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CONSTRUCTION OF A THERMOCOUPLE THERMOMETER
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CONSTRUCTION OF A THERMOCOUPLE THERMOMETER
By
Jim Roberts, Professor of Physics and Material Science
The University of North Texas
OBJECTIVE: This experiment is designed to show you how to construct a thermocouple or a device made of two (couple) of dissimilar metals that can produce a voltage when heat is applied, to collect voltage data, to plot the results on a CFX-9850GC Plus graphing calculator and make a thermometer using the results.
INTRODUCTION
There are two types of effects that arise when two dissimilar metals are
brought in contact with each other and the temperature is changed at the junction.
One effect produces an electrical potential (Seebeck effect) when heat is applied
and the other effect is to cool the junction when a current is passed through the
junction in the proper direction (Peltier effect). These two effects can be very
useful. Since the voltage at the junction depends upon the temperature of the end
points, we may generate a voltage by heating one junction while holding the other
constant in temperature, a source of electromotive force. The other effect is to
make a cooling device, a refrigerator, by passing a current through the junction in
the proper direction.
In figure 1 is shown a thermocouple. This is the structure of a commercial
thermocouple that is capable of sensing temperature changes at the junction. A
very practical usage of the thermocouple is to control the safety of gas delivering
systems. This is done by allowing the voltage produced to control the valve that
delivers gas to the burners. If the burner goes out, there is no heat to the
thermocouple and the voltage drops to release the valve and close off the gas flow,
thus preventing a potential explosion. All gas systems are now required, by law, to
have such safety valves.
The device shown in figure 1 can be produced by the use of two electrically
dissimilar metals such as copper and iron or copper and constantan. When the
device has been constructed, it can be calibrated to read voltage and convert this
into a temperature scale. If the amplifier gain is high enough the voltage can be
read using an EA-200 to collect the data points. When this has been completed,
the data are transferred into the graphing calculator for processing and testing for
linear behavior over the temperature range of interest.
Figure 1. A thermocouple mounted into a finger and the thermocouple wires and junction exposed so it can be seen what is in the sensing probe.
PROCEDURE
Construct your thermal junction by twisting a copper and an iron or
constantan wire together at the ends to form a closed loop. Cut the copper wire in
two pieces at the center and clean the surface to make good electrical contact with
the iron. The voltmeter mode of the EA-200 can now be used to measure any
potential difference at the terminals.
Figure 2. Schematic of the thermocouple set up for making a thermometer using the voltage generated by a thermocouple. The signal from the thermocouple was amplified to raise the voltage output and to match impedance to the EA-200 Data Collector/Analyzer
Place the thermocouple in a small test tube to isolate it from water. If the
temperature of air is to be measured, the isolation test tube is not needed. The
tube is to isolate the junction from electrical interaction with the water. Place the
thermocouple in the tube in about 250 ml of water in a beaker and place the beaker
on a hot plate. Connect the voltage probe to the terminals of the thermocouple to
warm it over a range of temperatures. You can also change the temperature by
using a hair dryer to blow hot air over the temperature probe and the thermocouple
junction. Put the temperature probe in channel 2 with the voltage probe in channel
1. You are ready to collect temperature and voltage data so the thermocouple
voltage can be calibrated to become temperature.
Figure 3. Left. An Excel plot of voltage versus centigrade temperature for the thermocouple with a reference bath of ice and water to reference to 0 degrees centigrade. Right. A picture of the CFX 9850 GB Plus calculator display window. The bold line is produced by equation 1 below.
Usually the voltage produced by the thermocouple junction is linear over a
reasonable range of temperatures. When the data has been transferred to the
CFX-9850 GC Plus graphing calculator it is tested for linear behavior. When you
finish the plot the statistics for a linear least squares routine can be analyzed to
determine how well the data fit a linear response by the r2 value. If the data do not
fit a linear response, the graphing calculator function for X2 is used. The number r2
should be very close to 1. The values change from +1…..-1. If the number is +1
the data has a perfectly linear response and the data are well correlated to a linear
fit. If the value is -1 the data and a linear response are dis-correlated and the data
have the greatest departure from a linear response.
The data in figure 1 were fit by equation 1 as given below:
T = -7.0592V + 33.524 (1)
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
14.8
19.1
23 28 33.6
39.5
46 52.7
59 64.9
TEMPERATURE (C)
VOLT
AGE
Figure 3. Left. An Excel plot of the amplified voltage versus centigrade temperature for the thermocouple with a reference bath of ice and water to reference to 0 degrees centigrade. Right. A picture of the CFX 9850 Ga Plus calculator display for the same experiment. Note the curvature is more pronounced and the data fit a quadratic model. This effect is due to the nonlinear property of the amplifier. The pictures were taken with a QV-7000-SX digital camera.
T = -104.849V2+331.175V -196.23 (2)
The thermocouple can be used to measure temperature by making a
voltage measurement and converting to centigrade temperature by using equation
1. The voltage depends on the gain of the amplifier so each unit must be calibrated
for the amplifier used in the measurement. The thermocouples used were obtained
from a hardware store and produce up to 30 millivolts when heated with a blue gas
flame. They serve as safety devices in conventional hot water heaters.
SUMMARY
The temperature sensor needs to be calibrated against a reference. This
may be ice and water at standard pressure or by use of a reference voltage against
which the instrument is calibrated. Both procedures were tested in this experiment.
The type thermocouple chosen must be one that will produce sufficient voltage to
activate the meter used to measure the output voltage. Of an amplifier is used, any
nonlinear response must be considered for the instrument to be accurate.
QUESTIONS
1. Can you save money by taking the energy from the Sun and converting it into
electricity by using a thermocouple? Discuss the costs involved in providing such
energy, if you answered yes to the question.
2. In the thermocouple part of this experiment you learned about converting heat to
electricity. Discuss how this may be done efficiently by using the Sun's energy.
Recall that focusing the rays of the Sun will multiply the heat energy falling on a
given area.
3. The apparatus shown in figure 1 of this exercise is a pyrometer or a device for
determining temperature. Discuss how you think this thing works.
4. One of the properties of nature is that if one process works, the reverse is true.
That is, the generator of electricity produces electricity when a magnet is moved in
a coil of wire. (The generator rule.) The inverse of this is that a current through a
wire will cause a magnet to move. (The motor rule.) Since heating the
thermocouple junction produces a voltage, might a current through the junction cool
it? Look up the Seebeck Effect and the Peltier Effect on the internet and discuss
these in the light of the “two faces of scientific processes”.
5. How many thermocouples of the composition studied above will need to be
placed in series to light a 120 volt light bulb?
6. Since the energy from the Sun can be used to heat the thermocouple to produce
electricity in the day time, discuss how we can store this electrical energy to be
used when the Sun is not shining.
7. The reference junction of the thermocouple system needs to be kept at a fixed
temperature to provide a reference for the second junction. Describe how the fact
that the temperature of the soil at the surface of the Earth relative to a few feet
below the surface is several degrees higher can be used to provide a temperature
change that can drive the thermocouple system.
8. Based on what you learned about the voltage potential, how much voltage can
be produced by the temperature difference found at a depth of one meter relative to
the surface temperature. Determine the temperature difference by using a
temperature probe and the EA-200 Data Collector/Analyzer.