ME 368
Laboratory 4
Thermocouples
Laboratory 4
Thermocouples
Equipment needed:
 myDAQ connected to computer running LabVIEW
 Traceable® thermometer
 2 lengths each of copper wire and 1 length of constantan wire (these look just like ordinary
wires, except one appears copper and the other appears silver)
 1/8” diameter stainless-sheathed thermocouple and associated extension-wire jack
 Ice water bath in red plastic cooler made from distilled water and non-distilled ice
 Room temperature water bath in white Styrofoam cup bath made from tap water
 2 Thermo Scientific 280 series water baths filled with distilled water
 Boiling water bath made using distilled water in a hot pot
Goals and Objectives
 Gain exposure to thermocouples and the thermocouple effect
 Understand differences between thermocouples and thermistors, including:
1. One is a temperature-dependent voltage-generating device, the other is a temperaturesensitive resistance
2. One has highly nonlinear temperature behavior, the other has approximately linear
temperature behavior
 Understand linear regressions and related statistics
1.1 Thermocouple operation
Thermocouples are formed whenever two dissimilar metals are in contact. Some metal pairs are
more sensitive to temperature than others, but all generate a small voltage based on the temperature
difference between the points on the metals where the voltage is being measured and the junction
between the metals. A great deal more background on thermocouples is available online and in various
text books and references and you may want to review some such as:
 Wikipedia (http://en.wikipedia.org/wiki/Thermocouple,
http://en.wikipedia.org/wiki/Thermoelectric_effect),
 Omega Engineering (http://www.omega.com/thermocouples.html) and
 efunda
(http://www.efunda.com/DesignStandards/sensors/thermocouples/thmcple_theory.cfm).
It is important to remember that it is the temperature gradient in the metal that causes the electrical
potential to arise. If you were to place a copper wire between two leads of a volt meter, and the entire
wire was at the same temperature, you would not be able to measure a potential difference between
them. Should you place a flame on the wire midway between the two voltmeter leads, you would
generate a thermoelectric potential between the midpoint of the wire and each of the wire ends.
However, the two potentials would cancel each other out, and the volt meter would again indicate that
there was no potential difference in the wire.
Instead of using a uniform wire made of copper, let’s imagine we splice a pure copper wire to a
copper-nickel alloy (55% Cu, 45% Ni, say) wire to form a wire composed of 2 segments. Now
imagine we heat the splice joint with a flame. Since each wire will generate a different thermoelectric
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Laboratory 4
Thermocouples
voltage, when a volt meter is connected to the ends of 2-segment wire (one connection to the pure
copper wire, far from the joint, and another connection to the copper-nickel alloy wire, far from the
joint) it will measure an electrical potential difference. The magnitude of the electric potential that is
generated is related to the difference of the Seebeck Coefficients of the two materials. The Seebeck
Coefficient of copper is approx. 6 V/°C and that of the copper-nickel alloy (called constantan) is
approx. -35 V/°C. Thus, the junction of the two should give an electrical potential of about 41 V for
every °C of temperature difference between the junction and the ends of the wire. Note that these
Seebeck Coefficient values are actually temperature dependent, so calibrations are made of careful
measurements of the junction voltage over a very wide range of temperatures instead of using this
constant value.
The copper-constantan combination is very common and is called a type-T thermocouple.
Internationally accepted convention states that pairs of wires leading to a type-T thermocouple should
be blue or marked with blue. There are several other common thermocouple wire combinations and
these are discussed in the references listed above.
From the description above, it seems that measuring a temperature with a type-T thermocouple
should be pretty straight forward. However, we ignored a couple very important issues. First, to use a
volt meter, we usually use copper wires to connect between the measurement circuitry and the device
to be measured. Thus, when the copper wire from the volt meter is connected with the constantan wire,
another type-T thermocouple is formed, and its potential will be summed with the potential of the
thermocouple we are trying to measure. In addition, the voltages are small so noise must be properly
managed. In this lab, we will investigate temperature measurement with thermocouples and how these
issues may be overcome.
lab completion a (2 pts): If a resolution of 0.1 °C is desired in temperature measurements made by a
type-T thermocouple, what voltage resolution is required in the DAQ system?
lab completion b (2 points): Compare your answer to lab completion a to a) the voltage resolution Q
of channel ai0 of the myDAQ setup for the +- 2 V range and to b) the noise level of 2.5 µV observed in
lecture when the +- 2 V range was sampled from ai0 at 200kS/s and sample-compressed by a factor of
6000 (see L8_Ex1.vi). Draw conclusions.
1.2 Connecting a homemade thermocouple
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Jumper ai0- to the nearest AGND terminal. Use the screwdriver at your station to connect one
of the copper wires to ai0+. Connect a constantan wire to ai0-.
Twist the free ends of the copper and constantan wires together to form a thermocouple.
Setup ai0 to monitor the +- 2V range at 200 kS/s with a sample-compression factor of 20,000 (1
data point every 0.1s), displaying the results on a waveform chart.
Run your code and observe the chart.
Place the thermocouple between your fingers and observe the voltage increase.
Allow the thermocouple to settle on the room temperature and determine the average voltage
measured when settled. An easy way to do this is to right click on the chart and select Export,
Export Data to Excel, and then use the average function within Excel.
Look up the voltage on the voltage chart for Type T thermocouples (see, e.g.,
type_T_thermocouple_chart.pdf on the course website in the equipment folder). What
temperature is represented by that voltage?
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1.3 Checking the accuracy of a temperature difference
We will now begin the process of correcting the voltage measurement so that the measured voltage
corresponds to the correct temperature according the international standard thermocouple voltage
tables.
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Place the thermocouple in an ice water bath, stirring it very gently
Record the average voltage along with the reference temperature
Repeat the above 2 steps for a room temperature bath
Subtract the ice bath temperature voltage from the room temperature water voltage
Look up the temperature value associated with the voltage difference in the Type T
thermocouple chart.
lab completion c (2 points): Does your temperature based on the voltage difference seem reasonable?
1.4 Using an ice bath reference
It would generally be annoying to have to always subtract off the ice bath temperature voltage for
your thermocouple, and sometimes it would be impossible to remove a thermocouple from its
installation to check the ice bath voltage. However, we can use a property of thermocouples that
allows us to add their voltages in series to automatically subtract the voltage using two thermocouples.
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Disconnect the constantan wire from the myDAQ; the copper wire should remain in its
terminal.
Place the second copper wire in terminal ai0-, leaving the jumper intact.
Twist the free constantan end to the free copper end. Your circuit now should be connected as
follows: AGND – jumper – ai0- – copper wire – T thermocouple junction – constantan wire – T
thermocouple junction – copper wire – ai0+.
Now, place the thermocouple with the copper wire connected to terminal ai0+ in an ice bath
and place the thermocouple connected to terminal ai0- in a room water bath.
Record the voltage measurement and look it up in the table.
lab completion d (2 points): How does this temperature compare with the temperature of the room
water bath measured using the reference thermometer?
Note that this type of thermocouple arrangement is called a thermopile. Although they are useful, we
will no longer use the thermopile in this lab.
1.5 Using the type-T thermocouple built into LabVIEW
LabVIEW is able to automatically convert measured voltages to temperatures (it has access to
built-in chart data so that you do not need to cross reference voltages to a chart to determine
temperatures)
 In the same code you’ve been working with, double-click on the DAQ Assistant to bring up the
configuration window
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Laboratory 4
Thermocouples
Highlight the “voltage” measurement channel and click the red X to delete it
Click the blue plus to “Add channel”
Select “Temperature”, then thermocouple and channel ai0
In the configuration window that comes up, set the maximum temperature to 200 °C and the
minimum temperature to -20 °C.
Select “T” as the thermocouple type and make sure that “constant” is selected for the CJC (Cold
Junction Compensation) Source.
Connect only one hand-twisted thermocouple to channel ai0 as you did with the first part of this
exercise. Look under the DAQ assistant, connection diagram tab, to be sure you have hooked
the thermocouple up correctly (although this diagram does not show the jumper you need
between ai0- and AGND)
Measure an average ice bath temperature with this approach. The measurement may be a bit off
initially. You can improve it by adjusting the CJC value within the DAQ Assistant
configuration window. Repeat until the average is very near 0 °C. Measure the room
temperature water and make sure it agrees (within 1.5 °C) with the reference thermometer
value.
Finally, unplug your homemade thermocouple and attach the commercial thermocouple. Check
the ice water temperature and adjust the CJC value if needed.
lab completion e (2 points): What value of CJC did you settle upon? When you now measure ice
and room-temp water with the commercial thermocouple, what is the average difference between
the measured and reference temperatures?
mini report 1: Determine the temperature precision interval [K] (P = 95%) of your thermocouple in
the room temperature bath, and compare it with the thermistor precision interval [K] in the same bath
at the same rate from the previous lab (mini report 5 in that lab).
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Using the commercial thermocouple, measure all 5 reference baths as you did in the previous
lab. Include sufficient averaging so that each temperature you measure in each bath has a
negligible precision interval. Negligible here means negligible compared to anticipated
systematic errors; a precision interval of 0.05 K or better (P = 95%) should be sufficient.
Record one data point (with a negligible precision interval) in each bath.
mini report 2: This is the preparation-for-report-writing component of this lab. Your goal is to
prepare a high-quality scientific plot. You will have to use Excel or other (non-LabVIEW) software.
Plot your 5 T_measured vs.T_reference datapoints along with a linear fit and the associated the xfrom-y estimate error bands. Example x-from-y error bands are shown as curves “c” in Figure 8.5
Dunn, and are computed using Equation 8.31 with n = 1 (one data point at each temperature on your
plot) and ν = N – 2. Use your x-from-y error bands to predict the temperature uncertainty of your
thermocouple for the 50 degrees C bath, following the approach associated with the vertical arrows
shown in Figure 8.5
mini report 3: Use the highest -order polynomial fit possible to fit your data. This can be done in Excel
using the “trendline” functionality. Describe which fit, the high-order polynomial or the linear fit, is
most appropriate for use as a calibration function for your thermocouple and why. Note: you may
need to plot your polynomial as x vs y to get the appropriate function.
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Based on your linear fit, program a calibration (a.k.a. correction) function into LabVIEW so that
LabVIEW displays the calibrated thermocouple result as well as the original thermocouple result. Test
the calibrated thermocouple in all 5 water baths.
mini report 4: Quantify how much better or worse the accuracy of the calibrated thermocouple results
are, compared to the original thermocouple results.
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