas and leak detection method is inert gases having specific
influenced by the following factors: charge-to-mass properties that permit
(1) the size of the leaks to be detected and their sensitive detection by mass
located; (2) the nature and accuracy of spectrometers and include gaseous
leak test information required; (3) the size radioactive isotopes detectable with
and accessibility of the system being particle counters and radiation
tested; (4) the system operating detectors.
conditions that influence leakage; (5) the
hazards associated with specific leak Techniques for Leak
location methods; and (6) the ambient Location with Dyed Liquid
conditions under which leak location tests Tracers
are required to be carried out. Wind,
stratification effects and lack of air Testing for leaks by use of dyed liquid
circulation can influence leak sensitivity tracers is a nondestructive testing process
and personnel. closely related to the liquid penetrant
testing process used to detect
Classification of Methods discontinuities open to the surface in test
for Locating and objects. For leak testing, however, the
Evaluating Individual Leaks liquid penetrant is applied to one side of
the enclosing wall of a test object or test
Methods for locating and evaluating system and, after allowing adequate time
individual leaks can be categorized in for the penetrant to seep through leaks,
various ways, including by types of leak visual testing for leak location is carried
tracer used in the detection, location and out on the opposite side of the enclosure
possible measurements of individual leaks. wall. Note that, in this type of test,
A primary division is that between liquid pressurization of the test object is not
tracers and more sensitive gaseous tracers. usually required. The migration of the
Leak location techniques that depend on liquid tracer through the leak passageway
tracer gas properties are listed below in is not due to an applied pressure
general categories, in order of increasing differential. Instead, the physical forces
leak sensitivity and complexity of test required to cause the liquid to penetrate
techniques. through the leak are provided by surface
wetting capillary action and by surface
tension effects characteristic of the liquid

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penetrant tracer and of the material more. Color former dyes are materials used
surfaces that this tracer touches. mainly in combination with sensitizer
dyes for the purpose of shifting the color
Significant differences exist between response by cascading the fluorescence.
liquid tracers used in leak testing and Color former dyes, when used alone,
liquid penetrants used for detection of usually do not provide an acceptable level
surface discontinuities in test objects. of thin film response but are sometimes
These differences exist mainly in extremely effective as bulk fluorescence
connection with the way applicable tracers. The classification of dyes used in
indicator dyes are used, their fluorescent tracers follows: (1) water
concentrations and the techniques for soluble sensitizer dyes, (2) water soluble
augmenting the detectability of leak color former dyes, (3) oil soluble sensitizer
indications. dyes and (4) oil soluble color former dyes.

Characteristics of Liquid Tracers Advantages of Water Phase
for Locating Leaks Fluorescent Leak Tracers

Liquid leak tracers are typically composed Water is the most plentiful and the least
of a vehicle or carrier such as oil or water expensive solvent liquid available. Where
and of a tracer dye system incorporated
into the liquid carrier to enhance visibility FIGURE 1. Leak testing using fluorescent tracer liquids:
of leakage indications. The dye solubility (a) fluid is added to air conditioning system; (b) ultraviolet
in various carrier liquids as well as the radiation causes leaking fluid to fluoresce.
coloration power of the dyes in dilute or (a)
in concentrated solutions in the carriers
become important factors that control the (b)
sensitivity and ease of use of the liquid
tracers. Nonfluorescent visible color dyes
that may exhibit intense coloration in
concentrated solutions usually lose their
coloration power rapidly as they are
extensively diluted in a solvent. For water
base tracers, basic dyes provide the most
intense colors. Solvent dyes usually yield
the strongest colors for oil soluble tracer
systems.

However, a fluorescent dye that has a
reasonable efficiency in the conversion of
ultraviolet radiation to visible light is
usually much more effective as a leak
tracer than is the average visible color
dye. This occurs partly because of the
greater response of fluorescent dyes in
dilute solutions and partly because of the
enhanced visibility or brightness contrast
of fluorescent leak indications seen
against a dark background (see Fig. 1). In
closeup applications where leak testing
can be carried out with an ultraviolet
lamp under subdued white light
conditions, it is often possible to get
brightness contrast values for fluorescent
leak indications that exceed several
hundred to one. These conditions provide
high levels of tracer sensitivity and
excellent visibility of the leak indications.

Components of
Fluorescent Tracer Dyes

Two broad categories are used in
classifying fluorescent dyes with respect to
their effectiveness in tracer usage in leak
testing. Sensitizer dyes are materials that
can yield strong fluorescent response in
thin liquid films and at practical dye
concentrations of about 0.5 percent or

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the use of water as a tracer carrier is Solvent Developers to Enhance
permissible, economic considerations Leak Indication of Water Phase
alone would call for its use. Other Fluorescent Tracer
considerations in favor of water include
cases where water would ultimately be A useful technique for detecting points of
used as the contained liquid in a tank or leakage in boilers, pipelines and valves
pipeline being tested. In such cases, the uses a water phase fluorescent tracer
use of an oil phase tracer liquid would be containing a special dye system that can
undesirable in view of the necessity for be developed by application of a suitable
cleaning the oily tracer liquid off the test spray solvent developer material. This
surfaces before letting water in. When the process will work if a water activated color
mode of bulk fluorescence is used in leak dye such as fluorescein, a water soluble
testing, water phase tracers are often fluorescent dye, is used. However,
preferred. With water phase leak tracers, fluorescein yields indications having
the fluorescence response can be detected brightness response much lower than that
at dye concentrations on the order of a of more efficient sensitizer dyes.
few parts per million or less. This level of
leak (dye) sensitivity is greater by several In this leak locating technique, the
orders of magnitude than the sensitivity fluorescent tracer dye is dissolved in
attainable with oil phase tracer materials. water, this fluorescent water is introduced
into the system under test and the system
Applications of Water is pressurized. If necessary, the test system
Phase Fluorescent Leak is allowed to stand under pressure for
Tracers in Leak Testing of several days. Then a liquid solvent
Boilers and Tanks developer is sprayed lightly over the
external surfaces, with particular attention
Water phase fluorescent racers are to joints, weldments and other regions
frequently used for detecting microleaks where leaks might occur. Inspection is
in water or steam systems. In this use, carried out in darkness or under subdued
water containing a dissolved fluorescent white light while the fluorescence is
tracer dye is introduced into the boiler, excited with ultraviolet radiation (similar
tank or system under test. The outside of to those widely used in fluorescent liquid
the test object is then inspected in near penetrant testing for discontinuities in
ultraviolet radiation (365 nm wavelength test objects).
mercury vapor radiation) for locations of
points of leakage. In the case of large Microtraces of dye accumulations at
tanks, it is often found that leaks will points of leakage will dissolve in the thin
show up only when the large tank is filled coating of developer liquid applied to
with water. In this case, the filled tank is surfaces under inspection. These minute
subject to the stresses of weight and amounts of fluorescent tracer dye
hydrostatic pressure incurred in normal dissolved in the developer can then
usage of the tank. Stress may act to open undergo a transition, from the
leaks that otherwise are too small for easy nonfluorescent solid state where
detection or that exist only under stress. fluorescence response is quenched to a
Pipelines, boilers, valves and other high level of fluorescent brilliance. The
systems may also exhibit leakage only technique of using liquid developers to
under conditions of pressure. enhance leakage indications obtained
with water phase fluorescent tracers is
Large leaks may be readily detectable sensitive enough for detection of leaks
by a wetness that surrounds the points of with submicroscopic cross sections.
leakage. Small leaks are more difficult to
localize because the water carrier may Characteristics of Oil Phase
evaporate as it exudes from the leakage Fluorescent Leak Tracers
point. Such small leaks will, of course,
carry out some of the tracer dye, causing a The second important category of dyed
buildup of dry dye around the point of liquid leak tracers is that of the so-called
leakage. In many cases, this accumulation oil phase tracer. This type of leak tracer
of dye cannot be readily detected, because uses as its vehicle or carrier a low viscosity
its fluorescence is quenched in the solid solvent liquid, preferably an aromatic oil.
state. Virtually all of the useful water The fluorescent dye or dyes used in such
soluble fluorescent dyes exhibit tracers must be oil soluble in nature. Most
concentration quenching or solid state of the available oil soluble sensitizer and
quenching of fluorescence, at least to color former dyes are readily soluble in
some degree. aromatic solvents, much more so than
they are in aliphatic oils or distillates.
Some of the most efficient fluorescent
sensitizer dyes are oil soluble in character.
An oil-and-solvent soluble coumarin dye
is used in many liquid penetrant and leak

582 Leak Testing

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tracer formulations; however, certain that no films of the dyed liquid can
vehicle materials such as the silicones and appear at the exit point of leakage. If the
fluorocarbons will not support solvent liquid of the tracer system is
fluorescence with this coumarin type of volatile, it will evaporate as it exudes from
oil phase fluorescent dye. Other the leak outlet and a deposit of dry dye
proprietary dyes are available that require will accumulate. This state is sometimes
little or no support for their fluorescence. obtained only after a time longer than
projected for the test. The length of time
Significance of Thin Film required for leak indications to form by
Fluorescence Response solvent evaporation will depend on the
Characteristics of Leak Tracers size of the leak and the concentration of
dye in the tracer liquid. Small leaks in
In cases where leaks are extremely small, large tanks can be economically detected
the amount of dyed liquid tracer that can by taking advantage of the evaporative
migrate through the leakage path is leak tracer technique. Fairly low
correspondingly small. Detection of such concentrations of dye can be used and it
small leaks often depends on the is necessary only to allow sufficient time
fluorescence of extremely thin layers of for an accumulation of dye to leak around
dyed liquid that form indications at each a point of exit. In many cases, it may be
point of leakage. The visible color necessary to use a solvent developer to
response, as well as the fluorescence enhance water phase evaporative tracer
response of thin films of leaking dyed leak indications, as described earlier.
liquids, follows the same laws as similar
films of color contrast or fluorescent Limitations of Dyed Liquid
liquid penetrants. The tracer sensitivity Tracers for Leak Detection
can be measured and calibrated as a and Location
function of the concentration of the
indicator dye. The fluorescence response Liquid tracers, generally analogous to
is about a linear function of the dye liquid penetrant testing media, have
concentration. In some cases, it is even several basic limitations in comparison
possible to estimate the dimensional with gaseous leak tracers. In many cases,
magnitude of the leak through techniques liquid cannot be introduced into the tank,
similar to those used in evaluation of pipeline or component under test.
liquid penetrant testing. Vacuum chambers or vacuum pump
systems fall into this category because
The detectability of a given leak liquid contamination and residues left
condition by dyed liquid tracers depends after evaporation might serve as sources of
on at least two factors. First, the leakage virtual leaks or cause outgassing, which
passageway must be large enough to could be very hard to clean up from the
permit a visible amount of the dyed tracer vacuum system. As noted earlier, liquids
liquid to pass through the leak during the can act to clog leaks so that more
test. Second, the dye solution (its sensitive gas tracers cannot pass through
brightness in thin films) must be the leaks. Refrigeration systems or
dimensionally sensitive so that equipment and air ducts may require leak
fluorescence or color response can be seen testing without the use of liquids.
in the thin film exudation from the point
of leakage. It is obvious that a large An obvious limitation results from the
leakage path cross section will permit a similarity of liquid penetrant tracers and
greater flow of tracer liquid than would a liquid leak tracers. Liquid penetrants can
small or restricted leakage path. be applied to test surfaces, where they
Accordingly, when the detection of enter surface connected discontinuities
extremely small leaks with liquid tracers is such as cracks and porosity. After cleaning
desired, it is often necessary to use a dye of excess penetrant from these surfaces,
tracer system that is characterized by a the liquid penetrant tracer then exudes
high level of sensitivity. (This is not from the discontinuities (which typically
always required, however, as explained do not penetrate through the wall
below.) thickness of the test material). Exudations
and indications produced from
Evaporative Leak Tracers discontinuities that do not penetrate
to Detect Very Small Leaks through the wall or pressure boundary
can form indications that are similar in
Sometimes microleaks are found in appearance and characteristics to leakage
connection with surface porosity or other indications formed by similar tracers.
conditions under which it is extremely
difficult to verify the leak condition. Certain types of hermetically sealed
Where the leak is extremely minute, the components, such as electronic devices,
rate of migration of the dyed liquid tracer are often tested by applying pressurized
through the leakage path may be slow tracer systems to their external surfaces (as
in helium bombing) so that some tracer

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can enter the test part through any leak powdered dye from a light yellowish
passageways. Later, the tracer can escape green color to vivid purple. After testing,
from the interior volume and pass the dye is removed with an air hose, by a
outward through the pressure boundary water wash or by wiping the test surface
to form leakage indications. with a cloth. The concentration of
ammonia gas can be as small as 50 µL of
This technique is less feasible when ammonia gas to 1 L of air (50 µL·L–1).
liquid tracers are used for leak testing. In Problems encountered with this leak
most cases, the liquid would be an testing technique are ensuring uniform
undesirable contaminant of the interior adherence of the dye to the weld areas
regions of sealed test objects. Secondly, and the complete removal of the dye after
the entry and later exudation of liquid completion of the leak test.
tracer from leaks through the pressure
boundary cannot be reliably discriminated Gas Phase Leak Testing
from surface indications that do not with pH Sensitive Dye
penetrate the enclosure wall. Finally, as Indicators
noted previously, exposure of test surfaces
to liquids that might seep into and clog Various visible color and fluorescent dyes
leaks can result in severe difficulties if are sensitive to the concentration of
later leak tests are conducted with gaseous active ions, either acid or alkaline. These
tracers, because prior leak clogging with ions determine the hydrogen potential (or
liquids can prevent the gas tracer from pH), which indicates the ion content of
passing through the leaks. pure water. The pH value for a solution
can be shifted by addition of an acid or
Techniques for Leak an alkali. A number of useful indicator
Location with Gas Phase dyes are quite sensitive to small changes
Dye Tracers in pH of the liquid in which they are
dissolved. While oxidation reduced
Gas phase tracers, used with dye indicators are feasible, the most direct
indicators, have been developed to a high way to use these effects is to use a pH
level of leak detection sensitivity. Either a sensitive liquid and a tracer gas that will
visible color or a fluorescent dye can be produce a change in pH of the liquid it
used to augment the sensitivity of a gas contacts. This type of gas phase leak
phase leak tracer process. Detectors indicator is a liquid material coated onto
consist of a dye and a tracer gas that will areas suspected of having leaks. The
react with each other to produce a change indicator material then reveals the
of color or fluorescence in the dye. A presence of leaks by fluorescent or visible
simple technique of gas phase leak testing color indications. A suitable indicator dye
with dye indicators is to pressurize the for producing visible color leak
system to be tested with ammonia gas. indications is phenolphthalein. However,
Note that ammonia gas is possibly numerous proprietary dye indicators are
dangerous to use in leak testing unless it available that yield satisfactory
is carefully controlled and exhausted from fluorescence or color response as the
work areas to avoid injury to personnel. result of very small changes in pH.
With ammonia gas as the leak tracer,
strips of pH indicator paper, moistened Most of the pH sensitive dyes can be
with water, can be used as probes to dissolved in pure water or in water and
search out any points of ammonia gas alcohol mixtures. Glycols are effective as
leakage. This technique provides a solvents for these dyes in many cases. In
medium-to-high leak test sensitivity for formulating the dye indicators, sufficient
the detection of gas leaks but is awkward acid (such as hydrochloric acid) or base
to use and ammonia fumes are unpleasant (such as sodium hydroxide) is added to
or toxic for test operations. the dye solution to shift the pH to a value
close to the color change point of the
Ammonia Gas Leak Detection with indicator dye. The concentration of the
Purple Dye Indicators indicator dye may be varied within
substantial limits. At the lower extreme,
Bromocresol purple dye is used in changes of color or fluorescence may be
chemical reaction leak testing with detected with dye concentrations as low
ammonia gas tracers. The dye is sprayed as one part per million (1 µL·L–1) in the
or brushed onto the outside surfaces of solvent. The practical upper limit is a
pressure vessel welds and allowed to dry. saturated solution of the dye. The solvent
Upon drying, bromocresol purple turns must be polar in nature so that it can
into a yellow chalklike powder coating. ionize dissolved acids or bases. Examples
The vessel under test is then pressurized of suitable solvents include various lower
with ammonia gas. If a leak exists, it is alcohols, glycols, glycol ethers and water.
indicated by a change in the color of the Alcohols and glycols are useful with water

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insoluble indicator dyes or where the test surfaces where it dries to a pale
volatility or refractive index properties are blue greaselike coating. Vapors of the
advantageous. It has also been found that amine tracer that diffuse through leakage
the sensitivity of the leak detection points act to trigger a color change in the
process can be greatly enhanced by pasty coating so that bright red spots are
dissolving the indicator dye in a highly formed to indicate leak locations.
purified, deionized water from which all
traces of buffer salt residues have been Chemical Fumes Leak
removed. This allows the pH sensitive Locator Techniques
tracer dye to switch easily from one color
or fluorescent state to another. The chemical smoke or fumes leak
indicator technique depends on a
Selection of Tracer Gas for Leak chemical reaction between a tracer fluid
Testing with pH Sensitive Dye and a chemical reagent to produce visible
Indicators leak indications. Various nonstandardized
chemical indicator leak tests exist, of
Two basic materials are used in leak which a few representative examples are
testing and location with the gas phase described next. Tracer gases or vapors used
tracer and pH sensitive dye indicator: (1) a in chemical indicator leak testing include
vapor source and (2) an indicator paint. ammonia, hydrogen sulfide and carbon
Ammonia vapor could be used, of course, dioxide. Both ammonia and hydrogen
but it represents a severe personnel sulfide are considered hazardous or toxic
hazard. Proprietary gas phase tracers have gases and should be used with proper
been developed that will yield a color safety precautions. The chemical indicator
change in response to a very minute techniques are static leak testing
quantity of vapors given off by simple techniques used fundamentally as leak
amine liquids such as ethylene diamine or location procedures. The longer the leak
propylenediamine. Many simple amines test is run, the more sensitive it becomes.
have vapor pressures on the order of 1 kPa A major advantage of these tests is their
(10 torr) at room temperature. In normal relatively low cost because no expensive
use, the vapor concentration of such instrumentation is required. Major
tracers cannot exceed 10 parts in 760 (the limitations of these tests include the
partial pressure ratio) or about 1.3 possibility that reactive chemicals might
percent. Even this concentration would damage materials or parts of systems
normally occur only quite close to the being tested or present hazards to
work area or within a pressure chamber. personnel.
Thus, for low vapor pressure amines, the
hazard to leak testing personnel may be Chemical Indicator Leak Detector
considered to be negligible. During leak Techniques Using Ammonia Tracer
testing, the vapor source may be placed Gas
inside the test chamber or system, either
in a shallow cup or on a cloth pad or Ammonia (NH3) gas has found industrial
sponge. Even without pressurization of use as a gas phase leak tracer. It is
the chamber, the vapors from the amine chemically basic and corrosive and it is
liquid may often diffuse through usually prohibited at concentrations
extremely small microleaks, pinholes, exceeding 75 µL·L–1. The corrosive action
porosities in seals, welds and brazed of ammonia is exhibited strongly on brass
joints. Of course, a small amount of parts. The ammonia tracer can be
pressurization will act to accelerate the introduced as an anhydrous ammonia gas
leakage of the vapors. or by placing a cloth saturated with liquid
ammonia in the pressurized space. The
Formulation of pH Sensitive sensitivity of the leak tests will depend on
Indicator Paints for Coating Test the concentration of the ammonia gas
Surfaces and will increase with higher
concentrations. Hydrogen chloride vapor
The indicator paint used to indicate leak can be introduced by probing the
locations must contain a suitable atmosphere in the vicinity of a leak with
indicator dye. The unbuffered pH of the an open bottle of hydrochloric acid or
dye solution must be carefully adjusted to with a swab wetted with 0.1 normal
a point just below the critical pH of hydrochloric acid. (Note that
fluorescence or color change. Also, a hydrochloric acid is a dangerous chemical
thickening agent and an evaporative and should not be brought into contact
dilutent should be included so that a with the skin or eyes or inhaled through
paint coating that is applied to a test nose or mouth.) Where leaks are present,
surface will become partially dry and form the leakage of ammonia tracer gas can be
a greasy or pasty coating on the test revealed by a white chemical fog or mist
surface. A typical proprietary leak tracer of ammonium chloride that forms when
paint is a liquid that can be brushed onto

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the ammonia comes into contact with the Formulation of Agar-Agar Film
hydrogen chloride vapor. This technique Solution Sensitive to Carbon
should be used only with good ventilation Dioxide Tracer Gas
because of the obnoxious characteristics
of both hydrogen chloride vapor and The spray solution sensitive to leaking
ammonia gas. carbon dioxide tracer gas is formulated
(by weight) of 1.0 part of agar-agar to 0.15
Another technique for use of ammonia part of phenolphthalein. It is important
in a chemical indicator leak test is to use to obtain the proper viscosity of agar-agar
sulfur dioxide (SO2), as from a sulfur solution, which should be between 0.5
candle, as the revealing reactant. and 1 mPa·s (5 and 10 atm) at the
Ammonia combines with sulfur dioxide spraying temperature, about 65 to 70 °C
gas to produce a white mist of (149 to 158 °F). To prepare the spray
ammonium sulfide. Sulfur dioxide is not solution, the dry powders of agar-agar,
quite so irritating or corrosive as sodium carbonate and phenolphthalein
hydrogen chloride; however, sulfur should be blended thoroughly in proper
dioxide is still an obnoxious gas and proportions. The dry mixed powders
should be used only in well ventilated test should be added to cool distilled water
areas. while stirring to disperse the solid
constituents. The resulting mixture should
A third gas that can be used as a be stirred constantly while heating to
revealing reactant for leaking ammonia temperatures between 96 and 98 °C (205
vapor is carbon dioxide (CO2). Carbon to 208 °F), either on a hot plate or with a
dioxide is not as sensitive as either steam bath. When the solids have
hydrogen chloride or sulfur dioxide when dissolved completely and a clear solution
used for leak location with ammonia has been obtained, it should be allowed to
tracer gas but has the advantage of being cool to a temperature between 65 and
noncorrosive. 70 °C (149 to 158 °F). At this storage
temperature, the indicator film solution
Chemical Indicator Leak can be stored in a closed glass container
Detector Techniques Using sealed to exclude air and the small
Carbon Dioxide Tracer Gas quantities of carbon dioxide in the
atmosphere.
Another chemical indicator leak location
technique uses carbon dioxide tracer gas Miscellaneous Additional
to fill the vessel or system under test. The Leak Detector Techniques
leak indicator consists of agar-agar Using Chemical Indicators
solution loaded with sodium carbonate
and phenolphthalein. This bright red Numerous additional techniques of leak
spray solution yields a stable red film testing using chemical indicators of
when sprayed onto test surfaces. Leaking leaking tracer gases of liquids have been
carbon dioxide causes the formation of proposed and used on occasion. Several of
white spots at points of leakage. The the following examples involve use of
volume of red agar-agar film that is hazardous tracer gases such as hydrogen
changed to a white color is directly sulfide or acetylene and should be
proportional to the amount of leaking avoided where feasible. Hydrogen sulfide
carbon dioxide that enters the indicator has been used as a tracer gas to locate
film. To apply this indicator film, the leaks in containers. The indicator solution
agar-agar solution is preheated and stored responsive to hydrogen sulfide is a five
at temperatures of 65 to 70 °C (149 to percent solution of stannous chloride. The
158 °F), at which temperature the agar is leak locations are then shown by brown
liquid. For application to test surfaces, the stains of stannous sulfide. Because of the
hot agar solution is poured into a poisonous nature of hydrogen sulfide, this
preheated sprayer bottle and sprayed with is not a very popular leak testing
preheated compressed air. The spray technique. An alternative chemical that
nozzle should be held about 0.6 m (2 ft) responds to hydrogen sulfide by the
from the test surface. A single coating formation of precipitates consists of
should be applied in one pass and indicator solutions containing silver salts
multiple coatings should be avoided. or lead salts.
Spraying should always be done in a
horizontal direction, never vertically. After Leaks in natural gas pipelines have
completion of leak testing, the agar-agar been located by the reactions of silver salt
film can be removed by a jet of high solutions with acetylene tracer gas.
velocity air. The air jet will lift the agar Acetylene is also a hazardous gas that
film from the test surface, leaving the explodes spontaneously at pressures above
surface in a clean, dry condition. 200 kPa (30 lbf·in–2). Acetylene forms
explosive mixtures with air or oxygen in
almost any proportions.

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Anhydrous copper sulfate has been and placing it inside the vessel to be
used as a chemical leak indicator where tested. All openings in the test system
water is used as the tracer fluid (as in should then be closed. Almost
detection of leaks from water lines or immediately, smoke will be seen issuing
water leaking during hydrostatic tests). from the larger leaks present in the test
When leaking water contacts a developer object. The order of escaping smoke will
film of anhydrous copper sulfate, it shows also assist in pinpointing the locations of
blue indications at locations of water leak exits. When smoke tests are made on
leakage. An alternative technique of steam generating boilers and similar
indicating water leakage is a lime wash or equipment with volumes of 2.8 × 103 m3
similar type of coating applied over (105 ft3) or larger, it is frequently desirable
external surfaces of test vessels or test to apply air pressure to the interior
systems within 24 h after hydrostatic volume of the system under test.
pressure testing.
Smoke candles can be obtained that
Typically, only large leaks such as provide from 100 m3 (4 × 103 ft3) of
centerline cracks or pinholes can be smoke in 30 s to 4 × 103 m3 (1.4 × 105 ft3)
indicated by this technique. Fine check of smoke in 2 or 3 min. The color of the
cracks may not be indicated when water smoke may vary from white to gray,
under pressure is used as a tracer. depending on the smoke density and
Sometimes nearly penetrating available lighting. The smoke used in leak
discontinuities may enlarge sufficiently location is generated by chemical
under hydrostatic pressure to seep water. reaction, contains no explosive materials
This water seepage will produce wetness and has been assumed to be nontoxic.
indications in the lime wash indicator,
but interpretation of these indications
may be difficult.

Fluorescent or Visible FIGURE 2. Smoke generators: (a) titanium
Tracer Dyes in Hydrostatic tetrachloride smoke stick; (b) dye based
Test Fluids smoke candles.
(a)
Fluorescing dye indicators can be added to
the pressurized liquid (usually water) used (b)
in hydrostatic pressure tests. The dye may
provide bulk fluorescence or can more
appropriately be of the type that provides
intensified fluorescence after evaporation
of the carrier liquid. In some cases, a
developing fluid or film may be applied to
the external surfaces of the test object
where leakage is possible or suspected.
During or following hydrostatic pressure
tests (which often serve as proof tests
simulating application of service stresses),
the test operator can visually examine all
welds under strong white light (if visible
color dyes were used) or under near
ultraviolet radiation (if fluorescent dyes
have been used in the pressurizing fluid).
Slow, continuing seepage from small leaks
thus can be indicated by brilliant
fluorescent indications.

Smoke Bomb Techniques
for Locating Leaks

Gas and smoke bombs (Fig. 2) can be used
for detecting and locating leaks. Generally
speaking, a volume of smoke sufficient to
fill a volume five or six times larger than
the volume of the vessel or system to be
tested is required for a smoke test.
Medium size volumes such as pressure
vessels can be tested by closing all vents,
igniting a smoke candle or smoke bomb

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