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Experiment setup for thermocouple calibration
Objectives
The objectives of this experiment are: (1) to calibrate two thermocouples, and (2) to examine some
of the properties and behavior of thermocouples. In Part A, you will calibrate two thermocouples by
comparing induced voltages measured with a data acquisition (DAQ) system and temperatures measured
with a thermometer. In Part B, you will examine the effect of inserting an intermediate metal into a
thermocouple. Finally, Part C will more closely examine the relationship between the voltages and
temperatures of a thermocouple.
1 Introduction: The Thermocouple
The thermocouple is the most common type of temperature sensor, primarily because it is
inexpensive and easy to use. In fact, it is used in many places familiar to you: in the home, it is used to
control the temperature of the furnace, water heater, and the kitchen oven; in the automobile, it is used to
monitor coolant and oil temperature, and even to control the air conditioner. It is not the most accurate
technique available to measure temperature – typical thermocouples are accurate to around ±0.5°C – but
for many applications this accuracy is acceptable.
Thomas Johann Seebeck (1770-1831) discovered that a circuit comprised of dissimilar metals
produces a voltage (and current) when the two dissimilar junctions are exposed to different temperatures.
This phenomenon, called the Seebeck Effect, is depicted in Figure 2.1. The voltage produced is
proportional to the temperature difference between the junctions. The voltage produced is small, on the
order of millivolts, so it is not very suitable for producing power1. But the device can easily be calibrated
to measure temperature.
1
If you put enough thermocouples together, and expose them to a high enough temperature difference, you can
generate enough power to, say, power a space probe. This is called thermoelectric power (or a thermopile), and is
used in deep space probes where solar power is not sufficient.
metal A
T2 ≠ T1
T1
metal B
metal B
voltmeter
leads
voltmeter
Figure 2.1. Thermocouple circuit connected to a voltmeter.
In order to calibrate a thermocouple, one of the junctions must be held at constant temperature.
For the sake of simplicity, let us set the reference temperature, T1, to be 0°C. This can easily be achieved
by submersing the junction in a bath of ice water, commonly called an ice bath reference. In Part A of this
experiment, we will use this technique to calibrate two thermocouples. Later in this course, we will
develop a simpler method that eliminates the ice bath.
There are many types of thermocouples, categorized by the two dissimilar metals used. Appendix A
lists the output voltage as a function of temperature for three common types of thermocouples. This data
is published by the National Institute of Standards and Testing (NIST), and is available through textbooks
and vendor catalogs. Note that these thermocouples are referenced to an ice bath.
2 Experiment
Part A. Thermocouple Calibration
Equipment
Thermocouple Kit, including:
Glass thermometer
24-gage type J (red/white wire) and type T (red/blue wire) thermocouples
Thermos
“hot hands” mitt
Data acquisition (DAQ) system
electric hot plate
two 250 ml beakers
chemistry stand
Procedure
1. Make an ice bath by filling the Thermos with crushed ice and water.
2. You are given two thermocouples, each made from 24-gage wire: one type J (Iron-Constantan)
and one type T (Copper-Constantan). For both thermocouples the Constantan wire is the wire
with the red stripe, and for both it is the negative lead. Connect these thermocouples to the
voltage terminals on the DAQ system labeled Channel 5 and Channel 6. WARNING: Record
which type thermocouple (type J or T) is connected to which channel.
3. Place the two junctions leading from the positive terminals in the ice bath. Wrap the remaining
two junctions together with the bulb of the thermometer and place them in the beaker of water
using the chemistry stand so that all three are approximately 2 inches above the bottom of the
beaker. None of the three sensors should touch the side or bottom of the beaker, and make sure
the bare wires of the thermocouples do not touch each other: otherwise they will short out.
Finally, make sure that the water level reaches the solid line on the thermometer, about an inch or
two above the bulb; this line is called the calibration line, and will ensure the accuracy of your
temperature readings. Refer to Figure 2.2 for the complete experimental layout.
blue wire
thermometer
red
red
white
Calibration
Line
red
red
- + - +
DAQ System
ice bath
hot plate
Figure 2.2. Experimental apparatus for thermocouple calibration. NOTE: If the
recorder does not read a positive voltage from one or both of the thermocouples,
check that the junctions leading from the positive terminals go to the ice bath.
4. Log into the computer using the login name “thermal” and the password “thermal.” Begin the
DAQ software program by double-clicking on the “THERMAL LAB” icon. For more
information about the DAQ software, refer to Appendix B.
5. Configure the software by selecting the following options (many of which should be the default
options):
Channel A:
Channel B:
Sample Rate (ms):
Oversampling (scans/sample):
Scan Rate (scans/s):
Save Data:
File Name:
CH5 Voltage (50mV)
CH6 Voltage (50mV)
1000
10
1000
ON
C:\usrtmp\data.txt
The software will count the data point being recorded (saved as “data point” number) and
the readings from the two channels every second. DON’T FORGET TO NOTE WHICH
THERMOCOUPLE IS CONNECTED TO WHICH CHANNEL!
6. You are now ready to begin collecting data. Record the current water temperature indicated on
the thermometer. Start the DAQ software by clicking the run button at the top left corner of the
window ( ). Data point zero (0) in the saved data file will then correspond to the initial water
temperature.
7. Begin heating the water, setting the hot plate to 50% power. At increments of approximately
10C, record the “Data Point” number from the DAQ software and the corresponding temperature
indicated by the thermometer. Later, you will match your recorded temperatures to the recorded
voltages in the file by using the data point number you wrote down.
8. At about 80 C, increase the hot plate setting to high. Continue taking data for about a minute
after the water begins boiling. Record the temperature of the water at full boil, and write down the
data point number. Then, halt execution by pressing the “STOP” button on the front panel.
9. The data recorded by the DAQ system will have been saved in the file “data.txt” in the
“C:\usrtmp” directory. The first line will have the following headings: “Data Point”, “Time (s)”,
“Channel A”, and “Channel B.” Open this file in Excel to verify that your data has been recorded.
10. Copy the file to a 3.5-in diskette, zip disk, or USB flash drive, or e-mail the file to yourself and
your teammates through the internet.
Part B. Effect of Intermediate Metal
Equipment
Digital thermometer
Ice bath
Type K (chromel-alumel) thermocouple with quick connector on one end.
Procedure
In this part of the experiment, you will use a thermocouple together with a digital thermocouple
reader to measure temperature. However, the thermocouple itself has been modified by splicing a copper
wire into one of the legs of the circuit. The effect of this modification will reveal several key behaviors of
thermocouples.
1. Take the thermocouple (the one with the yellow quick-connect plug) and plug it into the digital
thermometer.
2. Record the temperature reading for the following conditions:
a. All junctions exposed to room-temperature air.
b. End junction exposed to the ice bath. When finished, dry off the junction and let it come
back to room temperature.
c. Place one of the splice junctions into the ice bath as shown in Figure 2.3, and record the
temperature again. Remove the junction, dry it off, and let it return to room temperature.
d. Repeat step c for the other junction.
e. Repeat step c, this time submerging the center of the spliced wire into the ice bath, taking
care not to submerge either junction.
f. Repeat step c once more, this time submerging both junctions into the ice bath.
end
junction
splice
digital
thermometer
ice bath
Figure 2.3 Experimental set-up for Part B.
Part C. Effect of Junction Temperatures on Voltage Output
Equipment
Data acquisition (DAQ) system
Two beakers
Ice bath
Hot plate
Glass thermometer
Type J thermocouple
Procedure
In this experiment, you will more closely examine the effect of junction temperatures on the
thermocouple voltage using water at three known temperatures: tap, boiling, and ice-water
temperature.
1. Connect the J-type thermocouple (red and white wires) to the voltage terminals on the DAQ
system labeled Channel 5.
2. Fill two beakers with tap water. Heat one of the beakers of water to boiling. Use the ice bath from
Part A for the third bath of water.
3. Reconfigure the DAQ software program as follows:
Channel A:
Channel B:
Sample Rate (ms):
Oversampling (scans/sample):
Scan Rate (scans/s):
Save Data:
File Name:
CH5 Voltage (50mV)
CH6 Voltage (50mV)
1000
100
1000
OFF
c:\usrtmp\eraseme.txt
(This is just a dummy file; you will be recording data
by hand.)
4. Start the DAQ software by pressing on the run button at the top left corner of the window (
).
The voltage reading is displayed in millivolts next to the legend in the upper right hand corner of
the window (see Appendix B). Use this display to record voltage readings.
5. Record the voltage for each combination of junction temperatures using the three baths: (a)
Boiling water and ice water, (b) ice water and tap water, and (c) boiling water and tap water. The
experimental set-up is shown below in Figure 2.4. You will use the same thermocouple for all
three measurements. Be careful to keep the polarity of the thermocouple as indicated in the
figure. (For example, when measuring V1, the junction leading from the negative port on the
DAQ system is placed in the boiling water. The other junction is placed in the ice bath. For V2,
the junction leading from the negative port on the DAQ system is placed in the ice bath.) At the
same time, record the temperatures of the three baths using the glass thermometer.
6. Finally, place both junctions into the tap water, and record the indicated voltage. Repeat for the
ice water and the boiling water.
7. Halt execution of the DAQ software by pressing the “STOP” button on the front panel.
a.
Boiling
water
Ice bath
-
+
V1
b.
Ice bath
Tap water
-
+
V2
c.
Boiling
water
Tap water
-
+
V3
Figure 2.4 Thermocouple configurations for Part C.