ASNT NDT Handbook Volume 3 Infrared and Thermal Testing

various thermocouples in sequence. All of thermocouple type by simply changing
the volt meter and scanner wires arc gain resistors.
copper, independent of the type of
thermocouple chosen. In fact, as long as The advantage of the hardware
each thermocouple is known, compcmation circuit or electronic ice
thermocouple types can he mixed on the point reference is that either reference
same isothermal junction block (often ohviates computation of the reference
called a zone box) and the appropriate temperature. This saves two computation
modifications in software can be made. steps and makes a hardware
The junction hlock temperature sensor RT compensation temperature measurement
is located at the center of the block to somewhat faster than a software
minimize errors from thermal gradients. compensation measurement. However,
faster microprocessors and advanced data
Software compensation is the most acquisition designs have blurred the line
versatile technique for measuring between the two techniques, with
thermocouples. Many thermocouples are software compensation speeds challenging
connected on the same block, copper
leads are used throughout the scanner and FIGURE 13. Equivalent circuits illustrate
the technique is independent of the types hardware compensation: (a) circuit with
of thermocouples chosen. In addition, battery insertedi (b) circuit with ice bathi
when using a data acquisition system (c) diagram of equivalent circuit, showing
with a built~in zone box, the compensation voltage e.
thermocouple is simply connected as a
pair of test leads would be. All of the (a}
conversions are performed by the
instrument software. The one
disadvantage is that it requires a small
amount of additional time to calculate the
reference junction temperature. For
maximum speed hardware compensation
can be used instead.

Hardware Compensation (b)

Rather than measuring the temperature of + o-'C:"u'--j, Fe
the reference junction and computing its
equivalent voltage as with software ,, I T
compensation, a battery could be inserted
to cancel the offset voltage of the 1\-~1 Fe Cn
reference junction. The combination of I...----+-+--~
this hardware compensation voltage and
the reference junction voltage is equal to Ice bath at 273 K (0 oc = 32 °f-)
that of a 273 K (0 oc ~ 32 °F) junction (c)
(Fig. 13).
legend
The compensation voltage e is a
function of the temperature sensing Cn = constantan resistance alloy (50 to 65 percent
resistor, RT. The voltage F is now copper, 35 to 50 pNcent nickel)
referenced to 273 K (0 °C ~ 32 °F) and
may be read directly and converted to Cu = copper
temperature by using tables from the e = compensation voltage (V)
National Institute of Standards and
Technology. Fe = iron
R1 = resistance measurement (0)
Another name for this circuit is the T = electrical terminal
electronic ice point.6 These circuits are V = voltage 01)
commercially available for use with any
volt meter and with a \Vide variety of
thermocouples. The major drawback is
that a unique ice point reference circuit is
usually needed for each individual
thermocouple type.

Figure 14 shows a practical ice point
referencE' circuil that can be used in
con junction with a relay scanner to
compensate an entire block of
thermocouple inputs. All the
thermocouples in the block must be of
the same type but each block of inputs
can accommodate a different

236 Infrared and Thermal Testing

those of hardware compensation in accurate direct readout of temperature,
practical applications (Table 4). That is, the temperature display involves
only a scale factor.
Voltage-to-Temperature
Conversion By examining the variations in the
thermoelectric coefficient, it can easily be
Hardware and software compensation has seen that using one consttmt scale factor
been used to synthesize an ice point would limit the temperature range of the
reference. Now all that is necessary is to system and restrict tlw system accmacy.
read the digital volt meter and convert Better conversion accuracy can be
the voltage reading to a temperature. obtained by reading the volt meter and
Unfortunately, a thermocouples consulting the thermocouple tables of the
relationship of temperature versus voltage National Institute of Standards and
is nOt linear. Output voltages for some Technology. 4•5
popular thermocouples are plotted as a
function of temperature in Fig. 15. If the These lookup table values could he
slope of the curve (the thermoelectric stored in a computer but they would
coefficient) is plotted versus temperature, consume an inordinate amount of
as in Fig. 16, it becomes quite obvious memory. A more practical approach is to
that the thermocouple is a nonlinear approximate the table values using a
device. power series polynomial:

A horizontal line in Fig. 16 would (7) r0 + c1x + r2 x2
indicate a constant slope a and hence a + ({-\.} + ... + cll;e'
linear device. The slope of the type K
thermocouple approaches a constant over FIGURE 15. Thermocouple temperature versus voltage.
a temperature range from 273 to 1273 K
(0 "C to 1000 "C; 32 "F to 1832 "F).
Consequently the type K can be used with
a multiplying volt meter and an external
ice point reference to obtain a moderately

fiGURE 14. Practical hardware compensation.

Cu Fe
Cu Cn

Ru

Cu

Volt meter I

Integrated 0 773 1273 1773 2273 2773
temperature
sensor (500) (1000) (1500) (2000) (2500)

legend [932) [1832) [2732) [3632) {4532]

Cn =constantan resistance aHoy (50 to 65 percent copper, 35 to Temperature K C'C) [0 F}
50 pNcent nkke!)
legend
Cu =copper
Fe= iron E =chrome!(+) versus constantan (coppl:'r nickel alloy)(-)
J-= iron(+) versus constantan (copper nickel alloy){-)
i =electrical current (A) K ~ chromel (+)versus alumel (-)
R =platinum (+)versus platinum with 13 percent rhodium(-)
J = electrical junction S "'platinum(+) versus platinum with 10 percent rhodiwn (-)
T =-copper (-t) versus const;mtan copper nickel alloy,(-)
RH := hardware compensation resistance
V"' voltage Cl)

TABlE 4. Hardware compensation versus software compensation.

Compensation Speed Versatility Configuration Ease

Hardware fast one themocoup!e type hard to reconfigure- requires hardware
Software per reference junction change for new thermocouple type
requires more software
manipulation time versatile - accepts easy to reconfigure
any thermocouple

Contact Sensors for Thermal Testing and Monitoring 237

where c = polynomial coefficients unique higher system speed. ·nthle S is an
example of the polynomials used in
to each thermocouple; 11 = maximum conjunction with software compensation
order of polynomial; T90 = temperature fw a data acquisition sy~tem. Rather than
directly calculating tile exponentials, the
(kelvin); and x = thermocouple voltage software is programmed to use the nested
polynomial form to save execution time.
(volt). The polynomial fit rapidly degrades
As the polynomialn increases/ the outside the temperature wnge shown in
TableS and should not be extrapolated
accuracy of the polynomial improves. outside those limits.
Lower order polynomials may lJe used
over a narrow temperature range to obtain Table 5 uses the temperature
conversion equation:
FIGURE 16. Thermoelectric coefficient versus temperature.

(8) T90 Co + c1.\ + C2X?

+ C;~X3 + ... + Cg;\'9

1 and the following form for a nested
> polynomial (fourth order example):

3 The calculation of high order
polynomials is time consuming, even for
~c today's high powered microprocessors. As
mentioned above, time can be saved by
.~ using a lower order polynomial for a
smaller temperature range. In the software
~ 4o I-.ffli---.!:=-1-'='f----t: for one data acquisition system, the
thermocouple characteristic curve is
0 divided into eight sectors and each sector
u is approximated by a third order
polynomial (i'ig. 17).
~
The data acquisition system measures
~ the output voltage, categorizes it into one
of the eight sectors and chooses the
.0 appropriate coefficients for that sector.
This technique is both faster and more
~ accur<lte than the higher order
polynomial.
~
An even faster algorithm is used in
0 773 1273 1773 2273 many new data acquisition systems. Using

(500) (1000) (1500) (2000)

]932] [1832] [2732] [3632J

legend

E = chrome!(+) versus constantan (copper nickel alloy)(-)
J = iron(+) versus constantan (copper nickel alloy)(-)
K = chrome!(+) versus a!umel (-}
R = platinum(+) versus platinum with 13 percent rhodium(-)
S = platinum(+) versus platinum with 10 percent rhodium(-)
T =copper(+) versus constantan (copper nickel alloy)(-)

TABLE 5. NIST ITS-90 polynomial coefficients.'

Thermocouple Type J Thermocouple Type K

--~673~to~2~73~K---- 273to1033K 7=3~t-o~2=7~3~K~- 273 to 773 K

(-210 to 0 'C; (0 to 760 'C; (-200 to 0 'C; (0 to 500 'C;
-328 'F to +32 'F) 32 to 932 'F)
-346 'F to +32 'F) 32 to 1400 'F)
Polynomial ±0.04 K (0.04 'C = 0,07 'F) ±0.05 K (0.05 'C = 0.09 'F)
Order ±0.05 K (0.05 'C =0.09 'F) ±0.04 K (0.04 'C =O.o7 'F)
Eighth Order Ninth Order
Eighth Order Seventh Order

Co 0 0 0 0

c, -1.9528268 X lQ-2 -1.978425 X 10-2 -2.5173462 x 1o-2 -2.508355 X 10-2

c, -1.2286185 X lQ-6 -2.001204 X 10-7 -1.1662878 x 1o-6 -7.860106 X 10---8

(3 -1.0752178 X lQ-9 -1.036969 X lQ-11 -1.0833638 X 10·9 -2.503131 X 10-10

(4 -5.9086933 X 1Q-13 -2.549687 X 10-1 6 -8.9773540 X lQ-13 -8.315270 X lQ-14

w-c, -3.7342377 X lQ-16
-1.7256713 x 16 -3.585153 X 10-21 -1.228034 X 1Q-1

c, -2.8131513 X 10-20 -5.344285 X 1Q-26 -8.6632643 x 1o-2o -9.804036 X 10-2.?

c, -2.3963370 X lQ-24 -5.099890 X l0-31 -1.0450598 X 1Q-23 -4.413030 X 10-2(,

c, -8.3823321 x 1o- 29 -5.1920577 x w-28 -·1.057734 X lQ-30

c, -1.052755 X 10-35

238 Infrared and Thermal Testing

many more sectors ahd a series of first to solve unique measurement problems.
order equations, they can make hundreds, Idiosyncrasies of the more common
even thousands, of internal calculations thermocouples are discussed here.
per second.
The term stnndard wire error refers to
All the foregoing procedures assume the common commercial specifications
the thermocouple voltage can be published by the American Society of
measured accurately and easily; however, Testing and lvfaterialsFJ The standard
requirements of the system volt meter wire error represents the allowable
{Table 6) show that thermocouple output deviation between the actual
voltages are very small. thermocouple output voltage and the
voltage predicted by tables published by
Even for the common type K the National Institute of Standards and
thermocouple, the volt meter must be Technology:t
able to resolve 4 pV to detect a 0.1 K
(0.1 "C ~ 0.18 "F) change. This demands Noble Metal Thermocouples

both excellent resolution (the more bits, The noble metal thermocouples, types B,
the better) and measurement accuracy R and S, are CJII platinum or platinum
from the digital multimeter. The rhodium thermocouples and hence share
magnitude of this signal is an open many of the same characteristics.
invitation for noise to creep into any
system. For this reason instrument Diffusion. Tvfetallir vapor diffusion at high
designers use several fundamental noise temperatures can readily change the
rejection techniques, including tree platinum wire calibration, hence platinum
switching, normal mode filtering, wires should only be used inside a
integration and isolation. nonmetallic sheath such as high purity
alumina. The one exception to this rule is
Thermocouple prohibitively expensive- a sheath made
Characteristics of platinum.

Over the years specific pairs of Stability. The platinum based couples are
thermocouple alloys have been developed by far the most stable of all the common
thermocouples. Type S is so stable that it
FIGURE 17. junction voltage cancellation: V1 :::: V if V3 = V4. is specified as the standard for
Curve divided into sectors for polynomial calculation. temperature calibration between the
Ta = bx + cx2 + dx3. antimony point, 903.89 K (630.74 "C ~
1167.33 "F), and the gold point,
I I 1337.58 K (1064.43 "C ~ 1947.97 °F).

I ·-~~ Type B. The type B thermocouple i~ the
only common thermocouple that exhibits
I I a double-valued ambiguity (Fig. 18).
II I Because of the double-valued curve and
r----,--, - · r I the extremely low thermoelectric
coefficient at lmv temperatures, type B is
III I virtually useless below 323 K
II (50 "C ~ 122 "1'). Hecause the output is
I nearly zero from 273 K (0 "C ~ 32 "I') to
315 K (42 "C ~ 108 "F), type B has the
Voltage (relative scale) unique advantage that the reference

fiGURE 18. Double valued ambiguity of
type B thermocouple.

TABlE 6. Required sensitivity for digital volt meter.

Thermocouple Seebeck Digital Volt
Type Meter Sensitivity
Coefficient
for 0.1 K
at 278 K (0.1 °Co0.18°F)

(25 oc 0 77 °F) (~V)

(~V-K- 1 )

E 61 6.1 273 31S
5.2 (0) (42)
I 52 4.0 132) 1108)
0.6
K 40 0.6 Temperature, K (0 C) [°FJ
4.1
R6

s6

T 41

Contact Sensors for Thermal Testing and Monitoring 239

junction temperature is almost Types K and N. Type K has long been a
immaterial, as long as it is het\veen 273 K popular thermocouple. It is especially
suited to high temperature applications
(0 'C =32 'F) and 313 K (40 'C = 104 'F). lJecaus.e of its reslstance to oxidation.

Of course, the measurillg jundioll The type N thermocouple is gaining
temperature is typically very high. popularity tiS a replacement for type K. It
has a slightly lower output (smaller
Base Metal Thermocouples thermoelectric coefficient) than type K
but an even higher resistance to
Unlike the noble metal thermocouples, oxidation. The type N thermocouple
the base metal couples have no specified output curve depends on wire size and
chemical composition. Any combination there are two distinct nicrosil/nisil
of metals may be used that results in a characteristic curves, the differences being
voltage-versus-temperature curve fit that wire size and temperature range. 10 (The
is within the standard wire errors. This term nisi/ refers to a nickel chrome
leads to some rather interesting metal thermal alloy; nicrosil, to a nickel silicone
combinations. Constantan, for example, is thermal alloy. These alloys are used to
not a specific metal alloy at all but a measure high temperatures and exhibit
generic name for a whole series of copper inconsistencies in thermoelectric voltage
nickel alloys. Incredibly, the constantan in wire gages.)
used in a type T (copper constantan)
thermocouple is not the same as the Tungsten. There arc three common types
of tungsten thermocouples (Table 7). All
constantan used in the type 1 (iron are alloyed with rhenium to make the
metal more malleable. Tungsten
constantan) thermocouple:" thermocouples are used for measuring
very high temperatures in either a
Type E. Although type E standard wire vacuum or an inert atmosphere.
errors are not specified below 273 K
(0 'C = 32 'F), the type E thermocouple is Practical Thermocouple
ideally suited for low temperature Measurements
measurements because of its high
thermoelectric coefficient (58 V·K-1), low Noise Rejection
thermal conductivity and high corrosion
resistance. Tree Switching. Tree switching is a
technique for organizing the channels of
The thermoelectric coefficient for type a scanner into groups, each lvith its own
E is greater than all other standard main switch.
couples, which makes it useful for
detecting small temperature changes. \-\'ithout tree switching, every channel
can contribute noise directly through its
Type J. Iron, the positive element in a stray capacitance. \'\1ith tree switching,
J thermocouple is an inexpensive metal groups of parallel channel capacitance~
are in series with a single tree switch
rarely manufactured in pure form. capacitance. The result is greatly reduced
crosstalk in a large data acquisition
J thermocouples arc subject to poor system, because of the reduced
interchannel capacitance {Fig. 19).
conformance characteristics because of
Analog Filter. A filter may be used dir<.."rt!y
impurities in the iron. Even so, the J at the input of a volt meter to reduce

thermocouple is popular because of its TABLE 7. Percentage of rhenium in
high thermoelectric coefficient and lm\' tungsten terminals. Tungsten
price. thermocouples are used for measuring
very high temperatures in either vacuum
The 1 thermocouple should never be or inert atmosphere.

used above 1033 K (760 'C = 1400 °F) Type Rhenium (percent)
because an abrupt magnetic
transformation can cause decalibration Type c~ 5 versus 26
even when returned to lower Type o~ 3 versus 25
temperatures. Type GJ 0 versus 26

Type T. This is the only thermocouple a. Not symbols of Amercan National Standard~
with published standard wire errors for Institute.
the temperature region below 273 K

(0 oc ~ 32 °F); however, type E is actually

more suitable at very low temperatures
because of its higher thermoelectric
coefficient and lower thermal
conductivity.

'l)rpe T has the unique distinction of
having one copper lead. Thh can be nn
advantage in a specialized monitoring
situation ·where a temperatme difference
is t~ll that is desired.

The advantt~ge is that the copper
thermocouple leads are the same metal as
the digital volt meter terminals, making
lead compensation unnecessary.

240 Infrared and T/lermal Testing

noise. It reduces interference dramatically analog-to-digital converter to measure the
but causes the volt meter to respond more thermocouple voltage. Integration is an
slowly to step inputs (l'ig. 20). especially attractive analog-to-digital
technique in light of recent innovations
Integration. Integration is an have brought the cost in line with
analog-to-digital technique that historically less expensive
essentially averages noise over a full line analog-to-digital lt'chnologies.
cycle- thus pmver line related noise and Isolation. A noise source common to both
its harmonics are virtually eliminated. lf high and low measurement leads is called
the integration period is chosen to be less common mode noise. Isolated inputs help
than an integer line cycle, its noise to reduce this noise as well as protect the
rejection properties are essentially measurement system from ground loops
negated. and transients (Fig. 21).

Because thermocouple drcuHs that A thermocouple wire may he pulled
cover long distances are especially through the same conduit as a 220 V
susceptible to power line related noise, it alternating current supply line. The
is advisable to use nn integrating capacitance between the power lines and
the thermocouple lines will create an
FIGURE 19. Tree switching: (a) circuit before alternating current signal of
tree sWitching; (b) stray capacitance reduced approximately equal magnitude on both
nearly 20:1 by leaving tree switch 2 open; thermocouple wires. This is not a problem
(c) approximately equivalent circuit. in an ideal circuit but the volt meter is
not ideal. It has some capacitance
(a) between its low terminal and safety
ground (earth). Current flows through
Digital this capacitance and through the
volt thermocouple lead resistance, creating a
normal mode signal that appears as
TSl meter measurement error.

(20 channels) This error is reduced by isolating the
input terminals from safety ground with a
sNouorcel;e~·~~~~ TS2 careful design that minimizes the low
earth capacitance. Nonisolated or ground
Next 20 channels c referenced inputs (single ended inputs are

Digital FIGURE 20. Analog filter.
volt
J-"'""(b) Legend
l~T+~'f92'"'0-CJHC 0 t "' time (s)
I' = voltage (V)
High

Noise
source

~ i(b) r-:----~D~-~~;\''_j_-:. meter

T~~g~~f--- .

Noise "'9"

source

Legend Distributed
resistJnce
C = channel
TS = tree ~witch Digital
volt meter

Contact Sensors tor Thermal Testing and Monitoring 241

often ground referenced) do not have the square common mode noise source. They
must withstand a peak offset of ±170 V
ability to reject common mode noise. from ground and still make accurate
Instead, the common mode current flows measurements. An isolated system with
through the !v\; !cad directly to ground, electronic FET switches typically can only
handle ±12 V of offset from earth; if used
causing potentially large reading errors. in this application, the inputs would be
Isolated inputs are particularly useful in damaged.

eliminating ground loops created when The solution is to use commercially
the thermocouple junction comes into available external signal conditioning
(isolation transformers and amplifiers)
direct contact lvith a common mode noise that buffer the inputs and reject the
source. common mode voltage. Another easy
alternative is to use a data acquisition
In Hg. 22 the temperature is measured system that can float several hundred
at the center of a molten metal bath that volts.
is being heated by electric current. The
Notice that the noise can also be
potential at the center of the bath is minimized by minimizing series resistance
120 V root mean square. The equivalent R5• This minimizing can be done by using
circuit is shown in Fig. 23. larger thermocouple wire that has a
smaller series resistance. Also, to reduce
Isolated inputs reject the noise current the possibility of magnetically induced
noise, the thermocouple should be
by maintaining a high impedance twisted in a uniform manner.
between low and earth. A nonisolatcd Thermocouple extension wires are
system (Fig. 24) completes the path to available commercially in a twisted pair
earth and results in a ground loop. The configuration.

resulting currents can be dangerously Practical Precautions

high and can be harmful to both The following concepts have been
instrument and operator. Isolated inputs discussed: (1) the reference junction,
(2) how to use a polynomial to extract
are required for making measurements absolute temperature data and (3) what to
with high common mode noise. look for in a data acquisition system to
minimize the effects of noise. Now it is
Sometimes having isolated inputs is useful to consider the thermocouple wire
not enough. In Fig. 23, the volt meter itself. The polynomial curve fit relies on
inputs are floating on a 120 V root mean the thermocouple wire being perfect; that
is, it must not become decalibrated during
FIGURE 22. Measurement of temperature and the act of measuring temperature.
center of molten metal bath heated by
electric current. The pitfalls of thermocouple
thermometry deserve consideration. Aside
240Y ~ from the specified accuracies of the data
root acquisition system and its isothermal
mean reference junction, most measurement
square error may be traced to one of these
primary sources: (1) poor junction
]II v~ ,'o-
FIGURE 24. Nonisolated system completes
FIGURE 23. Inputs floating on 120 V root the path to earth, resulting in a ground
loop.
mean square common mode noise source
must withstand peak offset of ±170 V from
ground and still make accurate
measurements.

120V Series High Series High
root low resistance low
,-------------,
mean l20V Nolse current
square ':'''' ~------J''''' root
'
mean
Noise current square

242 Infrared and Thermal Testing

connection, (2) decalibration of unintentionally altering the physical
thermocouple wire, (3) shunt impedance makeup of the thermocouple wire so that
and galvanic action, (4) thermal shunting, it no longer conforms to the National
(5) noise and leakage currents, Institute of Standards and Technology
(6) thermocouple specifications and polynomial within specified limits.
(7) documentation. Decalibration can result from diffusion of
atmospheric particles into the metal,
Poor junction Connection caused by temperature extremes. It can be
caused by high temperature annealing or
There arc a number of acceptable ways to by cold working the metal, an effect that
connect two thermocouple wires, can occur when the wire is drawn
especially soldering, silver soldering and through a conduit or strained by rough
welding. When the thermocouple wires handling or vibration. Annealing can
arc soldered together, a third metal is occur within the section of wire that
introduced into the thermocouple circuit. undergoes a temperature gradient.
As long as the temperatures on both sides
of the thermocouple are the same, the Robert J. Moffat explains that the
solder should not introduce an error. The
solder does limit the maximum thermocouple voltage is actually
temperature to which this junction can be generated by the section of wire that
subjected (Fig. 25). To reach a high contains a temperature gradient- and
measurement temperature, the joint must not necessarily by the junction .12 For
be welded. But welding is not a process to example, if a thermal probe is in a molten
be taken lightly. 11 Overheating can metal bath, there will be t"wo regions that
degrade the wire; the welding gas and the are virtually isothermal and one that has a
atmosphere in which the wire is welded large gradient.
can both diffuse into the thermocouple
metal, changing its characteristics. The In Fig. 26, the thermocouple junction
difficulty is compounded by the very will not produce any part of the output
different nature of the two metals being voltage. The shaded section will produce
joined. Commercial thermocouples are virtually the entire thermocouple output
welded on expensive machinery using a voltage. If, because of aging or annealing,
capacitive discharge technique to ensure the output of this thermocouple were
uniformity. found to be drifting, then replacing only
the thermocouple junction would not
A poor weld can, of course, result in an solve the problem. The entire shaded
open connection, which can be detected section would have to be replaced because
in a measurement situation by performing it is the source of the thermocouple
an open thermocouple check. This is a voltage.
common test function available with
many data loggers and data acquisition Thermocouple wire obviously cannot
systems. be manufactured perfectly; there will be
some discontinuities that will cause
Decalibration output voltage errors. These
heterogeneities can be especially
Decalibration is a far more serious fault disruptive if they occur in a region of
condition than the open thermocouple steep temperature gradient.
because it can result in a temperature
reading that appears to be correct. FIGURE 26. Gradient produces voltage.
Decalibration describes the process of

fiGURE 25. Soldering of thermocouple point ! ! 1-;:: 473 K (200 "C = 392 ~F)
(lead, tin). junctions of iron to lead and of ~ 573 K (300 "C "'572 "F)
tin to constantan approximate that of iron - -~I I ---· 673 K(400 "C"'" 752 "F)
to constantan. - -- 773 K(5 00 "C "'932 GF}

Fe - -- -

Cn Metal bath of 773 K (500 "C == 932 or)
Solder (Pb, Sn)

legend
Cn = constantan resistance alloy (50 to 65 percent
copper, 35' to 50 percent nickel}
Fe= iron
Pb = lead
5n= tin

Contact Sensors for Thermal Testing and Monitoring 243

Because it is not known when.:'·<m deviate from the published values. \'\1hen
imperfection will occur within a wire, the using thermocouples at high
temperatures, the insulation should be
best thing is to avoid creating a steep chosen carefully. Atmospheric cffecb can
be minimized by choosing the proper
gradient. Gradients can be reduced by protective metallic or ceramic sheath.
using metallic sleeving or by careful
placement of the thermocouple wire. Galvanic Action

Shunt Impedance The dyes used in some thermocouple
insulation form an electrolyte in the
High temperatures can also take their toll presence of water. This creates a galvanic
on thermocouple wire insulators. action, with a resultant output hundreds
Insulation resistance decreases of times greater than the thermoelectric
exponentially with increasing effect. Precautions should be taken to
temperature, even to the point that it shield the thermocouple wires from all
creates a virtual junction. Assume a harsh atmospheres and liquids.
completely open thermocouple operating
at a high temperature (Fig. 27). Thermal Shunting

The leakage resistance RL can be No thermocouple can be made without
sufficiently low to complete the circuit mass. Because it takes energy to heat any
path and give an improper voltage mass, the thermocouple will slightly alter
reading. Now assume that the the temperature it was meant to measure.
thermocouple is not open but a very long If the mass to be measured is small. the
section of snwll diameter wire is being thermocouple must naturally be small.
used (Fig. 28). Hut a thermocouple made with small wire
is far more susceptible to the problems of
If the thermocouple wire is small, its contamination, annealing, strain and
series resistance Rs will be quite high and shunt impedance. 13 To minimize these
under extreme conditions Rt. << Rs This effects, thermocouple extension wire can
means that the thermocouple junction be used. Extension ·wire is commercially
will appear to be at Rt and the output will available wire primarily intended to cover
be proportional to T1 , not T2 . long distances between the measuring
thermocouple and the volt meter.
High temperatures have other
detrimental effects on thermocouple wire. Extension wire is made of metals
The impurities and chemicals within the having thermoelectric coefficients very
insulation can actually diffuse into the similar to a particular thermocouple type.
thermocouple metal and cause the It is generally larger in size so that its
dependence of voltage on temperature to series resistance does not become a factor
when traversing long distances. It can also
FIGURE 27. Leakage resistance. be pulled more readily through conduit
than very small thermocouple wire. It
Open generally is specified over a much lower
temperature range than premium grade
1- thermocouple ·wire. In addition to offering
a practical size advantage, extension wire
To digital volt meter is Jess expensive than standard
thermocouple wire, especially in the case
legend of platinum based thermocouples.

Rl"' leakage resistance Because the extension wire is specified
T ==electrical terminal over a narrower temperature range and is
more likely to receive mechanical stress,
FIGURE 28. Virtual junction. the temperature gradient across the
extension wire should be kept to a
R, R, minimum. This expedient, according to
the gradient theory, ensures that virtually
~·.---~Ar--~ none of the output signal will be affected
by the extension wire.
To digital
volt meter Noise

R, T, R, Line related noise as it pertains to the
data acquisition system has already been
legend discussed. The techniques of integration,
R1 ==leakage resistance (O) tree switching and isolation serve to
Rs == series resistance cancel most line related interference.
T ==electrical terminal Hroadband noise can be rejected with an
analog filter.

244 Infrared and Thermal Testing

The one type of noise the dat<i can an operator tell when the
acquisition system cannot reject is a direct thermocouple is producing erroneous
current offset caused by a direct current results? It is necessary to develop a
leakage current in the system. Although it reliable set of diagnostic procedures.
is less common to see direct current
leakage currents of magnitude sufficient Through diagnostic techniques, R.P.
to cause appreciable error1 the possibility Reed has developed an excellent system
of their presence should be noted and for detecting a faulty thermocouple and
prevented, especially if the thermocouple data channels. 15 Three components of this
\Vire is very small and the related series system are the event record, the zom' box
impedance is high. test and the thermocouple resistance
history.
Wire Calibration
Event Record. The first diagnostic is not a
Thermocouple wire is manufactured to a test at all but a recording of all pertinent
certain specification signifying its events that could even remotely affect the
conformance with thermocouple tables. measurements. Table 8 shows an example.
The specification can sometimes be
enhanced by calibrating the wire (testing Examination of the sample program
it at known temperatures). Consecutive listing reveals that measurand :tvf821 uses
pieces of. wire on a continuous spool will
generally track each other more closely a type J thermocouple and that the data
than the specified tolerance1 although
their output voltages may be slightly acquisition program interprets it as type .J.
removed from the center of the absolute But from the event record, apparently
specification. If the wire is calibrated in an thermocouple l'vf821 was changed to a
effort to improve its fundamental type K and the change was not entered
specifications, it becomes even more into the program. Although most
imperative that all of the aforementioned anomalies are not discovered this easily,
conditions be heeded to avoid the event record can provide valuable
decalibration. insight into the reason for an unexplained
change in a system measurement. This is
Documentation especially true in a system configured to
measure hundreds of data points.
It may seem incongruous to speak of
documentation as being a source of Zone Box Test
voltage measurement error but the fact is
that thermocouple systems, by their very The zone box is an isothermal terminal
ease of usc, invite a large number of data
points. The sheer multitude of data can block with a known temperature ust·d in
become unwieldy. \Vhen many data are
taken, there is an increased probability of place of an ice hath reference. If the
error from mislabeling of lines1 using the
wrong thermocouple curve etc. thermocouple is temporarily short

Because channel numbers invariably circuited directly at the zone box, the
change, data should be categorized by
measurand, not just by channel number. H system should read a temperature very
Information about any given measurand1
such as transducer type, output voltage, close to that of the zone box, that is, close
typical value and location can be
maintained in a data file. This can be to room temperature (Fig. 29).
done under personal computer control or
simply by filling out a preprinted form. If the thermocouple lead resistance is
No matter hmv the data are maintained,
the importance of a concise system much greater than the shunting
should not be underestimated1 especially
at the outset of a complex data gathering resistance, the copper wire shunt forces
project.
F = 0. In the normal unshorted case, T1 is
Diagnostics measured: ·

1-·fost of the sources of error mentioned (10) \f (( (T_t - '/~el )
arc aggravated by using the thermocouple
near its temperature limits. These But, for the functional test, the
conditions will be encountered terminals are shorted so that V = 0 V. The
infrequently in most applications. But indicated temperature "f1 is thus:
what about the situation where small
thermocouples are being used in a harsh (11) 0
atmosphere at high temperatures? How
TABLE 8. Example of event record.

Time Event

10:43 power failure
10:47 system power returned
11:05 changed M821 to type K thermocouple
13:51 new data acquisition program
16:07 M821 appears to be bad reading

Contact Sensors for Thermal Testing and Monitoring 245

Thus, for a digital volt meter reading of it is possible to deduce what has artually
\1 == 0, the system will indicate the zone_,
box temperature. First the temperature 11 happened (Fig. 31 ).
(forced to be different from Trer) is The resistance of the thermocouple will
observed; then the thermocouple is
shorted with a copper wire and it is naturally change with time as the
verHied that the system indicates the zone resistivity of the wire changes because of
varying temperatures. But a sudden
box temperature instead of 7J. change in resistance is an indication that
something is wrong. In this case, the
This simple test verifies that the resistance has dropped abruptly,
controller, scanner, volt meter and zone indicating that the insulation has failed,
box compensation are all operating effectively shortening the thermocouple
correctly. In fact, this simple procedure
tests everything but the thermocouple loop (Fig. 32).
wire itself. The new junction will measure

Thermocouple Resistance temperature T5, not T1. The resistance
measurement has given additional
A sudden change in the resistance of a information to help interpret the physical
thermocouple circuit can act as a warning phenomenon that has occurred. This
indicator. If resistance versus time is
plotted for each set of thermocouple FIGURE 30. Burning coal seam.
wires, a sudden resistance change can
immediately be spotted, which could be To data => r,
an indication of an open wire, a wire acquisition system
shorted because of insulation failure,
changes caused by vibration fatigue or T= 573 K
one of many other failure mechanisms.
(300 <c = 572 'F)
For example, assume the thermocouple
measurement shown in Fig. 30. For T= 1473 K
example, suppose it is desired to measure
the temperature profile of an (1200 oc"" 2192 "F)
underground seam of coal that has been
ignited. The wire passes through a high legend
temperature region, into a cooler region. T := temperature (K)
Suddenly, the temperature rises from
573 to 1473 K (300 octo 1200 oc; 572 °F rl == electrical terminal
to 2192 °F). Has the burning section of
the coal seam migrated to a different FIGURE 31. Thermocouple resistance versus
location or has the thermocouple time.
insulation failed, thus causing a short
circuit between the two wires at the point
of a hot spot?

H a continuous history of the
thermocouple wire resistance is available,

FIGURE 29. Shorting of thermocouples at terminals. ,,L_-----+----~

Time (relative scale)

Fe r,
Copper wire short
FIGURE 32. Cause of resistance change.
Cn

Volt meter l Cu

Zone box isothermal block

legend -~,,~,,

Cn "'constantan resistance alloy (50 to 65 percent copper, 35 to legend "" Short
50 percent nickel)
T, =temperature at terminal of short
Cu =copper T1 =-temperature at initial terminal
Fe= iron
T =electrical terminal
V = voltage (V)

246 Infrared and Thermal Testing

failure would not have been detected by a Summary
standard open thermocouple check.
In summary, 'the integnty of <1
Measuring Resistance thermocouple system may he improved
by following these precautions.
The checking of the thermocouple wire
resistance has been mentioned casually, as 1. Use the largest wire possible that will
if it were a straightforward measurement. not shunt heat <~.way from the
\o\'hen the thermocouple is producing a measurement area.
voltage, however, this voltage can cause a
large resistance measurement error. 2. If small wire is required, use it only in
Measuring the resistance of a the region of the measurement and
thermocouple is akin to measuring the use extension wire for the region with
internal resistance of a battery. A no temperature gradient.
technique known as offset compensated
olim measurement can solve the problem. 3. Avoid mechanical stress and
vibration, which could strain the
As the name implies, the data wires.
acquisition unit first measures the
thermocouple offset voltage ·without the 4. \A/hen using long thermocouple wires,
ohm current applied. Then the ohm use shielded, twisted pair extension
current source is switched on and the wire.
voltage across the resistance is again
measured. The instrument firmware 5. Avoid steep temperature gradients.
compensates for the offset voltage of the 6. Try to use the thermocouple wire well
thermocouple and calculates the actual
thermocouple source resistance. within its temperature rating.
7. Use an integrating analog-to*digital
Special Thermocouples
converter with high resolution and
Under extreme conditions, diagnostic good accuracy.
thermocouple circuit configurations can 8. Use isolated inputs ·with ample offset
even be used. Tip branched and leg capability.
branched thermocouples are four*wire 9. Use the proper sheathing material in
thermocouple circuits that allow hostile environments to protect the
redundant measurement of temperature, thermocouple wire.
noise voltage and resistance for checking 10. Use extension wire only at low
wire integrity (Fig. 33). Their respective temperatures and only in regions of
merits are discussed in detail elsewhere.~-'' smal1 gradients.
11. Keep an event log and a continuous
Only severe thermocouple applications record of thermocouple resistance.
require such extensive diagnostics but it is
comforting to know that there are
procedures that can be used to verify the
integrity of an important thermocouple
measurement.

FIGURE 33. Thermocouples: (a) leg branched

thermocouple; (b) tip branched

thermocouple.

(b)

Contact Sensors for Thermal Testing and Monitoring 247

PART 3. Resistance Temperature Detectors

Background !vfeyers' desjgn has been replaced by
another laboratory standard: the bird cage
The same year that Seebeck made his element proposed by Evans and Burns. 17
discovery about thermoelectricity, The platinum element remains largely
Humphrey Davy announced that the unsupported, which allows it to move
resistivity of metals showed a marked freely when expanded or contracted by
temperature dependence. Fifty years later, temperature variations. Strain induced
\Villiam Siemens recommended platinum resistance changes caused by time and
as the element in a resistance temperature are thus minimized and the
thermometer. Platinum is used to this day bird cage becomes the ultimate laboratory
as the primary element in·all high standard. Because of the unsupported
accuracy resistance thermometers. In fact, structure and subsequent susceptibility to
the platinum resistance temperature vibration, this configuration is too fragile
detector (PRTD) is used today as an for industrial environments.
interpolation standard from the triple
point of equilibrium hydrogen, 13.803 K Rugged Designs
(-259.347 'C ~ -434.824 'F), to the
freezing point of silver, 1234.930 K A more rugged construction technique is
(961.78 'C = 1763.20 'F). Platinum is shown in Hg. 35a. The platinum wire is
especially suited to this purpose, as it can hifilar wound on a glass or ceramic
withstand high temperatures while bobbin. The bifilar winding reduces the
maintaining excellent stability. A nohle effective enclosed area of the coil to
metal, it shows limited susceptibi1ity to minimize magnetic pickup and its related
contamination. noise. Once the ·wire is wound onto the
bobbin, the assembly is then scaled with a
The classical resistance temperature coating of molten glass. Sealing ensures
detector (RTD) construction using that the resistance temperature detector
platinum was proposed by C. H. Ivfeyers in will maintain its integrity under extreme
1932. 16 He wound a helical coil of
platinum on a crossed mica web and FIGURE 35. Resistance temperature detectors:
mounted the assembly inside a glass tube. (a) sealed bifilar winding; (b) helical;
This construction minimized strain on the (c) film.
wire while maximizing resistance
(Fig. 34). (a)

Although this construction produces a
very stable element, the thermal contact
between the platinum and the measured
point is poor and results in a slow thermal
response time. The fragility of the
structure limits its use mainly to that of a
laboratory standard.

FIGURE 34. Meyers resistance temperature (b)
detector construction.

(c)

248 Infrared and Thermal Testing

vibration but sealing also limits the longer than a platinum element but its
expansion of the platinum metal at high linearity and very low cost make it an
temperatures. Unless the coefficients of economical alternative. Its upper
expansion of the platinum and the temperature limit is only about 393 K
bobbin match perfectly, stress will be (120 oc = 248 °F).
placed on the wire as the temperature
changes, resulting in a strain induced The most common resistance
resistance change. This may result in a temperature detectors are made of either
permanent change in the resistance of the platinum1 nickel or nickel alloys. The
wire. economical nickel derivative wires are
used over a limited temperature range.
There are partially supported versions They are quite nonlinear and tend to drift
of the resistance temperature detector that with time. For measurement integrity,
offer a compromise between the bird cage platinum is the obvious choice.
approach and the sealed helix. One such
approach uses a platinum helix threaded Resistance Measurement
through a ceramic cylinder and affixed via
glass frit (Fig. 35b). These devices will The common values of resistance for a
maintain excellent stability in moderately platinum film resistance temperature
rugged vibrational applications.
detector range from 10 n for the bird
Metal Film Resistance
Temperature Detectors cage model to ±3 kn for the film
resistance temperature detector. The single
In the ne'\\'est construction technique, a most common value is 100 Qat 273 K
platinum or metal glass slurry film is (0 °C = 32 °F). A representative standard
deposited or screened onto a small flat temperature coefficient of platinum wire
ceramic substrate, is etched with a laser
trimming system and is sealed (Fig. 35c). is a = 0.00385 K-1• For a 100 n wire this
The film resistance temperature detector
offers substantial reduction in assembly corresponds to +0.385 .Q.K-1 at 273 K
time and has the further advantage of (0 oc = 32 °F). This value for a is actually
increased resistance for a given size. the average slope from 273 to 373 K (0 to
100 oc; 32 to 212 °F). The more
Because of the manufacturing chemically pure platinum wire used in
technology, the small size of the device platinum resistance standards has an a of
itself means it can respond quickly to step +0.00392 K-1 (where increment of
changes in temperature. Film resistance 1.0 K = 1.0 oc " 1.8 °F).
temperature detectors are less stable than
their wire wound counterparts but are Both the slope and the absolute value
more popular because of their decided are small numbers, especially when it is
advantages in size, production cost and considered that the measurement wires
ruggedness. leading to the sensor may contribute

±3 n. A small lead impedance can
contribute a significant error to

temperature measurement (Fig. 36).

Metals TABLE 9. Resistivities of common materials
in resistance temperature detectors.
All metals produce a positive change in
resistance for a positive change in Resistivity
temperature. This1 of course, is the main
function of a film resistance temperature Metal n.mm 2·m-1 (See Note~)
detector. As will soon be seen1 system
error is minimized when the nominal Gold 21.61 13.00
value of the film resistance temperature Silver 14.63 8.80
detector resistance is large. This implies a Copper 15.39 9.26
metal wire with a high resistivity. The Platinum 98.08
lower the resistivity of the metal, the Tungsten 49.87 59.00
more material must be used. Nickel 59.85 30.00
36.00
Table 9 lists the resistivities of common
resistance temperature detector materials. a. English unit is kiloohm circular milliinch per foot.

Because of their lower resistivities, gold FIGURE 36. Effect of lead resistance.
and silver are rarely used for film lead 5 n
resistance temperature detector elements.
Tungsten has a relatively high resistivity 0-----'\1\f'v----
but is reserved for very high temperature
applications because it is extremely brittle lead sn lOOU
and difficult to work. resistance
temperature
Copper is used occasionally as a detector
resistance temperature detector element.
Its low resistivity forces the element to be

Contact Sensors for Thermal Testing and Monitoring 249

A 10 Q lead impedance implies resistors are replaced by one reference
10 + 0.385 at 299 K (26 'C = 79 'F) error resistor. The digital volt meter measures
in measurement. Even tlw t(•mperature only the voltage dropped across the
coefficient ot the lead wire can contribute resistance temperature detector and is
a measurable error. The classical means for insensitive to the length of the lead wires
avoiding this problem has been a bridge (Fig. 39).
(Fig. 37).
The one disadvantage of using a
The bridge output voltage is an indirect four-wire resistor is that it needs one more
indication of the resistance temperature extension wire than the three-wire bridge.
detector resistance. The bridge requires This is a small price to pay for accmacy in
four connection wires, an external source temperature measurement.
and three resistors that have the same
temperature coefficient. To avoid Conversion of Resistance
subjecting all three of the bridge to Temperature
completion resistors to the same
temperature as the resistance temperature The resistance temperature detector is a
detector, a pair of extension wires more linear device than the thermocouple
separates the resistance temperature but it still requires curve fitting. The
detector from the bridge (Fig. 38a). Callendar-Van Dusen equation has been
used for years to approximate the
These extension wires recreate the resistance temperature detector curve: 19,20
initial problem: the impedance of the
extension wires affects tlle temperature Ro a [T - s(....'!.:__ - 1)(....'!.:__)
reading. This effect can be minimized by 100 100
using a three-wire bridge configuration
(Fig. 38b). ~ ( 1T00 - 1)[ 1T0"0]]

If wires A and B are perfectly matched FIGURE 38. Extension wires separate
in length, their impedance effects will resistance temperature detectors from
cancel because each is in an opposite leg bridge to avoid subjecting it to same
of the bridge. The third wire C acts as a temperature as three bridge completion
sense lead and carries no current. resistors: (a) impedance from bridge of two
extension wires affects temperature
The wheatstone bridge shown in measurement; (b) three-wire bridge
Fig. 38b creates a nonlinear relationship minimizes impedance effects.
between resistance change and bridge
output voltage change. This bridge (a)
compounds the already nonlinear
temperature resistance characteristic of f:
the resistance temperature detector by
requiring an additional equation to (b)
convert bridge output voltage to
equivalent resistance temperature detector f:
impedance.

Four-Wire Resistor

The technique of using a current source
along with a remotely sensed digital volt
meter alleviates many problems associated
with the bridge. Because no current flows
through the voltage sense leads, there is
no drop in these leads and thus no lead
resistance error in the measurement.

The output voltage read by the digital
volt meter is directly proportional to
resistance temperature detector resistance,
so only one conversion equation is
necessary. The three bridge completion

FIGURE 37. Wheatstone bridge. FIGURE 39. Four-wire measurement of resistance (H).

Dig'1tal volt meter tCurrent i + -<(- Current i = 0 100 u
resistance
+ Digital -<---- Current fo= 0 temperature
volt detector
meter

250 Infrared and Thermal Testing

where Rr =resi stance at t emperat ur e T·1 Self-Heating
Rn =re sistance at T = 273 3 2 °F);
K (0 oc = Unlike the thermocouple, the resistance
a= tt?nlpernture coefficient at T = 0 oc temperature detector is not self-powered.
(typically+ 3.92 x 10-3 Q.Q-I.K-1 where A current must be passed through the
device to provide a voltage that can be
increment of 1.0 K = 1.0 oc = 1.8 °F); measured. The current causes joule (il"R)
heating within the resistance temperature
S = 1.49 (typical value for 3.92 x JO<< detector, changing its temperature. This
self-heating appears as a measurement
platinum); ~ = 0 when T > 273 K error. Consequently, attention must be
paid to the magnitude of the
(0 oc =32 °1') and typically p =0.11 when measurement current supplied by the
T < 273 K (0 °C = 32 °F). ohm meter. A typical value for
self-heating error is 0.5 K·mV\T-1
The exact values for coefficients a, ~ (0.5 oc.mvv-1 = 0.9 °F·m\,Y-1) in free air.

and 0 are determined by testing the Obviously, a resistance temperature
detector immersed in a thermally
resistance temperature detector at four conductive medium vvill distribute its
joule heat to the medium and the error
temperatures and solving the resultant from self-heating will be smaller. The
same resistance temperature detector that
equations. This familiar equation was rises 1 K·m\'\'-1 i~1 free ~ir will rise only
0.1 K·m\-\'-1 m atr flmvmg at the rate of
replaced in 1968 by a twentieth order 1 m-s-I.s

polynomial to provide a more accurate To reduce self-heating errors use the
minimum ohm measurement c'urrent that
curve fit. will still give the required resolution and
use the largest resistance temperature
The plot of this equation shows the detector available that will still give good
response time. Obviously, there are
resistance temperature detector to be a compromises to he considered.

more linear device than the thermocouple Thermal Shunting

(Fig. 40). Thermal shunting is the act of altering the
measurement temperature by inserting a
Practical Precautions measurement transducer. Thermal
shunting is a problem more with
The same practical precautions that apply resistance temperature detectors than with
to thermocouples also apply to resistance thermocouples, as the physical bulk of a
temperature detectors, i.e., use shields and resistance temperature detector is greater
twi~ted pair wire, use proper sheathing, than that of a thermocouple.
avOid ~tress ?nd steep gradients, use large
extenston \\'Ire, keep good documentation Thermal Electromotive Force
and use an integrating digital multimeter.
In addition, the following precautions The platinum-to-copper connection that
should be observed. is made when the resistance temperature
detector is measured can cause a thermal
Construction offset voltage. The offset compensated
ohm technique can be used to eliminate
Because of its construction, the resiStance this effect. (See "!able 10.)
temperature detector is somewhat more
fragile than the thermocouple and
precautions must be taken to protect it.

FIGURE 40. Resistance temperature detector more linear than
thermocouple.

16

r~-f-f-f-.f-.-f---i-!-

12-

~-r-~~-+-+~-~ 0.390
/ v - --s 1- I_...... -+---+---!":::- 1=-- 0.344
o.293

4 V+-+---1,---+-- -- TAB.LE 10. Comparison of small and large
resistance temperature detector.

- .. ... - -t---t-1---f-- !---!----j Performance Small large
Parameter Device Device
- - · -· --"--+---"---+·-
Response time fast stow
0 473 673 873 1073 Thermal shunting low poor
(200) (400) (600) (800) Self-heating error high low

)3921 [752] [11721 [14721

legend

type S thermocouple
- - - "' platinum resistance temperature detector

Contact Sensors for Thermal Testing and Monitoring 251

PART 4. Thermistors

Like the resistance temperature detector, (14) T 1
the thermistor is also a temperature
sensitive resistor. Although the .' 1
thermocouple is the most versatile A + H(ln R) + C(ln R)'
temperature transducer and the platinum
resistance temperature detector is the where Tis temperature (kelvin), R is
most stable, the ·word that best describes thermistor resistance (ohm) and A, Band
the thermistor is sensitil'e. Of the three C are curve fitting constants.
major categories of sensors, the thermistor
exhibits by far the largest parameter A, B and C are found by selecting three
change with temperature. data points on the published data curve
and solving the three simultaneous
Thermistors are generally composed of equations. \!\'hen the data points are
semiconductor materials. Although chosen to span no more than 100 K
positive temperature coefficient units are (100 °C == 180 °F) within the nominal
available, most thermistors have a center of the thermistor's temperature
negative temperature coefficient; that is, range, this equation approaches a rather
their resistance decreases with increasing
temperature. The negative temperature remarkable ±0.020 K (±0.020 oc ~
coefficient can be as large as several
percent per kelvin, allowing the ±0.036 °F) curve fit.
thermistor circuit to detect minute Somewhat faster computer execution
temperature changes that could not be
observed with a resistance temperature time is achieved through a simpler
detector or thermocouple circuit. equation:

The tradeoff for this increased (15) T ~
sensitivity is loss of linearity. The
thermistor is an extremely nonlinear [(In R)-Aj- 1l
device highly dependent on process
parameters. Consequently, manufacturers ·where A and n are again found by
have not standardized thermistor curves
to the extent that resistance temperature selecting two (H,T) data points and
detector and thermocouple curves have solving the two resultant simultaneous
been standardized (l'ig. 41). equations. This equation must be applied
over a narrower temperature range to
An individual thermistor curve can be approach the accuracy of the
very closely approximated through the Steinhart~Har~ equation. Microcontrollers
Steinhart~Hart equation:21 are well suited for thermistor interfacing.

FIGURE 41. Plots of voltage or resistance Measurement
versus temperature.
The high resistivity of the thermistor
Thermistor affords it a distinct measurement
advantage. The four~wire resistance
Resistance measurement may not be required as it is
temperature ·with resistance temperature detectors. For
detector example, a common thermistor value is

Thermocouple 5 kQ at 298 K (25 oc ~ 77 °F). With a

Temperature T (relative scale) typical Tc of 4 ·percent per kelvin, a

measurement lead resistance of 10 n

produces only 0.05 K (0.0.1 oc ~ 0.09 °F)

error. This error is a factor of 500 times
less than the equivalent resistance
temperature detector error.

Disadvantages

Because they are semiconductors,
thermistors are more susceptible to
permanent decalibration at high
temperatures than are resistance
temperature detectors or thermocouples.

252 Infrared and Thermal Testing

Thermistors are generally limited to a few
hundred kelvin and manufacturers warn
that extended exposures even well below
maximum operating limits will cause the
thermistor to drift out of its specified
tolerance.

Thermistors can be made very small,
·which means they will respond quickly to
temperature changes. Their small thermal
mass makes them especiaJiy susceptible to
self-heating errors.

Thermistors are a good deal more
fragile than resistance temperature
detectors or thermocouples and must be
carefully mounted to avoid crushing or
bond separation.

Contact Sensors for Thermal Testing and Monitoring 253

PART 5. Integrated Circuit Sensors and Data
Processing1

Monolithk linear A data acquisition and control system
Temperature Transducers provides high speed temperature
measurements where point count is high.
An innovation in thermometry is the \'\'hen configured for temperature
integrated circuit temperature transducer. measurements, it offers the follo\'·.'ing
These are available in both voltage and capabilities.
current output configurations. Both
supply an output linearly proportional to A scanning analog-to-digital convcrter
absolute temperature. Typical values are with 64 channels can be configured for
1 pA·K-1 and 10 mV·K-1 (Fig. 42). temperature measurements_ Scanning rate
is 56 000 channels per second. Several
Some integrated sensors represent hundred chmmel configurations are
temperature in a digital output format possible with multiple modules. A signal
that can be read directly by a conditioning plug-on rides pigb')'back on
microprocessor. the analog-to-digital converter module
and provides input for thermocouples.. An
Except that they offer a very linear external terminal block has a built-in
output with temperature, these integrated thermocouple reference junction and
circuit sensors share all the disadvantages terminal connections to the application.
of thermistors. They are semiconductor Four-wire resistor plug-on offers offset
devices and thus have a limited compensation for resistance ternperature
temperature range. The same problems of detector and thermistor measurements. A
self-heating and fragility are evident and built-in engineerlng unit offers
they require an external power source.
FIGURE 42. Integrated circuit sensors, also
These devices provide a convenient known as monolithic linear temperature
way to produce an output easy to read sensors: (a) sensor of current i; (b) voltage
and proportional to temperature. Such a sensor 1 K == 1 °C == 1.8 °F.
need arises in thermocouple reference
junction hardware and in fact these (a) +
devices are increasingly used for
thermocouple compensation.

Measurement System yI i"'-l !1A¥·1

Data processing systems are available for 10 kn lo digital volt meter
implementing thermocouple reference
junctions. Integrated circuit sensors can (b) +
be used to perform software thermocouple
compensation. Todigit<JI
volt meter
Conversion routines built into

firmware accept type B, E, J, K, N, R, S and

T thermocouples; 2.2, S and 10 kQ
thermistors; and a wide range of
resistance temperature detectors.

Data acquisition systems can
incorporate desirable features mentioned
above: internal digital multimeter;
integrated analog-to-digital conversion for
noise rejection; low thermal scanning
with built-in thermocouple reference
junctions; open thermocouple check;
built-in thermocouple, thermistor and
resistance temperature detector
linearization routines with conformity to
the International Temperature Scale;4
four-wire ohm function with offset
compensation; and isolated inputs that
float up to 300 V.

254 Infrared and Thermal Testing

conversions for thermocouple, thermistor
and resistance temperature detector
measurements.

Such a data processing system offers
more than temperature measurements. It
provides a wide variety of
analog~to·digital input and output
capability required by designers of
electromechanical products and by
manufacturers needing stringent
monitoring and control of physical
processes.

Instrumentation alternatives range
from small compact systems for portable
or remote operation to high speed
scanning systems that also provide
advanced control and analysis
capabilities.

Summary

Reliable temperature measurements
require a great deal of care in both
selecting and using the transducer, as well
as choosing the right measurement
system. \'\'ith proper precautions observed
for self-heating, thermal shunting,
transducer decalibration, specifications
and noise reduction, even the most
complex temperature monitoring project
will produce repeatable, reliable data. Data
acquisition systems assume a great deal of
this burden, allowing the operator to
concentrate on interpretation of test
results.

Contact Sensors for Thermal Testing and Monitoring 255

PART 6. liquid Crystals

More than a century ago, the discovery of Nematic phases are thus typified by a
two unusual chemical compounds22,Z.1 certain directional order hut lacking a
exhibiting unusual behavior between the layered structure, Nematic liquid cryst.:1ls,
crystalline solid and isotropic liquid state, in particular the twisted nematics formed
was to change the understanding of the by sandwiclling the liquid crystals
possible states of matter. Discovered between plates of glass l\'ith a special
around the time of the birth and surface finish, are very common in liquid
development of some of the world's crystal displays of watches, calculators
biggest chemical industries, this new field, .:md similar devices.
liquid crystals (LC), was to undergo a
long, slow gestation period, culminating Cholesteric or Chiral Nematic
over the last 30 years of the twentieth Phase
century in an amazing range of
inventions and industrial applications. The physical properties of cholesteric, or
Even the membranes in the human body clliraluematk~ liquid crystals, are in
depend on liquid crystals. The catalog of almost all aspects similar to nematics,
known liquid crystals has grown to over a except that the director axis assumes a
hundied thousand new substances,2'i helical form of finite pitch equal to the
associated ·with remarkable discoveries distance it takes for the director to rotate
and inventions in different branches of one full turn in the helix. A byproduct of
the material and life sciences and in the helical structure of the chiral nematic
engineering and industry.25-26 As pointed phase is its ability to reflect light
out by Collings and Patel,27 the sheer selectively at wavelengths equal to the
volume of material published on liquid pitch length. A color wlll be reflected
crystals is now so large that an exhaustive when the pitch is equal to the
summary and bibliography is virtually corresponding wavelength of light in the
impossible. visible spectrum. Temperature changes
modify the director orientation and pitch
Thermotropic liquid length, resulting in an alteration of the
Crystals wavelength of the reflected light, or color,
with temperature, thus providing a basis
Thermotropic liquid crystals are among for thermographic measurements.
the most widely used liquid crystals. They
exhibit various liquid phases as a function Smectic Phase
of temperature and their linear and
non1inear optical behavior has been The third group are the smectic
extensively studied.27·51 Although their mesophases. }..folecules in this state show
molecular structure can be complicated, a degree of positional order not present in
they are often represented as rigid rods. the nematic or chiral nematic phases,
The molecules interact with one another resulting in a more solid structure. Short
and, depending on the temperature and range, quasilong range and long range
their makeup, form distinctive ordered positional order all occur in smectic liquid
structured patterns, ranging over the crystals,
nematic, cholesteric or chiral nematic and
smectic rnesophases. A large number of smectic phases have
been discovered and classified, according
Nematic Phase to their molecular order and structural
symmetry properties (for example, smectic
Decreasing the temperature of a liquid A to smeclic K and smectic C* to smectic
crystal from the isotropic liquid state, in K*,27·52 The recent use of ferroelectric
which the molecules are randomly liquid crystals (also known as smcctic C*
positioned, changes the material to the liquid crystals) in providing time varying
nematic phase, where the molecules gain measurements of surface shear stress in
a certain amount of orientational order high speed wind tunnel testing is one
but no positional order. This orientational example of nonintrusive testing using
order permits definition of an average smeclic liquid crystals.sJ,S4
direction (director nxb:) of tl1e molecules.

256 Infrared and Thermal Testing

Liquid Crystal Practical Procedures in Liquid
Thermography
Crystal Thermography
By far the largest group of liquid crystals
A great deal of useful info_rmation is now
are the thermotropic liquid crystals readily accessible, on the mternet as well
as in publications, providing background
(TLCs), with over 78 000 compounds and references on types and formats of
thermochromic liquid crystals, 41 .47,48 and
registerect.24 From the early days, the practical procedures to follow. 49"51 •61 -h7

effect of temperature on the different Materials

mesophases was a factor of key interest. Thennochromic liquid crystals are
avili1ahle in several forms: (1) sprayable
As a result, it is not surprising that most microencapsulated liquid crystal coating
for solid surfaces In air or water,
of the work on liquid crystals in (2) coating systems for large area thermal
mapping in air, (3) microencapsulated
nonintrusive measurements or liquid crystal slurries for use as tracer
particles in flow field studies in aqueous
nondestructive testing have concentrated liquids, (4) polyester sheets coated with
microencapsulated liquid crystals for flat,
on temperature and heat transfer solid surfaces in air or water and
(5) unsealed liquid crystal compounds and
measurements. mixtures for microscopic evaluation of
small objects such as electronic
Early work suffered from the components.48

limitations of pure liquid crystals, which Unsealed compounds allow spatial
discrimination down to the micrometer
deteriorated rapidly and were particularly level. However, because of their inherent
oily form, pure thermotropic liquid
susceptible to degradation by crystals can be difficult to work with and
their thermal performance can degrade
contamination and ultraviolet light Many rapidly through contamination and
exposure to ultraviolet radi<ltion. On the
of these problems were eliminated by a other hand, microencapsulated liquid
crystals are protected by being enclosed in
microencapsulation process. a strong, thin transparent wall, forming
~vHcroencapsulation also possessed the small temperature sensing capsules of S to
10 pm in diameter. An tldvantage of this
added advantage of greatly reducing color medium is that different microcapsules
can be mixed together to provide
variation due to viewing angle. independent or overlapping regions of
color play, besides reducing dependency
A review29 of some of the work before on the lighting and viewing setup.

1975 includes applications of liquid The chemical makeup of a
thermotropic liquid crystal material fixes
crystals to check the following: _ its color temperature response. The
irregularities on bonded structures, 30-J2 manufacturers typically provide a
two-color temperature code, indie<lting
regions of overheating on electronic the starting temperature (el'mt
tempemture) and the red~to-blue start
equipment,:ll-33 flow blockages in heat bandwidth (W). For example, R35CSH'

exchangers,3~ crack detection on aircraft specifies a red (R) event temperature of
stwcture, 3~ the effectiveness of ·windshield
35 °C and a blue start, S "C higher, at
heater,33.J~ and thermal mapping in the
313 K (40 oc =- 104 "F). Thermochromic
medical fieldJS.:~o Over the next 15 years
liquid crystal formulations are available
many advances occurred in the tailoring with red event temperatures ranging from
243 K to 373 K (-30 octo +100 oc; -22 OF
of the liquid crystals, in _particular in to +212 "F); with narrow bandwidths
digital image processing.37•55 down to around O.S K (0.5 "C = 0.9 "F);
and with broad bandwidths up to 30 K
There was not only a movement from (30 °(: == 86 "F). This provides the user
·with a range of choices off the shelf but
qualitative visuaJizations to more reliable the manufacturers can also provide spec"Jal
and quantitative measurements3'H1 mixtures designed for a particular
application.
requiring suitable calibration techniques,

but also a demand to extend the work to

cover time dependent phenomena. The

latter highlighted the important question

of the thermal characteristics and the

time responses of various liquid crystal

materials. 40
Parsley41 summarized a number of

steady state and transient heat transfer

techniques, including studies of the

thermal performance of gas turbine

components,42,43 measurements of local

heat transfer coefficients on gas turbine

blades-14.45 and the response of

microencapsulated, chiral nematic, liquid

crystal films to surface temperature
changes.45,-J(, The experiments sh?w~d !he

response time of the thermotropic hqwd

crystal film to be no more than a few

millisecond. Liquid crystals developed for

surface shear stress measurements in

aeronautics are now also available with

response times in the submillisecond

rangeSJ-Oo and new ferroelectric liquid

crystals possess optical switching times
faster than a microsecond.27·52

Contact Sensors for Thermal Testing and Monitoring 257

Because thermochromic liquid crystal Wide Band Thermochromic liquid
thermography depends on assigning a Crystal Formulations
temperature to the color reflected from
thermotropic liquid crystal coated surface, An alternative to narrow band
care must be taken to avoid reflection of thermochromic liquid crystals is to use a
the light transmitted through the crystals, single wide band thermochromic liquid
by first coating the test surface with liquid crystals to map the isocolor or isotherm
crystal compatible, black backing paints. pattern of a surface from a single image.
In the case of liquid crystal sheets, a These wide band techniques are useful
nonreflecting backing material forms part when an object has large temperature
of the sheet, with the option of an variations and when accuracy and high
additional peelable, sticky back layer for spatial resolution are required. Practical
attaching the sheet directly to a flat test applications of wide band techniques
surface. include the investigation of surface
temperature distributions in such areas as
Qualitative Temperature gas turbine blade cooling, electronic
Visualization Techniques components and the study of boiling heat
transfer phenomena.
?vfany temperature applications only
require qualitative information. In this Farina61 has summarized the
case, the two-color temperature advantages and disadvantages of the two
descriptors of the thermochromic liquid systems: narrow band and ·wide band.
crystal material, combined with the color The advantages of the narrow band
response capabilities of human eyes, can thermochromic liquid crystal technique
readily provide a simple solution. are as follows.
Qualitative thermochromic liquid crystal
techniques are typically easy and 1. High accuracy is possible in both
inexpensive to implement and can absolute and relative temperature
provide high spatial resolution ·when measurements.
properly used in an application that
provides suitable optical access to the 2. Implementation is inexpensive and
thermochromic liquid crystal coated requires simple image processing
surface. A typical qualitative application systems.
might be a quick investigation of the
temperature response of electronic The disadvantages of the narrow band
components or electrical equipment to thermochromic liquid crystal technique
natural or forced convection and are as follows.
component orientation.61
1. Construction of an isotherm pattern
Quantitative Narrow Band can be tedious and time consuming.
Thermochromic Techniques
2. Full field capabilities of the
Most quantitative liquid crystal thermochromic liquid crystal coating
thermography applications have used are not used.
narrow band microencapsulated
thermochromic liquid crystal The advantages of the wide band
formulations or multievent temperature thermochromic liquid crystal technique
mixtures consisting of a number of are as follows.
narrow band thermochromic liquid
crystals, each having different event 1. It uses the thermochromic liquid
temperatures. Collected data from crystals bandwidth to map the entire
successive experiments at different surface isotherm pattern of a surface from a
temperatures facilitates construction of single image.
surface isotherm patterns using either
single or multievent formulations. Narrow 2. It is preferable in applications having
band thermochromic liquid crystal large temperature variations and
formulations permit (1) accurate requiring high spatial resolution.
verification of surface isotherms,
(2) relatively accurate surface temperature The disadvantages of the wide band
measurements using simple image thermochromic liquid crystal technique
processing systems and (3) higher are the following.
accuracy with some in situ color
temperature calibration. 1. Robust color temperature response
calibration is necessary for high
accuracy measurements.

2. It is more expensive to implement
because more sophisticated image
processing systems are required.

Color, lighting and Color
Temperature Calibration
Procedures

The interpretation of color is very difficult
and subjective.68-70 Basically color is
related to the wavelength of the reflected

258 Infrared and Thermal Testing

light. In human eyes the rods and cones the raw (red green blue) values would vary
decompose color into a combination of from 0 to 255, with their values then
the red, green and blue (RGB) primary riormalized by dividing by 255.
colors. This red green blue tristimulus
decomposition is regularly used in However, the red green blue
modern machine vision systems. representation is not the moS:t appropriate
model when it comes to calibrating color
A large selection of charge coupled versus temperature. A more convenient
device digital color cameras, both single and practical approach is to transform the
shot and video, are available with high basic red green blue data into the hue
pixel discrimination and good saturation intensity (HSl) color modeJM-7°
performance, providing red green blue
outputs. The latter basically represent the (Fig. 44):
color field in the form of three RGB
matrices defined by the outputs from the (16) R+G+B
light sensitive pixels (picture sensing 3
elements) of the camera. A wide range of
frame grabber cards also exist for seizing (17) s 3[min(R + G + JJ)j
this information and transferring it to the
computer for storage and analysis. In fact, R+ G+ B
many personal computers now come
equipped with direct digital camera input FIGURE 44. Hue saturation intensity (HSI):
ports and internal signal processing (a) color triangle; (b) color solid.'0
hardware and programs for treating the
data. (a) Blue

'Ib use color, some suitable color model Green
must first be chosen. Basically, a color
model specifies a three-dimensional (b)
coordinate system and a subspace within
that system where each color is I
represented by a point. The RGB color
model widely used in color monitors and
color video cameras uses a cartesian
coordinate system with the color subspace
consisting of a cube, with the values of
red, green and blue each ranging from 0
to I (Fig. 43). For example, using an
eight-bit (28 = 256) digital color camera

FIGURE 43. Red green blue (RGB) color cube. Points along the
main diagonal have gray values, from black at the origin to

white at point (1,1,1 ).7°

B

Blue_;!_(O_.o_,1_>______, Cyan

(1, 1.1) (0,1.0)

Magenta f.-----.-----c{. __........... G

White Green
I
I
I
I

I

I Gray scale
Black~<- ____ _

(1.0.0) / legend
/
/ C"' central color point
/ H =dimensionless number representing hue
I = dimensionless number representing intensity
/ P = two-dimensional point in color triangle
/

./.._/ _ _ _ _ _ _ _ _y

/Red Yellow

R

Contact Sensors for Thermal Testing and Monitoring 259

(18) H LR-G) I(R-:_ll) Summary of liquid Crystal
Test Procedure
COS-I -c~~-~2~-~-~
l:arina49/i3,6.'l has summarized the main
frR-G)2 +(11-B)(G-H) guidelines to follnw when using liquid
crystals for thermographic measurements.
where H =hue (dimensionless); J is
1. Determine the expected minimum
intensity (dimensionless); Pis color point and maximum temperatures of the
inside two-dimensional H and S color surface(s) under investigation.
triangle; Sis saturation (dimensionless);
and R, G and R respectively represent red, 2. Select the appropriate thermochrom ic
liquid crystal mixture for this
green and blue data from the digital temperature range ....,.. for exarnple,
camera. narrow band, multievent or \Vide
band.
Assuming that 1-/ is calculated in
degrees, replacing H by (360- H) if 3. Properly prepare and apply tlle black
background and thermochromic liquid
(JJ.J-') > (G·Y'), the hue is then crystal material to the test surface.
normalized to the standard range fO, 1J by
dividing by 360. Note that H is not 4. Provide adequate optical access to the
defined when S = 0. 70 surface and minimize unwanted
reflections.
A bright, stable light source is required.
5. Provide an online lighting and
\VIlite light sources that remove infrared vie\Ving arrangement if possible with
and ultraviolet radiation from their crossed polarizers.
output spectrum are preferable, to avoid
unwanted radiant heating of the test 6. lVfinimize infrared and ultraviolet
radiation to the test surface.
surface and deterioration of the liquid
crystals from ultraviolet light. 7. Calibrate the color versus temperature
response (for example, hue versus
If possible, color temperature temperature) of the thermochromic
calibrations should be carried out with liquid crystal and imaging system if
preca1ibrated secondary standards, such as quantitative measurements are
thennocouplcs or thermistors, mounted required.
on the test plate or test object. It is
important to ensure consistent light 8. Image or visualize the active
source settings and lighting viewing thermochromic liquid crystal test
arrangements between calibration and surface.
actual testing to minimize errors. A
coaligned primary lighting viewing 9. Oetennine the surface temperature
system, without any background or distribution using the appropriate
secondary lighting, is recommended, color versus temperature calibration.
whenever possible. 63•65
Conclusions
A typical calibration procedure using
hue versus temperature is described in The widespread application of liquid
detail in the Jiterature.fi6 A thermistor was crystals is relatively modern (3ince 1970)
first calibrated against a platinum and depends to a great extent on
resistance thermometer in a constant developments and requirements in
temperature oven. The thermistor was completely different branches of science
then mounted on a heated copper block and engineering, such as computers,
with the liquid crystal coatings optics and chemistry. The great increase
illuminated by white lights with in the number, variety and knowlt>dge of
ultraviolet filters. Hue-versus~temperature liquid crystal compounds, together with
calibrations were carried out using a the rapid emergence of new technologies
charge coupled device camera with a red such as microelectromechanical
green blue frame grabber and image mechanisms and microelectrooptical
processing sofhvare. Figures 45 and 4666 devices, can facilitate the integration of
illustrate respectively three typical and liquid crystals into many monitoring and
virtually identical initial calibrations and measurement devices in the field of
a subsequent final calibration after three temperature measurement.
weeks of testing. The calibrations exhibit
differences of two percent or less of the
temperature h<HHhvidth of the liquid
crystals.

260 Infrared and Thermal Testing

fiGURE 45. Three identical initial temperature versus hue calibrations for runs 9, 10 and 11.ti6

r313 (40) {104] ----,----

311 (38) {100] I '

j 1I ---

I

E ----r---

E 309 (36) (97] - - - - "

l1' 1

0 '
I
307 (34) [93) 1 - - - - - t - - - -

E I
~

305 (32) (90]

303 (30) (86]
0 0.1 0.2 03 OA 0.5 0.6

Hue (relative unit)

FIGURE 46, Calibration of temperature versus hue at start (run 11) and end (run 15) of test
program. 66

313 (40) (104] f- -! -- - - - 1 - ------~---- I. ------~
i I
1I i
I I
II
- - - - - t -311 (38) [100) C"i i
"'
1
.. II - - - - -'r - - -
I
i
"' 309 (36) (97] I

'-" !

I
I

~~ 307 (34) {93) I

~ I

0 ·I ~----~---
OL
0.2 0.3 OA
E
~ Hue (relative unit)

305 (32) (90]

303 (30) (86] 0.1 ___j
0
0.5 0.6
legend
- n· = run 11

~=-runl5

Contact Sensors for Thermal Testing and Monitoring 261

PART 7. Media with Calibrated Melting Points

Temperature Indicating Temperature ratings are in increments
Media as small as 3.4 K (3.4 "C ~ 6.1 "F) but
increments ranging from 14 K (14 oc =
Certain materials are used by industry to 25 "F) to 28 K (28 "C ~ SO "F) are typically
indicate temperature according to their used for welding applications. For most
predictable melting points. These applications, a jump of 28 K (28 oc =
temperature indicating materials are FIGURE 47. Temperature indicating materials:
available in several media - typically as (a) lacquer; (b) pellets; (c) sticks of differing
lacquer1 pellets or sticks (Fig. 47). temperature sensitivity.
(a)
The lacquer is brushed on before
welding starts and is useful on highly (b)
polished surfaces or for making large
marks vielved at a distance. (c)

Heat indicating pellets, about the size
and shape of an aspirin1 have greater mass
than stick or lacquer marks (see Fig. 47b).
Pe1lets are sometimes selected for large,
heavy pieces requiring prolonged heating
- applications where stick or lacquer
marks could fade with time.

A temperature indicating stick (chalk or
crayon) is typically made of materials
\'l'ith calibrated melting points and
temperature measuring accuracies to
±1 percent.

Indicators are available in closely
spaced increments over a range from
311 K (38 "C ~ 100 "F) to 1643 K
(1370 "C ~ 2500 "1').

Stick Medium Procedure

The work piece to be tested is marked
with the stick. VVhen the work piece
attains the predetermined melting point
of the indicator mark, the mark instantly
liquefies, notifying the observer that the
work piece has reached that temperature.

Premarking with a stick is not practical
under certain circumstances- ·when a
heating period is prolonged a highly
polished surface does not readily accept a
mark or the marked material gradually
absorbs the liquid phase of the indicator.
In such instances, the operator frequently
marks the work piece with the stick. The
desired temperaturr is noted ·when the
stick ceases to make dry marks and begins
to leave a liquid smear. A similar
procedure can be used to indicate
temperature during a cooling cycle. But a
melted mark, on cooling, will not solidify
at exactly the same temperature at which
it melted, so solidification of a melted
indicator mark cannot be relied on for
temperature indication.

262 Infrared and Thermal Testing

50 °F) to 56 K (56 oc = I 00 °F) and a range Marking materials tised on austenitic
of sticks up to 923 K (650 °C = 1200 °F) stainless steels typically have a certified
are usually adequate. analysis that meets the following specified
maximum amounts of detrimental
Temperature indicating sticks were contaminants: (1) inorganic halogen
developed in America by a metallurgist content less than 200 pg-g-1; (2) halogen
working on submarine hulls in the 1930s. (inorganic and organic) content Jess than
At the lime, preheat was measured with one percent by weight; (3) sulfur content
so-called melting point standards, less than one percent by weight measured
granules of substances with known in accordance with ASTM D 129;71 and
melting points used to calibrate heat (4) total content of lm\' melting point
sensing instruments. The engineer used metal (lead, bismuth, zinc, mercury,
the granules directly, spreading them on antimony and tin) Jess than 200 pg·g-1 by
the preheated metal and using their melt weight and no ii1dividual metal content
as a signal to proceed with welding. greater than 50 pg·g-1• The certification
typically indicates the techniques and
The melting point granules were next accuracy of analysis and the name of the
formed into sticks held together ·with testing laboratory.
organic binders. Different temperature
ratings were added and some refinements Applications for
have been made but the principle of Temperature Indicators
indicators has remained unchanged. The
sticks make physical contact with the Temperature indicators can be used for a
heated test object, reach thermal variety of applications, especially in
equilibrium rapidly and do not conduct process control: preheat temperature tests
heat away from the test surface. and in annealing and stress relieving
procedures, hardfacing, overlaying for
For temperature ratings less than 613 K corrosion resistance, flame cutting, flame
(340 °C :::: 644 °F), indicator marks can conditioning, heat treating, pipe bending,
usually be removed with water or alcohol. shearing of bar steel, straightening
For ratings above 613 K (340 oc = 644 oF), hardened parts, shrink fitting, brazing,
water is preferred. If the mark has been soldering and nonferrous fabrication. The
heated well above the rated temperature indicators can help find hot spots in
and has become charred, abrasion may be insulation and engines, help monitor
needed for complete removal. temperatures in curing and bonding
operations and help check pyrometric
Certification of calibration.
Temperature Indicators
Tests of Railway Bearings
Temperature indicating sticks are mixtures
of organic and inorganic compounds. The Bearing breakdown can be detected by
purity of the source materials directly using fluorescent temperature indicating
affects the accuracy of the predicted pellets as heat sensors for inboard journal
melting point. There is the possibility of boxes. The pellets are inserted in a
contamination with trace quantities of specia1ly fabricated stainless stec1 holder
other elements, which may be detrimental that contains two pellets. The holder is
to the accuracy of the indicator. In some inserted into the hollow axle of each rail
cases, low melting point materials (lead, car with an insertion tool. The tool has a
tin, sulfur, halogenated compounds) may mechanical stop to ensure that the holder
be undesirable for the welding procedure. is located at a predetermined depth. This
permits proper monitoring of journal box
Most manufacturers can provide operating temperatures.
certification supported by analyses of
typical batches; Documentation indicates Once a specified temperature is
which temperature ratings may contain exceeded, in this case 373 K (100 °C =
contaminants that can be avoided by the 212 °F), the pellets melt and fluw
user. completely out of the holder. The
fluorescent material is easy to detect and
In some critical applications (nuclear clearly indicates that excessive heat has
fabrication, aircraft assembly), actual been conducted 'from the bearing to the
chemical analysis of the specific lot axle.
number of the temperature indicators
may be required. If the customer supplies Verifying Oven Temperatures
a written certification requirement listing
the compounds to be tested for, most Technicians can determine if self-cleaning
manufacturers will send lot numbered ovens reach the proper cleaning
samples for laboratory analysis. The temperature using pellets with
customer is usually expected to pay precalibrated melting points at 723 K
laboratory charges for such specialized
requirements.

Contact Sensors for Thermal Testing and Monitoring 263

(450 "C = 842 "F). The pellets are placed sealing tape used in production. A visual
on a flat piece of aluminum foil situated test of each seam after sealing indicates
on the oven's center rack (see Fig. 48). The whether the seam temperature was ·within
cleaning cycle iS activated and as the the required range, allowing visual
verification of conditions for all dielectric
temperature reaches 723 K (450 oc = seams.

842 "F), the peilets begin to melt. Precise Postforming Heat Control
\-\1hen the cleaning cycle is completed
Temperature indicating materials are
and the oven has cooled, the pellets are incorporated into many industrial
examined - complete melting of the applications where- an indication is
tablet verifies that the nominal cleaning needed to show that a critical temperature
temperature has been achieved. has or has not been reached. A phase
changing fusible liquid is used to indicate
Process Control of Ceramic optimum postforming temperatures when
lnsulat'ron bending decorative laminate for the
contoured edges of countertops, desks,
A gas tight seal is needed to prevent tables and other surfaces (Fig. 49).
leakage of combustion gases through the
glass portion of a spark plug. lb obtain Postforming is the process of bending a
optimum fusion properties, it is important flat sheet of laminate around a radiused
to know and control the temperature core material (particle board, plywood or
inside the ceramic insulator and this can fiber board). The process is typically done
be done using a temperature indicating after controlled heating monitored with
pellet. Sample insulators are loaded with temperature sensitive liquids. Postfonning
pellets and processed with production can be a manual or mechanical operation.
parts. Information obtained from Hand postforming is used for unusual
analyzing the samples is used to adjust configurations or limited quantity
furnace conveyor speed and temperature. production; mechanical postforming is
used for high quantity production. Both
Monitoring Fabric Seam techniques need a heat source, prepared
Temperature cores, postforming grades of decorative
laminate, pressuring guides and evenly
In the making of specialized doth applied pressure.
(protective clotlling, aerostat balloons),
seam integrity iS an important A core is prepared by first shaping the
manufacturing function. A good edges to be laminated. The core and
radiofrequency seal can be achieved on a laminate are evenly coated with a contact
given fabric substrate only within a
specific temperature range, determined by FIGURE 49. Postforming of laminate around
the minimum temperature needed to radiused core.
ensure a complete seal and the maximum
temperature possible before material
degradation. Constant temperature
control and verification are required.

This can be achieved using temperature
sensitive strips (one for the upper limit,
one for the lower limit) applied to the

FIGURE 48. Pellets used to verify oven
temperatures over 723 K (450 "C = 842 "F).

264 Infrared and Thermal Testing

adhesive, preferably a spray. The laminate that the correct temperature for coating
is positioned and registered with the core, has been reached.
allowing the laminate to overhang the
radius. Postforming grades of decorative Preheating before Welding
laminate are formable between
temperatures of 429 K (156 oc = 313 oF) Heating to the proper temperature before
and 436 K (163 oc = 325 °F). welding lessens the danger of crack
formation and shrinkage stresses in many
A popular example of hand metals. Hard zones near the weld are
postfonning is the 180 degree edge wrap. reduced and lessen the possibility of
In this example, radiant heat is applied to distortion. Preheating also helps diffuse
the decorative surface of the laminate hydrogen from steel and helps reduce the
with the work supported over the heat. To likelihood of subsequent hydrogen
determine the proper postforming inclusions.
temperature, the temperature indicating
liquid is painted in stripes onto the The need for preheating increases with
laminate. \'\'hen the liquid changes from a the mass of the material being \Yelded. It
dry (matte) to a wet (melted) appearance, is most useful for the thkk, heavy
the assembly is wiped into the cavity of a weldments used in bridge construction,
fixture to form the 180 degree radius. The shipbuilding, pipelines and pressure
fixture is a U channel made by two boards vessels. Preheating is also recommended
attached to a base. The dimenSion of the for (1) welding done at or below 25.1 K
U channel is the thickness of the core plus (-18 oc = 0 °F), (2) when the electrode is a
the thickness of the laminate, allowing small diameter, (3) when the joined pieces
about 0.5 mm (0.02 in.) clearance. are of different masses, (4) when the
joined pieces are of complex cross section
Another example of handforming is and (S) for welding of high carbon or
known as a full wrap. In this application, manganese steels.
the core is positioned over radiant heaters
with temperature indicating stripes The most common use for temperature
painted on the adhesive in the area of the indicators is the measurement of preheat,
radius. When the melt indicates forming postheat and interpass temperatures for
temperature has been reached, the welding. In a typical application, the
assembly is moved back onto a flat welder marks the test surface \Vith an
supporting surface. The wrapping action indicating stick of a specific temperature
uses the flat surface as a pressure point. rating (Fig. 47c). \'\'hen the mark changes
phase (melts), the material has reached
An example of mechanical postforming the correct temperature and is ready for
is the roll forming machine. Radiant welding. It is important for the user to
heaters are located above an assembly understand that change of color has no
supported by a moving carrier. VVhen the significance; only the actual melting of
forming temperature has been reached, the mark should be considered.
slanted forming bars wipe the laminate
over the radius. After the laminate has Oxyacetylene equipment cannot be
been formed, a succession of rollers used for welding or cutting of high
maintains pressure until the assembly has strength steels used in automotive
cooled. In this application, temperature components because too much heat can
sensitive liquid is painted onto the reduce their structural strength. However,
laminate to verify that the dwell time in some instances an oxyacetylene torch
under heat has been sufficient for may he used if the critical temperature of
reaching forming temperature. 1033 K (760 oc = 1400 °F) for high
strength steel is not exceeded.
Pipeline Coatings
VVhen preheat temperatures are 643 K
Epoxy powders are specially formulated to (3 70 °C = 700 °F) or when heating is
enhance corrosionproof resistance of prolonged, an indicating mark could
utility pipe: that is, pipe usually buried evaporate or cOuld be absorbed by the test
underground, where it is subject to widely material. Under these conditions, marks
varying pipeline operating conditions. should be added periodically during
Intimately bonded to the pipe, the heating. \'\'hen the rated temperature i:c.
bonded epoxy is unaffected by widely reached, the stick leaves a liquid streak
varying soil compaction, moisture instead of a dry mark and ·welding can
penetration, fungus attack, soil acids and begin.
chemical degradation.
To ensure accurate temperature
To achieve a long lasting bond of indication with no override, two or more
epoxy coating to metal pipe, the pipe indicators can be used to alert the
must be preheated very carefully to the operator that the test object is
recommended preheat of 273 K (0 °C = approaching the correct temperature.
32 °F). A spot on the pipe needs to be \o\1hen a range of recommended preheat
touched with the stick; its melting shows temperatures is given, several indicators
might be appropriate. For example,
carbon molybdenum steel should be

Contact Sensors for Thermal Testing and Monitoring 265

preheated to between 366 K (93 oc =
200 °F) and 478 K (205 oc = 400 °F). A
bundle of indicators with ratings at 366,
393, 448 and 478 K ('J.l, 120, 150, 175
and 205 °C; or 200, 250, 300, 350 and
400 °F) might be useful for determining
how much of the test object is within the
preheat temperature range.

266 Infrared and Thermal Testing

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270 Infrared and Thermal Testing

CHAPTER

Equipment for Infrared
and Thermal Testing

Herbert Kaplan, Honeyhill Technical Company,
Norwalk, Connecticut (Parts 1 to 4)

Leonard J. Bonnell, Vipera Systems, Incorporated,

Huntingdon Valley, Pennsylvania (Part 5)

Thomas J. Brukilacchio, Innovations in Optics,

Incorporated, Woburn, Massachusetts (Part 5)
Dennis C. Leiner, Light House Imaging, Peterborough,
New Hampshire (Part 5)

The United States government retains a nonexclusive right to reproduce Part 5.

PART 1. Infrared and Thermal Instrumentation

Equipment for temperature measurement Thermochromic Liquid Crystals
and thermography includes contacting as
well as noncontacting devices. Contacting Thermochronlic liquid crystals (also called
devices for temperature measurement cholesterol crystals) change color lvith
include thermopiles, thermocouples, temperature. Coatings made of liquid
liquid thermometers, gas expansion crystals arc commonly used as
devices (bourcton gas thermometers), temperature threshold indicators.
liquid crystals (cholesterol crystals), heat Depending on the mixture, a coating
flux indicators and fiber optic sensors. applied to a surface will change color
Aside from some specialized instruments, predictably when the surface exceeds a
the vast majority of noncontacting threshold temperature. The color change
temperature measurement devices are may be reversible or irreversible, and the
infrared radiation sensing instruments sensing range for most mixtures is limited
and systems. to a narrow temperature span. Typically, a
set of liquid crystal markers provides a
Infrared sensing instruments and selection of transition temperatures. This
systems are divided into point sensors allows the user to select the appropriate
(radiation thermometers), line scanners marker for the desired temperature.
and thermal imagers.
Thermocouples and Thermopiles
This chapter begins with a review of
contacting thermal measurement Thermocouples are contact temperature
instruments and a discussion of the basic sensors based on the tllermoelectric effixt,
configurations of infrared sensing and or see/Jeck effect. Thomas Seebeck
imaging instruments. This is followed by a discovered that, vvhen two dissimilar
discussion of performance parameters metals are joined at both ends and these
and, finally, descriptions of currently ends are at different temperatures, a
available commercial thermal sensing and predictable direct current will flow
imaging equipment, thermographic irnage through the circuit. The thermoelectric
processing software and image hard copy coefficient determines the relationship
recording accessories. 1 between this current and the temperature
difference between the two junctions.
Contacting Thermal This coefficient is known for each type of
Measuring Devices thermocouple.

The most commonly used contacting To configure a thermometer, the circuit
devices include bimetallic thermometers, is broken and the open circuit voltage h
thermochromic liquid cryStals, measured by a volt meter. One of the two
thermocouples, resistance thermometers junctions is then held at a reference
(R'I'Ds), thermistors and heat flux temperature, such as an ice bath, and the
indicators. These devices are discussed voltage is calibrated to indicate the
briefly here and in more detail in a temperature of the other junction, which
separate chapter. then becomes the temperature sensing
junction.
Bimetallic Thermometers
'fhermopiles are banks of
Bimetallic thermometers are sensors thermocouples connected in parallel or in
constructed of dissimilar metallic strips series to increase the output gradient. The
bonded together. Typically, different iron reference temperature is important
nickel alloys are used. The strips differ in because of the thermocouples' nonlinear
temperature coefficient of expansion such response.
that temperature changes result in
predictable bending of the assembly. Resistance Thermometers
Arranged in a spiral or helical
configuration, one end of the bimetallic Rcsi:;;tance temperature detectors (RTDs)
element is fixed and the other end is are contact sensors that measure
attached to a pointer. Properly calibrated, temperature by a prcdictt1blc change in
tile angular position of the pointer can he resistance as a function of temperature.
made to indicate temperature on a scale. Platinum is Uw most popular resistance
temperature detector material because of
its excellent stability and its linecu

272 Infrared and Thermal Testing

response to temperature change. Other device. Transient heat flux can be related
materials used include nickel, copper, to the transient thermopile output and to
tungsten and iridium. In operation, the the geometry of the device.
resistance temperature detector may be
placed in a bridge circuit such that the Optical Pyrometers
bridge output voltage is a measure of the
resistance and hence the temperature at Optical pyrometers include brightness
the resistance temperature detector. A pyrometers and infrared pyrometers.
more accurate measurement may be Infrared pyrometers are also called
achieved by using a constant current infrared radiation thermometers. V<iriou:-.
source and a digital volt meter (DVM), types are discussed else\vhere.
such that the digital volt meter reading is
proportional to the resistance temperature Brightness pyrometers arc also called
detector resistance and hence the matching pyromctcrs. They incorporate a
temperature at the resistance temperature calibrated light source (lamp) powered by
detector. a calibrated current supply. Looking
through a viewer, the operator matches
Thermistors are also sensors that the brightness of the target to be
measure temperature by a predictable measured with the brightness of the
change in resistance as a function of calibrated lamp. The adjustment control h
temperature. Thermistors are made of calibrated in temperature units, such that
semiconductor materials. \'\1hereas when the brightnesses are matched, the
resistance temperature detectors are low control indicates the temperature of the
impedance devices, thermistors are high target to be measured.
impedance devices. Thermistors are
typically more sensitive to temperature Basic Configurations of
changes than resistance temperature Infrared Radiation Sensing
detectors are but thermistors are not as and Imaging Instruments
stable.
In terms of configuration and operation,
Heat Flux Indicators most thermal imagers are considered to be
extensions of radiation thennorneters or
Heat flux indicators are heat flow meters radiation thermometers plus scanning
and arc used to measure rates in optics. The performance parameters of
conduction, convection, radiation and thermal imagers arc extensions of the
phase change systems such as building performance parameters of radiation
walls, boiler tubes and air conditioning thermometers. To aid comprehension, the
ducts. A typical heat flux indicator basic measurement problem is discu~sed
consists of a sensitive thermopile, in this chapter in terms of the
composed of many fine gage measurement of a single point. It is then
thermocouples connected in series on expanded to cover thermal scanning and
opposite sides of a flat core with known imaging.
and stable thermal resistance. The entire
assembly is covered with protective Figure 1 illustrates the basic
material. configuration of an infrared sensing
instrument (infrared radiation
The voltage generated across the
thermopile is calibrated to be a measure of
the steady state heat flux through the

FiGURE 1. Basic configuration of infrared radiation thermometer.

Target surface emits Optics Detector
converts
infrared energy ~ infrared Electronics
Lens Filter passes energy to amplifies
I
and
collects selected conditions Dete<:t
the signal
energy spectral band

I II \ '"electrical

Target II I'-. Held I Isignal J Measure
size
1 of Output

J / view Monitor

II

Control

Equipment for Infrared and Thermal Testing 273

thermometer), showing the components detectors usually operate at or near room
necessary to make measurements. temperature, whereas photon detectors
CoHecting optics (an infrared lens, for are usually cooled to optimize
example) are necessary for gathering the performance.
energy emitted by the target spot and
focusing this energy onto the sensitive The mercury cadmium telluride
surface of an infrared detector. The (HgCdTe) detectors in Fig. 2 are photon
processing electronics unit amplifies and
conditions the signal from the infrared detectors cooled to 77 K (-196 oc =
detector and introduces corrections for
such factors as detector ambient -321 "I:) for operation from 8 to 12 ~m
temperature drift and target effective
surface emissivity. Generally, a readout, and to 195 K (-78 oc = -109 °F) for
such as a meter, indicates the target
temperature and an analog output is operation from 3 to 5 ~m. Because of
provided. The output signal is used to their fast response these detectors are used
record, display, alarm, control, correct or extensively in high speed scanning and
any combination of these. imaging applications.

Infrared Detector In contrast to the mercury cadmium
telluride detector, the radiation
An infrared detector is at the heart of thermopile shown in l;ig. 2, is a broad
every infrared sensing and imaging band thermal detector operating
instrument, whatever its configuration. uncoolcd. It is used extensively for spot
Infrared detectors can sense infrared measurements. Because it generates a
radiant energy and produce useful direct current electromotive force
electrical signals proportional to the proportional to the radiant energy
temperature of target surfaces. reaching its surface, it is ideal for use in
Instruments using infrared detectors and portable, battery powered instruments.
optics to gather and focus energy from
the targets onto these detectors are The lead sulfide detector is typical of
capable of measuring target surface those used in radiation thermometers that
temperatures with sensitivities better than measure and control the temperature of
very hot targets. Its peak sensitivity at
0.10 K (0.10 oc = 0.18 °F) and with 3 pm matches the peak energy emitted by
a 1000 K (727 °C = 1340 °F) graybody.
response times in the microsecond range. Because of the atmospheric absorption
An instrument that measures the considerations discussed above, most
temperature of a spot on a target in this infrared thermal imagers operate in either
manner is called an infrared radiation the 3 to 5 pm or the 8 to 12 pm spectral
thermometer. An instrument that region.
combines this measurement capability
with a means or mechanism for scanning FIGURE 2. Response curves of various infrared detectors.
the target surface is called an infrared
thermal imager. It can produce thermal } 1Q12
maps, or thermograms, where the
brightness intensity or color hue of any ~
spot on the map represents the apparent
temperature of the surface at that point. -?

Figure 2 illustrates the spectral E
responses of various infrared radiation
detectors. Radiant energy impinging on -'0
their sensitive surfaces causes all infrared
detectors to respond with some kind of 0.51 2 3 4 5 6 7 8 9 10 11 12 13 14
electrical change. This may be an
impedance change, a capacitance change, Wavelength ),, ).Jm
the generation of an electromotive force
(EMF), known as voltage, or the release of legend
photons, depending on the type of A Silicon, 298 K (25 ~c"' 77 cf).
detector. Infrared detectors are divided B. Indium antimonide, 77 K(-196 ~c = -321 ~F).
into thermal detectors and photon detectors. C. lead sulfide, 298 K(25 oc"' 77 or).
Thermal detectors have broad, uniform
spectral responses, somewhat lower D. Mercury cadmium le!luride, 77 K(-196 cc =. 321 'F).
sensitivities and slower response times
(measured in millisecond); photon E. lead selenide, 243 K (-30 "C = -22 "F).
detectors (also called plwtodete(tors) have F. Mercury cadmium telluride, 215 K {-58 oc-"- -73 or).
limited spectral responses, higher peak G. Thermopile; 298 K(25 °( = 77 <f).
sensitivities and faster response times
(measured in microsecond). Thermal

274 Infrared and Thermal Testing

Infrared Optics - Lenses, Mirrors corrections for factors such as detector
and Filters ambient temperature drift and effective
target surface emissivity. In radiation
There are two types of infrared optics; thermometers, a meter is usually provided
refractive (lenses, filters, windows) and to indicate the target's apparent
reflective (mirrors). Refractive optics temperature. An analog or dig!taLoutput
transmit infrared wavelengths of interest. signal is provided to record, display,
\-\'hen used for higher temperature alarm, control, correct or any
applications, their throughput losses can combination of these.
usually be ignored. When used in low
temperature measurement instruments Scanning and Imaging
and imagers, absorption is often
substantial and must be considered when When problems -in temperature
making accurate measurements. Reflective monitoring and control cannot be solved
optics, which are more efficient, are not by the measurement of one or several
spectrally selective and somewhat discrete points on a target surface, it
complicate the optical path. Reflective becomes necessary to spatially scan -
optics are used more often for low that is, to move the collecting beam or
temperature applications, where the the instrument's field of view - relative
energy levels cannot warrant throughput to the target. This is usually done by
energy losses. inserting a movable optical element into
the collecting beam (Pig. 3).
\Vhen an infrared radiation
thermometer is aimed at a target, energy line Scanning
is collected by the optics in the shape of a
solid angle determined by the \'\'hen the measurement of a single spot
configuration of the optics and the on a target surface is not sufficient,
detector. The cross section of this infrared line scanners can be used to
collecting beam is called the field of view assemble information concerning the
(FOV) of the instrument and it determines distribution of radiant energy along a
the size of the area (spot size) on the single straight line. Quite often, this is all
target surface that is measured by the that is necessary to locate a critical
instrument at any given working distance. thermal anomaly. The instantaneous
On scanning and imaging instruments position of the scanning clement is
this is called the instantaneous field of usually controlled or sensed by an
view (IFOV) and becomes one picture encoder or potentiometer so that the
element on the thermogram. An infrared radiometric output signal can be
interference filter is often placed in front accompanied by a position signal output
of the detector to limit the spectral range and be displayed on a recording device
of the energy reaching the detector. The and/or fed out to a computer based
reasons for spectral selectivity will be process control system.
discussed later in this chapter.
A typical high speed commercial line
Processing Electronics scanner develops a high resolution
thermal map by scanning normal to the
The processing electronics unit amplifies motion of a moving target such as a paper
and conditions the signal from the
infrared detector and _introduces

...- ,- ~

FIGURE 3. Infrared radiation thermometer with addition of scanning element(s) for imaging.

Optics
,.....----..,Target surface emits
/infrared energy Detector Electronics
Filter converts amplifies
infrared ood
Lens passes radiation to conditions Detect
electrical signal Measure
collects selected signal
I
r 'spectral I
band
Target Output

size

l II Monitor

scanners or 1mager~ Control

Equipment for Infrared and Thermal Testing 275

web or a strip steel process. The resulting real time scanning rate is increased.
output is a thermal strip map of the Ivfultidetector scanners reduce the
process as it moves normal to the scan constraints on detector performance by
line. The scanning contiguration is adding detector elements that share the
illustrated in Fig. 4. The output signal temporal spatial burden, allowing for
information is in a real time computer faster frame rates with no reduction in
compatible format and can be used to signal~to~noise ratio or improving the
monitor, control or predict the behavior signal~to~noise ratio with no decrease in
of the target. frame rate.

Two-Dimensional Scanning Electronic Scanning - Pyroelectric
- Thermal Imaging Vidicon Thermal Imagers

Three common imaging configurations Electronically scanned thermal imaging
that produce infrared thermograms are systems based on pyrovidicons and
optomeclwnical scanning, electronic scmmi11g operating primarily in the 8 to 14 pm
and focal plaue array imaging. Of the three, atmospheric window are widely used.
optomechanical scanning was the most They provide qualitative thermal images
common until the mid~1990s. Focal plane and are classified as thermal vie\vers. A
array imagers have replaced scanning pyroelectric vidicon or pyrovidicon is
imagers in most applications. configured the same as a conventional
video camera tube except that it operates
Optomechanical Scanning in the infrared (2 to 20 pm) region instead
of the visible spectrum. Image scanning is
To scan optomechanically in two accomplished electronically in the same
dimensions generally requires hvo manner as in a video camera tube.
scanning elements. Although an almost
infinite variety of scanning patterns can Focal Plane Array Imaging
be generated using two moving elements,
the most common pattern is rectilinear. First introduced to the commercial market
This scanning pattern is most often in 1987, cooled infrared focal plane army
accomplished by two elements, each imagers have evolved into compact,
scanning a line normal to the other. A qualitative and quantitative thermal
representative rectilinear scanner is imagers wHhout scanning optics. These
illustrated in Fig. 5. Its scanning devices have been replacing
mechanism comprises two oscillating optomechanically scanned imagers for
mirrors behind the primary lens, a high many applications. The first uncooled
speed horizontal scanning mirror and a infrared focal plane array imagers lwve
slmvcr speed vertical scanning mirror. been used by the military for several years
and became available to thcrmographers
One performance limitation of in 1997. Hgure 6 is a schematic of a
single~detector optomechanical scanners representative uncooled infrared focal
is a tradeoff between speed of response plane array imager.
and signal~to~noise ratio of the detector.
These instruments require high speed FIGURE 5. Optomechanically scanned infrared imager.
cooled photodetectors that are pushed to
their performance limits as the desired Horizontal

FIGURE 4. line scanner scanning mirror
configuration.
Incoming
Target infrared
distance
radi~
Target
plane Objective
lens

sector

Scan line width= spot size

276 Infrared and Thermal Testing

Performance Parameters thermographic instruments, therefore, do
of Infrared Sensing and not include temperature accuracy,
Imaging Instruments temperature repeatability and
measurement spatial resolution
To select an appropriate instrument for an (IFOVmeas).
application or to determine whether an
available instrument will pefform Generally, instruments that include the
adequately, it is necessary for the capability to produce quantitative
thennographer to understand its thermograms are more costly than
performance parameters. qualitative instruments and require
periodic recalibration. Many applications
The performance parameters for point can be solved without the time and
sensing instruments {infrared radiation expense of quantitative thermography;
thermometers) are temperature range, others require true temperature mapping.
absolute accuracy, repeatability,
temperature sensitivity, speed of response, Performance
target spot size and working distance Characteristics of Point
(field-of-view spatial resolution), output Sensing Instruments
requirements, sensor environment and (Radiation Thermometers)
spectral range.
The American Society for Testing and
For scanners and imagers the :~vlaterials defines infrared point sensing
performance parameters include instruments as infrared radiation
temperature range1 absolute accuracy, thermometers even though they do not
repeatability, temperature sensitivity, total always read out in temperature units.
field of view (TFOV}, instantaneous field Some read out directly in apparent radi;:mt
of view (IFOV), measurement spatial power units such as \'\'·nr2·s-J (or
resolution (IFOVmeas), frame repetition llTU·ft-2.Jr1), some provide a closure or
rate, minimum resolvable temperature alarm signal at a selectable temperature
(MRT), temperature sensitivity, image and others others provide only difference
processing software, sensor environment indications on an light emitting diode
and spectral range. display. 2

Qualitative versus Quantitative Temperature Range
Thermography
Temperature range is a statement of the
For scanners and imagers, one distinction high and lnw limits over which the target
based on instrument performance temperature can be measured by the
limitations is that between qualitative and instrument. A typical specification would
quantitative thermography. A qualitative be, for example, "temperature range 273
thermogram displays the distribution of to 1273 K (0 to 1000 "C; 32 to 1832 "F)."
infrared radiance over the target surface,
uncorrected for target, instrument and Absolute Accuracy
media characteristics. A quantitative
thermogram displays the distribution of Absolute accuracy, as defined by the
infrared radiosity over the surface of the National Institute of Standards and
target, corrected for target, instrument TechnolObl)' (NIST) standard,3 entails the
and media characteristics so as to maximum error, over the full range, that
approach a graphic representation of true the measurement will have ·when
surface temperature distribution.
Performance parameters of qualitative

FIGURE 6. Typical uncooled infrared focal plane array imager.

Optics Iris Array bias
Window

-=-~ Array address generator

Preamplifiers Digital
processor

Infrared local plane array Analog.to.digital
converters

Equipment for Infrared and Thermal Testing 277

compared to this standard blackbody (0.25 oc = 0.45 °F) at a target temperature
reference. A typical specification would
be, for example, "absolute accuracy: of 298 K (25 oc = 77 °F)." In this case, the

±0.5 K (±O.S oc or ±0.9 °F) ±1 percent of sensitivity of the instrument would
improve for targets hotter than 275 K
full scale."
(2 oc = 36 °F).
Repeatability
Speed of Response
Repeatability describes how faithfully a
reading is repeated for the same target Speed of response is how long it takes for
over the short and long term. A typical an instrument to update a measurement.
specification would be, for example, It is defined as the time it takes the
"repeatability (short and long term) of instrument output to respond to a step
±0.25 K (±0.25 °C; ±0.45 °F)." change in temperature at the target
surface. Figure 7 sl1ows this graphically.
Temperature range and absolute 'fhe sensor time constant is defined by
accuracy will always be interrelated. For convention to be the time required for
example, the instrument might be the output signa) to reach 63 percent of a
expected to measure a range of step change in temperature at the target
temperatures from 273 to 473 K (0 to surface. Instrument speed of response is
usually specified in terms of a large
200 oc; 32 to 392 °F) ·with an absolute percentage of the fuH reading, such as
95 percent. As illustrated in Hg. 7, this
accuracy ±2 K (±2 oc = ±3.6 °1:) over the takes about five time constants and is
entire range. This could alternately be general1y limited by the detector used (on
specified as ±1 percent absolute accuracy the order of microseconds for
over full scale. photodetectors and milliseconds for
thermal detectors).
On the other hand, the best accuracy
might be required at some specific A typical speed of response
specification would be, for example
temperature, say at 373 K (100 oc =
"speed of response (to 95 percent)='
212 oF). In this case, the manufacturer
should be informed and the instrument 0.05 s."
could be calibrated to exactly match the It should be understood that there -is
manufacturer's laboratory calibration
standard at tlwt temperature. Because always a tradeoff between speed of
absolute accuracy is based on traceability response and temperature sensitivity. As
to the NIST standard,3 it is difficult for a in all instrumentation systems, as the
manufacturer to comply with a tight speed of response for a particular device
specification for absolute accuracy. An becomes faster (instrumentation engineers
absolute accuracy of ±0.5 K (±0.5 °C = call this a ·wider information bandwidth) the
±0.9 oF) or±1 percent of full scale is about sensitivity becomes poorer (lower
as tight as can he reasonably specified. signal-to-noise ratio). If the speed of
Repeatability, on the other hand, can be response is specified to be faster than h
more easily ensured by the manufacturer necessary for the application, the
and is usually more important to the user. instrument may not have as good a
temperature sensitivity as might be
Temperature Sensitivity possible otherwise.

Temperature sensitivity defines the FIGURE 7. Instrument speed of response and time constant.
smallest target temperature change the
instrument will detect. Temperature Time- About five
sensitivity is also called tllermal resolution constant time constants
or noise equil•alenl temperature difference
(NETD). It is the smallest temperature 100
change at the target surface that can be
clearly sensed at the output of the "•" 90
instrument. This is almost always closely
associated ·with the cost of the > 80
instrument, so unnecessarily fine 70
temperature sensitivity should not be
specified. An important rule to remember "c 60
is that, for any given instrument, target
sensitivity wHl improve for hotter targets 0"'
where there is more energy available for
the instrument to measure. Temperature ~c
sensitivity should therefore be specified at
a particular target temperature near the •~
low end of the range of interest.
•~
A typical specification for temperature
sensitivity would be, for example, 10 -
"temperature sensitivity of 0.25 K
Time (s)

278 Infrared and Thermal Testing

Target Spot Size and Working (radian). A 17.5 mrad (I degree) field of
Distance· view means a d·D- 1 ratio of 60 to 1 and a
35 mrad (2 degree) field of view means a
Target spot size D and working distance d d·D-1 ratio of 30 to 1.
define the spatial resolution of the
instrument. In a radiation thermometer, Output Requirements
spot size is the projection of the sensitive
area of the detector at the target plane. Jt Output requirements for radiation
may be specified directly, "10 mm at 1m thermometers can vary widely- from a
(0.4 in. at 3 ft)/' for example, but it is simple digital indicator and an analog
usually expressed in more general terms signal to a broad selection of output
such as a field of view solid angle functions, including digital outputs
(10 mrad, 1 degree, 2 degree) or a (binary coded decimal); high, low and
field-of-view ratio (ratio of spot size to proportional set points; signal peak or
working distance~ for example, d/15, valley sensors; sample and hold circuits;
d/30 or d/75). A milliradian (mrad) is an and even dosed loop controls for specific
angle with a tangent of 0.001. A d/15 applications. On-board microprocessors
ratio means that the instrument measures provide many of the above functions on
the emitted energy of a spot one-fifteenth even inexpensive standard portable
the size of the working distance: 30 mm models of radiation thermometers.
at 450 mm (1.2 in. at 18 in.), for example.
Figure 8 illustrates these relationships and Sensor Environment
shows hmv spot size can be approximated
quickly based on working distance and The sensor environment includes the
field-of-view information furnished by the ambient extremes under which the
manufacturer. A typical specification for instrument will perform within
spot size would be, for example, "target specifications and the extremes under
spot size= 2 degrees from 1.0 m (39 in.) which it can be stored without damage
when not in operation. For a portable
to=.'' radiation thermometer, a typical
This would take into account the specification for sensor environment
would be as follows.
shortest working distance at which the
instrument could be focused (1 m or 1. Operating temperature is 273 to 310 K
39 in.). For some instruments designed for (0 to 37 oc; 32 to 100 °!').
very close working distances, the simple
d·D- 1 ratio does not always apply. If 2. Humidity is at 20 to 80 percent
closeup information is not clearly relative (not condensing).
provided in the product literature, the
instrument manufacturer should be 3. Atmospheric pressure is at -610 m to
consulted. For most applications and for +2440 m (-2000 to +8000 ft) above sea
middle and long working distances level.
(greater than 1 m or than 3 ft), the
following simple calculation (Fig. 8) will 4. Storage (nonoperating) temperature
closely approximate target spot size: ranges from 258 to 333 K (-15 to

=(1) D u.d +60 oc; s to 140 °F).

where D is about spot size, dis distance to Frequently in process control
target and a is field-of-view plane angle applications, the sensor must be
permanently installed in a somewhat
more extreme environment of smoke,
soot, high temperature and even
radioactivity. For these applications,

fiGURE 8. Determination of instrument's field of view. Target

j~ d ~~

D ud

I

legend

D "' target spot size
d :=distance to target
u "'field of view (rad)

Equipment for Infrared and Thermal Testing 279

manufacturers provide a ·wide range of wavelengths to reach the detector. (A
enclosures that offer special protective combination of a spectrally selective
features such as air cooling, water cooling, detector and a filter can also be used.)
pressurization, pmge gases and shielding. This can make the instrume·nt highly
selective to a specific material whose
Spectral Range temperature is to be measured in the
presence of an intervening medium or an
Spectral range denotes the portion of the interfering background.
infrared spectrum over which the
instrument will operate. The operating For general purposes and for measuring
spectral range of the instrument is often
critical to its performance and, in many targets cooler than about 770 K (500 oc ~
applications, can be exploited to solve
difficult measurement problems. The 930 °F), most manufacturers of radiation
spectral range is determined by the thermometers offer instruments operating
detector and the instrument optics in the 8 to 14 pm atmospheric ·window.
(Fig. 9). Here, the flat spectral response of For dedicated use on hotter targets,
a radiation thermopile detector is shorter operating wavelengths are
combined with that of a germanium lens selected, usually shorter than 3 pm. One
and an 8 to 14 pm band pass filter. The reason for choosing shorter wavelengths is
instrument characterized is suitable for that this enables manufacturers to use
general purpose temperature commonly available and less expensive
measurement of cool targets through quartz and glass optics, which have the
atmosphere. The transmission spectrum of added benefit of being visibly transparent
a 0.3 km (0.19 mi) atmospheric ground for more convenient aiming and sighting.
level is also shown. Another reason is that estimating
emissivity incorrectly \'l'ill result in smaller
An infrared interference filter is often temperature errors 'vhen measurements
placed in front of the detector to limit the are made at shorter wavelengths.
spectral range of the energy reaching the Thermographers have learned that a good
detector. The follmving three classes of general rule to follow, particularly when
filters are common. dealing with targets of low or uncertain
emissivities, is to work at the shortest
1. High pass filters pass energy only at wavelengths possible without
·wavelengths longer than a designated compromising sensitivity or risking
\\'avelength. susceptibility to reflections from visible
energy sources.
2. Low pass filters pass energy only at
wavelengths shorter than a designated Performance Characteristics of
wavelength. Scanners and Imagers

3. Band pass filters (like that in Fig. 9) Because an infrared thermogram consists
pass radiation ·within a designated of a matrix of discrete point
spectral band (8 to 14 pm, for measurements, many of the performance
example). parameters of infrared thermal imagers are
the same as those of radiation
Spectrally selective instruments use thermometers. The output of an infrared
band pass filters to allow only a very Jine scanner can he considered as one line
specific broad or narrow band of

FIGURE 9. Spectral response of instrument determined by detector and optics spectra.

General purpose
band pass filter
(8 to 14 1-1m)

100

:!< 90 Them>C•pHc· (29H K)
80
cc 70 Transmission of
60 germanium
8.·~ 50 (uncoated single
40 optics)
~~ 30
20
~·~ \0

0c
v "~.

w~

~'0'

"De

wW

,_> ~

·~ ~

~

5 10 1S 20
Wavelength /,. (IJm)

280 Infrared and Thermal Testing

of discrete point measurements. The 35 percent of the modulation lmlls{i'r
parameters of temperature range, absolute (unction test used to check imaging spatial
accuracy, repeatability, sensor resolution. Verification of imaging spatial
environment and spectral range are resolution is described in detail later in
essentially the same for radiation this chapter. The simple expression, D =
thermometers, line scanners and imagers. adf can he used to estimate imaging spot
Others are derived from or are extensions size at the target plane from
of radiation thermometer performance manufacturer's published data by
parameters. substituting the published instantaneous
field of view for a.
Qualitative thermal imagers (also called
thermal viewers) differ from quantitative Measurement Spatial Resolution
thermal imagers (also called imaging (IFOVmeas)
radiometers) in that thermal viewers do
not provide temperature or thermal Measurement spatial resolution
energy measurements. For (IFOVmeas) is the spatial resolution of tlw
thennographers requiring qualitative minimum target spot size on which an
rather than quantitative thermal images, accurate measurement can be made in
therefore, some performance parameters terms of its distance from the instrument.
are unimportant. An example of a typical measurement
spatial resolution specification would be
Total Field of View uiFOVmeas = 3.5 mrad at 0.95 SRF." The
0.95 SllF refers to 95 percent of the slit
For scanners and imagers, the total field of response function test used to check
view (TFOV) denotes the image size in measurement spatial resolution. This is
terms of total scanning angles for any described in detail below. The simple
given lens. An example of a typical total expression, D =ad can again he used to
field of vie"w specification would be estimate measurement spot size at the
"TFOV = 20 degree vertical x 30 degrees target plane from manufacturer's
horizontal" ('with standard lx lens) and published data hy substituting published
would define the thermogram total target measurement spatial resolution for a.
size by a simple trigonometric
relationships: Frame Repetition Rate

(2) I' Frame repetition rate replaces speed of
response and is defined as the number of
times every point on the target is scanned

where d is working distance, H is total FIGURE 10. Determination of total field of view for infrared
horizontal image size, Vis total vertical imager.
image size, x is horizontal scanning angle
and y is vertical scanning angle. This is I
illustrated in Fig. 10.
~,.:r--·~~-~ r
The total field of view for a line
scanner consists of one scan line as shown legend
in Fig. 4. The horizontal image size His d =mean distance to the target (em, ft)
equal to the scan sector. The vertical H"' total horizontal image size=- d f2 tan (x/2}]
image size V equals the instantaneous
field of view. All other parameters are the IFOV "'instantaneous field of view
same as for an imager. TFOV "' total field of view (target size)"' V x II

Instantaneous Field of View V =total vertical image size"' d (2 tan (y/2)]
x =image horizontal angular subteme (degrees)
Instantaneous field of vie''' (IFOV) in an y =image vertical angular subtense (degr€es)
imager is very similar to that for a point
sensing instrument: it is tile angular
projection of the detector element at the
target plane. In an imager, however, it is
also called imaging spatial resolution and
represents the size of the smallest picture
element that can be imaged. An example
of a typical instantaneous field of vie\\'
specification would be "U:OV = 1.7 mrad
at 0.35 1vfTF. 11 The 0.35 MTF refers to

Equipment for Infrared and Thermal Testing 281

in one second. This should not be Descriptions of Thermal
confused with field rate. Some imagers are Sensing and Imaging
designed to interlace consecutive fields, Equipment
each consisting of alternate image lines.
This results in images less disconcerting to Point Sensors (Radiation
the human eye. The frame rate in this Thermometers)
case would be half the field rate. An
example of a typical frame repetition rate Point sensors (radiation thermometers)
specification for an imager would be can be further divided into temperature
"frame repetition rate::: 30 frames per probes, portable hand held devices, online
second.11 For a line scanner, the term line process control devices and specially
scan rate is used and is expressed in lines configured devices.
per second.
Temperature Probes
Minimum Resolvable Temperature
Difference Temperature probes are low priced, pocket
portable, battery powered devices that
Minimum resolvable temperature (MRT) usually feature a pencil shaped sensor
or minimum resolvable temperature connected to a small, basic readout unit.
difference (MRTD) replaces temperature Generally, they are optically preadjusted
sensitivity and is defined as the smallest for minimum spot size at a short vwrking
blackbody equivalent target temperature distance. A S.O mm (0.2 in.) spot at a
difference that can be observed out of 20 mm (0.8 in.) working distance is
system noise on a thermogram. As in typical. Temperature usually ranges from
radiation thermometry, this difference about 253 to 573 K (-20 to 300 "C; -4 to
improves (becomes smaller) with
increasing target temperature and is 570 "F) and a sensitivity of ±1 K ( ±1 oc =
expressed in those terms. An example of a
typical minimum resolvable temperature ±1.8 °F) is achieved easily. Probes are
difference specification for a line scanner designed for closeup measurements such
or an imager would be ''MRTD = 0.05 Kat as circuit board analysis, troubleshooting
297 K target temperature (0.05 "Cat of electrical connections, inspection of
25 "C; 0.09 "Fat 77 "F)." plumbing systems and biological and
medical studies.
Minimum resolvable temperature
difference may also depend on the spatial Portable Hand Held Devices
frequency imposed by the test discipline.
The test technique for checking minimum Portable hand held radiation
resolvable temperature difference is thermometers are designed for middle
described elsewhere. distance measurements and, with few
exceptions, operate in the 8 to 14 pm
Thermal Imaging Display and spectral region and are configured like a
Diagnostic Software Overview pistol for one-handed operation and
aiming. They are usually optically
Thermography applications often require preadjusted for infinity focus. A typical
extensive thermal imaging display and 2 degree field of view resolves a 75 mm
diagnostic software. Thermal imagers (3.0 in.) spot at a 1.50 m (60 in.) working
feature image processing capabilities that distance and a 300 mm (1 ft) spot at a
may be divided into five categories, one 9 m (30 ft) working distance. Most
or more of which may be used in the instruments in this group incorporate
same application. These categories are microcomputers with limited memory
quantitative thermal measurements of and some have data Jogging capabilities.
targets; detailed processing and image An open or enclosed aiming sight is
diagnostics; image recording, storage and provided and, in some more recent
recovery; image comparison (differential models, a projected laser beam is used to
or multispectral thermography); and facilitate aiming of the instntment as
database and documentation. shown (Fig. 1 1). Note that the laser beam
does not represent the field of view.

A measurement readout is always
provided and usually the temperature is
shown on a digital liquid crystal display.
These instruments are powered with
disposable batteries and have low power
drain. Temperature ranges are, typically,
from 270 to 1270 K (0 "C to 1000 "C;
30 oF to 1800 °1:). Temperature sensitivity
and readability are usually 1 percent of
scale (1 K or 1 oc or 2 °F) although

282 Infrared and Thermal Testing

sensitivities on the order of 0.1 K (0.1 oc specific target, this type of instrument
remains there for the life of the
or 0.2 oF) are achievable. Response times im.trument or process. With few
arc on the order of fractions of a second, exceptions, these instruments operate on
usually limited by the response of the line power. The measurement value can
readout. be observed on a meter but is more often
used to trigger a switch or relay or to feed
Hand held radiation thermometers are a simple or sophisticated process control
used extensively in applications where loop. Most of online monitoring and
spot checking of target temperatures is control sensors send signals to universal
sufficient and continuous monitoring is indicator/control units that accept inputs
not required. Hand held radiation from various types of industrial sensors.
thermometers have become an important
part of many plant energy conservation Because this instrument group is
programs. Process applications include selected to perform a specific task; a
monitoring mixing temperatures of food shopping list format is provided to the
products, cosmetics and industrial customer by the manufacturer so that all
solvents. lvHcrocomputers enable hand required features can be purchased,
held instruments to incorporate special including environmental features such as
features such as the ability to store 60 water cooled housings, air purge fittings
readings for future retrieval and printout. and air curtain devices. Emissivity set
controls, located in a prominent place on
Online Process Monitoring and a general purpose instrument, are more
Control Devices likely to be located behind a bezel on the
sensor on these dedicated units, where
Online monitoring and control sensors they are set once and then locked. The
arc for dedicated usc on a product or a spectral interval over which the sensing
process. Permanently installed where it head operates is selected to optimize the
can measure the temperature of one signal from the target, to reduce or
eliminate the effect of an interfering
FIGURE 11. Infrared radiation instruments: energy source or to enable the instrument
(a) hand held infrared radiation to measure the surface temperature of
thermometer with laser aiming and thin films of material that are largely
(b) infrared focal plane array imager for transparent to infrared radiation. The
qualitative thermography. capability for spectral selectivity has made
(a) these instruments important in the
manufacture of glass and thin film
(b) plastics.

Devices with Special
Configurations

Special configurations of infrared
radiation thermometers include (1) ratio
pyrometers (also called two·color
pyrometers), (2) infrared radiometric
microscopes, (3} laser reflection
pyrometers and (4) fiber optic coupled
pyrometers.

Equipment for Infrared and Thermal Testing 283

1. Two-color pyromders, or ratio pyrometrrs, line scanners are in this configuration.
are a special case of the online The output signal information is in a real
instrument. Ratio pyrometers are lime computer compatible format and can
be used to monitor/ control or predict the
particularly useful in high temperature behavior of the target.
applications above 573 K (300 °C =
Like the online point sensor, these line
572 °F) and in measuring small targets scanners are usually permanently installed
of unknown emissivity, provided the where they monitor the temperature
profile at one site of the process,
background is cool, constant and remaining there for the life of the
uniform. The emissivity of the target instrument or the process. Likewise, they
need not be known if it is constant are usually fitted with environmental
and reflections are controlled. The housings and preset emissivity
target does not need to fill the field of compensation sets. The best applications
for this scanner are in online, real time
vkw provided the background is cool, process monitoring and control
constant and uniform. The applications where they are integrated
measurement is based on the ratio of with the process host computer system. lt
energy in two spectral bands, so is not unusual to find line scanners at
multiple locations in a process with all of
impurities in the optical path resulting them linked to the host computer.
in broad band absorption do not affect
the measurement. Ratio pyrometers In the 1990s, infrared line scanners
are usually not applicable to based on linear focal plane arrays came
measurements below 573 K into use. This type of instrument
frequently uses an uncooled array of
(300 oc ~ 572 °F). thermal detectors (radiation thermopiles).
This scanner has no moving parts. The
2. Infrared radiometric microscopes are linear array is oriented perpendicular to a
configured like a conventional process or a target moving at a uniform
microscope and, by using reflective rate. The scanner output may be used to
microscope objectives and beam develop a thermogram or the data for
splitters, the operator can each pixel can he fed directly to a host
simultaneously view and measure computer and used to monitor and
targets down to 10 pm in diameter control the process. Instruments of this
with accuracy and resolution of about type have been used to monitor moving
0.5 K (0.5 °C" I 0 1'). railroad cars for overheated wheels and
brake assemblies.
3. Laser reflection pyrometers use the
reflected energy of an active laser to Special Purpose Devices
measure target reflectance. A built-in
microcomputer calculates target Special purpose configurations of line
effective emissivity and uses this figure scanners include one type of portable line
to provide a corrected true temperature scanner and a number of aerial mappers
reading. This instmment, though that scan a line normal to the motion of
expensive, is useful for measurement the aircraft and develop a thermal strip
of high temperature specular target map. ~,fany of these mappers have been
surfaces in adverse environments. replaced by low cost fonwmf looking
infrared scanners based on staring focal
4. Fiber optic coupled pyrometers make plane arrays.
possible the measurement of normally
inaccessible targets by replacing the
optic with a flexible or rigid fiber optic

bundle. This limits the spectral
performance and hence the

temperature range to the higher values
but has allowed temperature
measurements to be made ·when
previously none \vere possible.1

Line Scanners

Line scanners are divided into online
process control devices and special
purpose scanners.

Online Process Control Devices

Online (monitoring and control) line
scanners are high speed, online
commercial line scanners that develop
high resolution thermal maps by scanning
normal to the motion of a moving target
such as paper web or a strip steel process.
The vast majority of commercial infrared

284 Infrared and Thermal Testing

PART 2. Thermographic Imagers

Imagers (thermographic instruments) visible spectrum. Aside from the tuhe and
include both qualitative and quantitative germanium lens, which are expensive,
imagers. these systems use television recording
accessories. In comparison with other
Qualitative Thermographic infrared imaging systems, the picture
Imagers quality and resolutiot~ are good,_ .
approaching conventional televtswn
Qualitative thermal imagers are also called format. The thermal image can be viewed
thermal viewers. They include or videotaped with equal convenience
mechanically scanned, electronically and no cooling is required.
scanned (pyrovidicon) and staring focal
plane array imagers. Pyrovidicon systems do not
intrinsically offer quantitative
Mechanically Scanned Thermal measurement capability but some
Viewers manufacturers offer models in which an
integrated radiation thermometer is bore
Mechanically scanned thermal viewers are sighted with the scanner and its
moderately priced battery powered measurement is superimposed on the
scanning instruments that produce a video display along with a defining reticle
qualitative image of the radiosity over the in the center of the display. Thermal
surface of a target. The battery packs are resolution of flicker free pyrovidicon
rechargeable and usually provide 2 to 3 h instruments is between 0.2 and 0.4 K (0.2
of continuous operation. These one-piece, and 0.4 oc; 0.4 and 0. 7 °1').
lightweight instruments, designed to be
simple to operate, feature thermoelectric Pyroelectric devices have no direct
detector cooling provided by a battery current response and a basic pyrovidicon
powered cooler. Although not designed imager's display will fade when the device
for absolute temperature measurements, is aimed at an unchanging thermal scene.
they can demonstrably sense temperature Early pyrovidicon imagers needed to be
differences of tenths of degrees and can be panned to retain image definition. To
used for targets from below 273 to 1773 K enable fixed monitoring, crude, flag type
(0 to 1500 oc; 32 to 2372 °F). Typically, choppers were devised to interrupt the
the total field of view is from 6 to 8 image at adjustable chop rates. However,
degrees high and from 12 to 18 degrees this resulted in a blinking image that was
wide, with spatial resolution of 2 mrad, disconcerting to the eye. These choppers
10 mm at 2.0 m (0.4 in. at 7 ft). Images have been replaced by synchronous
are video recorded by means of a choppers that chop the image in
conventional video tape recorder output synchronism with the electronic scan rate
jack and video recorder accessories. and produce flicker free images on the
display. Pyrovidicon viewers operate well
The broad applications for thermal in the 8 to 14 pm atmospheric
viewers are generally limited only to those transmission window. Operating costs are
in which the temperature measurements very low because no cooler or coolant is
are not critical and recording quality does required.
not need to be optimum. The
combination of a thermal viewer (to Staring Infrared Focal Plane Array
locate thermal anomalies) and a hand Thermal Viewers
held thermometer (to quantify them) can
be a powerful and cost effective Staring infrared focal plane array thermal
combination. viewers arc direct adaptations of devices
developed for military and aerospace
Electronically Scanned Viewers night vision and missile tracking
(Pyrovidicon Imagers) applications. For these applications,
performance emphasis is on picture
Pyrovidicon (pyroelectric vidicon) imagers quality rather than measurement
are electronically scanned video cameras. capability.
The camera tube is sensitive to target
radiation in the infrared rather than the Instruments using cooled platinum
silicide {PtSi) staring arrays with as many
as 512 x 512 elements are available.
Instruments using cooled indium

Equipment for Infrared and Thermal Testing 285