Understanding the Challenges of Surface Temperature Measurement Using Thermocouples
Thursday, April 14, 2016
the Challenges of Surface Temperature Measurement Using Thermocouples
Ronald D. Lucier, ASNT NDT Level III, FLIR Systems, Inc.
Voltage from a thermocouple must eventually be converted to temperature. The potential
produced by a thermocouple, as a function of temperature difference between its
two ends, is very nearly linear over a very wide range. The following curve is
the "standard response curve" for a Type K thermocouple. This is the
process of calibration.
most common surface temperature measurements are made with thermocouples; but since
a thermocouple’s reading is actually a measurement of its own current
temperature, the challenge has always been to get the thermocouple to correctly
match the heat of the measured surface. Unfortunately, few infrared
thermographers take this measurement uncertainty into account when relying on
thermocouple measurements as a reference in determination of emissivity. This
paper will explain the theory behind thermocouples and, through demonstration,
illustrate many of the problems in their use. In addition, this paper will
highlight situations in which the combination of an infrared camera and a
thermocouple is preferred, and cases in which an infrared camera by itself is
the superior method for surface temperature measurement.
vast number of commercial and industrial processes rely on accurate temperature
measurements. But are the measurements – as performed – accurate? How temperature
is measured and what degree of accuracy is required are two very important
questions that need to be answered for every application. Entire textbooks are
devoted to this one subject!
paper focuses on one of the biggest challenges: accurate surface measurements
made with thermocouples. The author readily acknowledges that while
thermocouples provide accurate readings of liquids and gases, surface
measurements present a unique set of problems.
common question asked of an infrared thermography instructor is, “If we really
want to measure temperatures, why don’t we just use thermocouples?” This is
amusing considering the student is sitting in class with an infrared camera!
When asked about thermocouple mountings, most students suggest using electrical
tape because it is inexpensive, easy to install, and easy to remove. One
particular student employed in the HVAC industry related that he commonly
installed thermocouples on compressors using electrical tape and preferred to
rely on the thermocouples’ readings than those of his gauges. (Incidentally, he
explained that he was in the infrared class because he was replacing too many
compressors and hoped IR would give him the answer.)
attachment of thermocouples is probably the worst option, as the resulting
surface measurements will be inconsistent and inaccurate. Permanent attachment
by cementing is the preferred method for those who need consistent results.
When permanent attachment is inconvenient or impossible, infrared thermography
is the preferred – if not only – choice.
Thomas Seebeck is credited with the discovery of the “thermoelectric effect” in
1821. This states that any conductor subjected to a thermal gradient will
generate a voltage. Seebeck misinterpreted the effect, thinking the current was
magnetic rather than electric. Indeed, his reports to the Prussian Academy of
Sciences in 1822 and 1823 describe his observations as, “the magnetic
polarization of metals and ores produced by a temperature difference.”
Italian physicists, Leopoldi Nobili and Macedonio Melloni, continued Seebeck’s
work to create a thermoelectric battery. This battery is now called a
“thermopile.” When Nobili and Melloni coupled the thermopile with a
galvanometer, they were among the first physicists to measure infrared
The illustration in Figure 1 shows a
complete thermocouple circuit. Two dissimilar metals are connected in a circuit
and subjected to a thermal gradient that will cause a change in voltage.
Figure 1. A simple thermocouple circuit
change in the temperature gradient results in a change in voltage. These
voltages are generally in the microvolt-per-°C range. Higher temperature gradients
(indicating higher temperatures) generate higher voltages.
Seeback voltage change is linearly proportional with temperature when the
temperature changes are small. The traditional equation is represented as:
order to measure the voltage change caused by the temperature gradient, a volt
meter must be placed in the circuit. This adds two more electrical junctions:
one is copper-to-copper, the other is copper-to-dissimilar metal. Now that we
have two thermocouples in the circuit, how does the volt meter distinguish
between them? Notice the ice bath in Figure 1, which is assumed to be 0°C. It
is used as a “known reference junction” or known temperature. Once one junction
temperature is known, the other temperature – the one we are trying to measure
– can be determined mathematically.
you purchase a thermocouple and install it, where do you add the ice? For
factory-made thermocouples such as the Extech EA10, the manufacturer uses
hardware compensation and an internal temperature sensing resistor in place of
the ice bath. This is commonly known as an electronic ice point reference
circuit and is unique to each type of thermocouple.
VOLTAGE TO TEMPERATURE
Figure 2. Typical Type K calibration curve
the temperature-versus-voltage relationship of a thermocouple is not always
linear. The equation introduced previously shows an ideal relationship with the
Seebeck coefficient alpha being a constant. But this is actually not the case,
it is a variable represented by a polynomial.
process of calibrating a thermocouple produces an ideal curve such as presented
in Figure 2. The blue line represents an actual millivolt output versus
temperature and the dashed line is a “best fit” of the data. This type of
thermocouple is relatively linear over a wide temperature range although when
the actual data is examined there may be significant non-linear data at
specific points. This curve is for illustration purposes only.
coefficients of the polynomial representing the calibration curve are used with
the input of millivolts to determine the temperature readout on the
Like many other
19th century Industrial Revolution inventions, the thermocouple has
many everyday uses.
Figure 3. Extech EA10 dual-input thermometer
thermocouple shown in Figure 3 is typical of general purpose instruments. It’s
reliable, economical, and available through multiple outlets. Many
manufacturers produce instruments such as this, and the following comments are
not unique to Extech (a FLIR company).
stated accuracy of the EA10 in the user manual is +/- 0.3% + 2°F. The two Type
K thermocouples read within 0.4°F to 0.8°F of each other over a five minute
period, which is considered a good correlation.
instruction manual was very easy to understand, showed all of the features of
the instrument, and presented the instructions in English, Spanish, and French.
author downloaded the instruction manuals from several competitors of Extech
and found similar manuals for general purpose instruments. All manufacturers
included clear, concise instruction in multiple languages.
the copious information provided in the instructions, one very important piece
of information was missing: how to mount the thermocouple. Manufacturers offer
instruction for operating the instruments, but not for the most important step in
the measurement mission: how to make the measurement. This step requires an
understanding of the thermocouple environment. The only information on mounting
a thermocouple was found on a sheet of instructions that came from Omega
Engineering and referred to their epoxy adhesive. Refer to the latter section
on permanent thermocouples.
ENVIRONMENT – SURFACE CONDUCTION
by their very nature only indicate the temperature they have reached. In order
to measure a solid, liquid, or gas the challenge is to normalize the
thermocouple to the same temperature as the solid, liquid, or gas. This paper
explores the biggest challenge – solid surfaces.
exterior or upper boundary of an object or body
plane or curved two dimensional locus of points
external or superficial aspect of something
Dictionary (www.m-w.com) provides a simple definition of a surface important to
review here, as it is the very nature of the surfaces involved that present the
biggest challenges in temperature measurements. Describing a surface as “two
dimensional locus of points” relates best to the problems of thermocouples as
seen with infrared thermal imagers. Technically, points are infinitely small,
but, when collected in an area, define a surface.
Figure 4. Type J & K beaded wire-to-surface configuration
common forms of thermocouples are Type J and Type K beaded wire thermocouples,
which are typically attached to surfaces temporarily. These thermocouples can
also be permanently cemented in place.
using Type J or K thermocouples, either temporarily attached or cemented, an
obvious measurement issue is the problem of heat conduction. That’s because the
primary mode of heat transfer between the object surface and the thermocouples
is conduction. The Fourier Law of Conduction for a one dimensional surface is:
equation and terms should be familiar to all Level I, II, and III infrared
thermographers. What value do you use for A?
Technically, since it is a point, it must be zero. In reality, it cannot be
zero since we get a reading once we place a thermocouple on a flat or curved
surface. Try this and note how slowly the temperature increases. The area
available for heat conduction is very small.
the case of very complicated conduction heat transfer processes, simple
equations no longer work. Here we have to modify the above equation and
introduce the concept of the conduction shape factor. The conduction shape
factor is used to account for the specific geometry not accounted for in
general equations such as the one above. Therefore, we can rewrite that general
equation into a new form in which the term S is our conduction shape factor:
calculations involving a sphere and a plane, as represented in Figure 4, we get
a rather complicated formula where D is the diameter of the solder ball and r is the radius:
is obvious that if the value S is
large, the heat transfer will be larger and more heat will transfer into the
thermocouple, giving a more accurate response. So far we have accounted for the
conduction of the heat from the flat surface to the spherical surface. Now we
have to get the heat from the spherical surface to that dissimilar junction so
a thermal gradient can be established. This involves another equation:
the above equation, r1 is
the radius of the solder ball, r2 is the radius of the thermocouple wire and k is the thermal conductivity of the material in the solder ball.
transfer of heat from the surface we wish to measure into the device that will
be measuring it – even when approximated under steady state conditions and
equations – involves some interesting mathematical concepts. If this situation
is extended into transient conditions, where the temperatures are changing very
rapidly, the math becomes very complicated and the measurement challenges
become extremely difficult.
ENVIRONMENT – SURFACE CONVECTION
that a thermocouple has been placed on a surface and the complexity of the heat
conduction shape factor has been introduced, the problem becomes truly complex
with addition of another process—convection. All surfaces are subject to the
three forms of heat transfer: conduction, convection, and radiation. The
exception is when the surface exists in a vacuum, where convection cannot
thermal image in Figure 5 shows a Type K thermocouple attached to a surface
with a piece of electrical tape (the entire test setup is shown in Figure 7).
The thermal gradient causing the Seebeck effect is clearly shown, as are the
emissivity differences between the tape and the metal surface.
Figure 5. Thermal image of a Type K thermocouple on a warm surface
The differences in the radiation heat transfer are apparent.
The piece of tape securing the thermocouple to the surface has the effect of
insulating the flow of heat from the surface of the can. Because thermal
resistance to conduction heat transfer has slightly increased in that area,
this area will radiate and cool faster. You can see the results in Figure 5:
the tape appears considerably brighter than the metal surface.
To further confuse matters, the effects of convection have to
be recognized. Traditional infrared cameras have limited abilities to see small
temperature effects caused by convection. However, the FLIR GF320 camera has a
High Sensitivity Mode (HSM) with sequential image frame subtraction, allowing
for the detection of minute quantities of trace gases. Look at the very small
differences in the images in Figure 6. The surface in question was 112 °F (44.4
°C); temperature variation caused by convection across the surface was as much
Figure 6. HSM depiction of convection at a thermocouple surface
ENVIRONMENT – COMBINATION OF EFFECTS
net effect of all the above is shown in Figure 7. One thermocouple was placed
on the inside of the can and secured to the surface and one on the outside, in
approximately the same place. The expectation is that the two readings should
be the same, but they aren’t.
Figure 7. T1 and T2 measurements don’t match up
two thermocouples agreed within 0.5 °F most of the time, but there was
variation. The variation can be explained from the room convective currents, as
indicated in Figure 6. There are stronger convective currents inside the can of
water due to the density of water, but that would cause the temperature
indication of T2 to drift versus T1, but not cause the difference.
difference in the readings here is caused by all the effects previously mentioned,
but most significantly by conduction heat transfer. The shape of the solder
ball at the end of the beaded thermocouple provides very little area for heat
conduction. The magnitude of the conduction shape factor indicates the same.
that we’ve concluded that temporary attachment of thermocouples is a poor
practice, what about permanently mounted thermocouples?
8 is an image of a crude oil heater taken with a FLIR GF309 furnace camera. The
tube temperatures are measured to determine tube life and to determine
operating parameters of the heaters. One thermocouple was permanently fixed to
the surface of one newly installed tube and is in the extreme upper left of the
image, just out of view.
Figure 8. Crude oil heater
thermocouples in the heaters are subjected to an extreme environment: gas
temperatures are in excess of 2,800°F and a radiation environment in excess of
1,800°F (1,537°C and 982°C, respectively).
the thermocouples measure at a single point. While they provide useful data, at
least for a while, they do not indicate whether coke is building up inside the
tubes. Severe coking will eventually lead to tube ruptures. You can see the
coke build-up in Figure 8 as white hot spots in the thermal image.
severe environments obviously affect the thermocouples’ long term performance.
Unfortunately, they are not the only problem.
THERMOCOUPLES – BACK TO THE SMALL SCALE
results from using electrical tape as shown in Figure 7 are unacceptable. A
better method was investigated: cement-on-thermocouples. The author expected
that at nearly five times the price, the results would be much improved—and they
author chose two Omega CO1 Type K fast response thermocouples. The foils are
0.0005” thick and bonded between a very thin polymer/glass laminate. These
thermocouples are very thin and flat, ideal for mounting on curved surfaces.
Omega recommended OB200 Epoxy for mounting.
epoxy was mixed (two parts resin and catalyst) according to the manufacturer’s
recommendations, applied to both surfaces, and cured close to the recommended
temperature. Again, the author notes that the only place indicating how to
mount a thermocouple was found in the instructions for the epoxy, not for the thermocouple
Figure 9. Infrared results while curing
Figure 10. Visible (thermocouple) results while curing
results present a puzzle: what to believe? Each thermocouple was mounted in the
same fashion at the same
location inside and outside the container. The can was heated with a hot plate
and based on simple convection heat transfer—different film coefficients
between an enclosed space and an open one—one wouldn’t expect the two readings
to match up. There were many times during the day when the two thermocouples
read exactly the same, but there was an 8°F temperature difference with the
thermocouple attached to the outside of the container, and a 13°F reading
difference for the thermocouple inside of the container at the time that the
thermal image was obtained. The outside thermocouple was experiencing
additional room convection from the air conditioning. There was little
consistency in readings between the camera and the thermocouples, particularly
the outside one (smaller case reading T2 on the thermocouple readout).
the thermocouples were adequately cured and checked for integrity, a third
thermocouple was taped on the surface and hot water was added. Results below:
Figure 11. Infrared results with hot water in can
Figure 12. Thermocouple reading with hot water in can
that now with the water inside the can, the cemented-on thermocouple almost
exactly agrees with the infrared measurement of the tape. Both are within 1°F
of the inside water temperature (the water had to be mixed vigorously to limit
natural convection and decrease the convective boundary layer) but
significantly disagree with the taped-on thermocouple. Obviously, temporarily
attached thermocouples are completely unreliable and should never be used. In
contrast, the cemented-on thermocouple agrees with the infrared camera
measurement, but only when there is water in the can.
results illustrate the difficulty of seemingly simple applications. The images
in Figures 6, 9, and 11 provide clues as to the reason for the discrepancies
and why thermocouples and IR cameras sometimes don’t match: convection. In
Figure 11, the can was naturally cooling and was on a surface at room
temperature. In Figures 9 and 10, the can was empty and being heated from the
bottom by a very hot plate. Therefore, the two convective heat transfer
conditions are very different. Secondly, the thermal inertia of the two cans
(volumetric heat capacity) was significantly larger in the can with water as
opposed to the can with air. Lastly, as evidenced in Figures 9 and 10, there
was a much larger thermal gradient in Figure 9, making the selection of
measurement points all that more important!
LONG TERM PERFORMANCE
that it has been established that surface measurements with thermocouples are,
at best, very complex, what else is there to consider? There are several issues
concerning the long term performance of a thermocouple.
- Poor junction
connection – How the thermocouple is mounted is critical. Temporary connection via
electrical tape is a poor practice in just about every situation. A conductive
cemented connection or a thermocouple specifically designed to be cemented on
is probably a better choice, but its environment must be considered. In high
temperature environments, specialized thermocouples are welded to the surfaces.
These welds can separate under stress, making their readings no longer
reliable. This can be a sudden or gradual process.
- Decalibration – Operating a
thermocouple at very high temperatures or in corrosive atmospheres can cause
diffusion of atmospheric particles into the thermocouple metals, changing the
entire characteristics and calibration of the thermocouple. A high temperature
annealing can occur along the section of the thermocouple that has the highest
thermal gradient causing calibration issues.
- High temperatures – High
temperatures can affect the insulation around the thermocouples. In the case of
heaters, the thermocouple may actually measure gas resulting from insulation
breakdown rather than the surface itself.
- Galvanic action – If the insulation
contains certain dyes that leach out in the presence of water, electrolytes
will form. The resulting current can be hundreds of times greater than the
- Thermal lag – Ideally, we want
to have a small thermocouple so that it will not affect the surface temperature.
However, small thermocouples generally have small wires, which may significantly
affect response time.
SUMMARY AND CONCLUSION
reliably measure the temperature of themselves, within the accuracies stated by
accuracies stated by the manufacturers are not necessarily the accuracies of
the ambition is to get the thermocouple to the same temperature as the surface,
in reality, that is impossible.
offer the opportunity for an accurate measurement of liquids and gases, but
fall short of providing an accurate measurement of surface temperature.
thermocouple greatly affects the convective and radiation heat transfer locally
at the surface, thus affecting
attached thermocouples should be avoided.
installed correctly, cemented-on thermocouples are most likely reliable.
placement of thermocouples should be done with an IR camera to avoid strong
placing thermocouples where strong convective currents will affect the
nothing. Periodically check the calibration and integrity of permanently
REFERENCES – TECHNICAL
Extech EA10 Dual Input Thermometer
Omega Engineering OB-200 Epoxy Adhesive
Omega Engineering CO1, CO2, CO3 User’s
ASHRAE Handbook – 1985 Fundamentals (pages
13.6 & 13.7)
“Introduction to Infrared Systems
Engineering”, ©1969 Richard D. Hudson, reprinted in SPIE Milestone Series MS116
“Selected Papers on Temperature Sensing:
Optical Methods”, Ronald D. Lucier, Editor (1995)
to Infrared Systems Engineering”, ©1969 Richard D. Hudson, reprinted in SPIE
Milestone Series MS116
ABOUT THE AUTHOR
Lucier is a Mechanical Engineer with more than 32 years of engineering
experience, 27 of which have been devoted almost exclusively to the use of
infrared thermography. He holds an ASNT NDT Level III certificate (number
48004) and is a Senior Instructor for the Infrared Training Center of FLIR
Systems. Ron is also a licensed pyrotechnician and a Master Magician.