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CONSTRUCTION of a THERMOCOUPLE THERMOMETER by Jim Roberts, Professor of Physics and Material Science the University of North Texas

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CONSTRUCTION of a THERMOCOUPLE THERMOMETER by Jim Roberts, Professor of Physics and Material Science the University of North Texas SUPPORT MATERIALS EXploriNG SCIENCE COLLECT AND ANALYZE DATA USING THE CASIO FX-9750 AND EA-200 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 fx-9750G 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. 6 4 2 0 10.7 16.8 24.5 33.4 43.1 53.4 63.9 75.3 86.1 -2 -4 VOLTAGE -6 -8 -10 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 fx-9750G 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 fx-9750G 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) 2.4 2.3 2.2 2.1 2 1.9 VOLTAGE 1.8 1.7 1.6 1.5 14.8 19.1 23 28 33.6 39.5 46 52.7 59 64.9 TEMPERATURE (C) Figure 4. (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 fx-9750G 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. STUDYING VOLTAGE GENERATION BY THE FORCE OF GRAVITY By Jim Roberts, Professor of Physics and Material Science The University of North Texas OBJECTIVE: This activity is designed to see how we can generate electricity using gravity, to collect data with a Data Collector/Analyzer and display the data on a fx-9750G graphing calculator and to quantify the results.
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