EXPERIMENT 6 PHYSICS 250 THERMAL MEASUREMENTS

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1 EXPERIMENT 6 PHYSICS 250 THERMAL MEASUREMENTS Apparatus: Electronic multimeter Iron-constantan thermocouple Thermistor Hot plate Electronic thermometer with two leads Glass beaker Crushed ice Methyl alcohol Insulated cup CAUTION: The hot plate retains its heat long after the power is turned off. Always treat it as if it were very hot. Introduction Since so many physical and biological phenomena are sensitive to or depend on the temperature of the surroundings, the measurement of temperature is of importance in many areas of science. The common mercury-in-glass thermometer is very simple and inexpensive, but it is not very versatile and has a limited range of usefulness. During this laboratory period you will study the operation of two devices that are widely used in science to measure temperature: (1) the thermocouple and (2) the thermistor. Both devices involve an electrical measurement, are very simple in their construction, and can be considered to be temperature transducers. It is important to understand the limitations and the advantages of each. You must know the electrical response function that relates the voltage from the thermocouple to the temperature or the function that relates the resistance of the thermistor to the temperature. The first thing you will do in this experiment is to determine the response functions by reference to a standard thermometer (actually a thermocouple with a calibrated meter). This determination provides a calibration of the device. For the iron-constantan thermocouple, standard calibration tables are available that are accurate to approximately 0.5 percent, and these tables will be available in the laboratory. You may check two or three points to verify this calibration. Each individual thermistor is somewhat different and must be individually calibrated. With modern electronic meters, the measurements are very simple to make with good precision. It will not be necessary to understand in detail why the devices measure temperature, but it will be necessary to understand how to make the measurement and how to interpret the electrical data in terms of temperature. Each device will first be briefly described, and then some details of how to use it will be given. After you know the calibration curves for both the thermistor and thermocouple, you will 6-1

2 make a variety of simple measurements to illustrate their use. Once more, you will not be given detailed instructions but are encouraged to learn through investigation. A. The thermocouple A thermocouple is simply two wires of different metallic composition welded together to form a junction. The energy of the electrons within each metal is different, and this energy difference depends upon the temperature of the point where the junction is made. The energy difference can be measured as a voltage difference by using a very sensitive voltmeter. Since the metal used in the terminal wires of the voltmeter is different from the thermocouple wires, it is necessary for high precision measurements to have two junctions as shown in Fig. 1. One junction is placed at a point where you want to measure an unknown temperature, and the second junction is placed at a reference temperature (generally 0ºC). Note: If both junctions are at the same temperature, a zero voltage is measured. If the junction at an unknown temperature is hotter than the junction at the reference temperature, a positive voltage is detected. You will use thermocouples made of iron and constantan, a special alloy of nickel and copper. These metals develop a relatively large EMF compared to other metals, but Voltmeter the EMF is still so small that a 25ºC difference is required to yield one millivolt. The thermocouple has several significant advantages over a glass thermometer that should be emphasized: 1. It measures temperatures over a very broad range between -270ºC and +1300ºC (and higher depending upon the metals used). 2. It is very small and can thus pinpoint temperatures at different locations. 3. Because it involves an electrical measurement, the recording can be remote from the temperature region. These features will be illustrated in the experiments to follow. reference temperature unknown temperature metal 1 metal 2 metal 1 Figure 1. Illustration of the use of a thermocouple to measure temperatures. 6-2

3 B. The thermistor A thermistor is simply a small piece of semiconductor material with metal contacts, as illustrated in Fig. 2. The electrical resistance as read directly from a meter changes with temperature. The thermistor, unlike the thermocouple, has a very limited range between -20ºC and +150ºC. The thermistor is relatively small and is electrical in nature and thus has the advantages b and c listed above for the thermocouple. The thermistor, however, is very sensitive and allows you to measure temperatures to within 0.01ºC or better with relative ease. This sensitivity is the feature that makes the thermistor very valuable. You will first calibrate your thermistor at several points between 0ºC and 100ºC by reference to your standard thermometer. After determining the calibration curve at a few points, you can determine any temperature by Figure 2. The thermistor. interpolation between these points, but a curve-fitting procedure outlined below is more appropriate. The determination of a temperature vs. resistance calibration curve for the thermistor represents an excellent example of the use of curve-fitting techniques guided by a theoretical model. The resistance R of a piece of semiconductor is inversely proportional to the conductivity of the material, and the conductivity is proportional to the number n of free charge carriers in the material. For a semiconductor, n is given by an expression n = n e -E/KT where T is the absolute temperature and E is an electronic energy level difference. Thus, you theoretically expect the resistance to obey an equation of the type A + E / KT +B / T R = = A = A, (1) - E / KT e e e where, for each specific thermistor, A and B are constants that reflect the dimensions of the piece of semi-conductor and the type of semiconductor used in the construction of the thermistor. Based on the above analysis, ln R = ln B A+ T Thus a plot of ln(r) vs 1/T is a straight line while the plot of R vs 1/T will be an exponential curve. Using P-Lab or Excel you can determine the parameters A and B by either entering 1/T as x and ln(r) as y and fitting to a straight line or by entering 1/T as x and R as y and fitting to an exponential curve.. (2) 6-3

4 You can gain insight into why the thermistor has high sensitivity by mathematically determining the ratio For the thermistor used, the value of A is approximately 0.02 ohms and R at room temperature is approximately 3000 ohms, so ln(r/a) for measurements near room temperature is approximately 10. Using the electronic meters available, you can determine R with a precision of R/R of approximately Thus you can obtain the temperature T with a precision T/T of approximately If T 300 K, T will be approximately K. You should realize that equation (3) does not give the absolute value of T with great accuracy since the values of A and B are not truly accurate; nevertheless, using the empirical equation (3) allows you to determine small differences with great precision. One specific objective during this laboratory period is for you to demonstrate the sensitivity of the thermistor by making a few measurements of very small temperature differences. This problem illustrates a significant concept in experimental measurement. If you desire to know only how much one value differs from another (that is, if only the difference is important), a highly sensitive but not highly accurate measuring device is necessary. This example helps clarify the difference between precision (sensitivity) and accuracy (fundamental calibration). The thermistor you have calibrated is a very precise, sensitive device, but its accuracy is dependent upon the accuracy of your calibration technique, which is not very good. Nevertheless, you can measure small differences very well. A second idea in measurement techniques becomes apparent as you attempt these precise measurements. It is foolish to try to determine a temperature to a precision of 0.01ºC if the temperature you are measuring is changing in time or is different in different locations by 0.1ºC or more. Stability and equilibrium are important and necessary. Objective: To become acquainted with two very commonly used temperature-measuring devices, the thermocouple and the thermistor. Procedure T T You have four temperature probes. You will need to be able to distinguish between them. Take a moment before you start to be sure you have no questions. Thermistor: The thermistor has a double banana plug with a single lead (twisted wire of two colors with a clear insulator around them). The thermistor probe has a glassy appearance. Two-couple thermocouple: This has a double banana plug attached to two leads, one short and one long. The short lead is the reference lead and the long lead is the probe. Standard digital thermometer probe: d T T = R ln A d R R. (3)

5 This has a small plug (with a red dot) for use with the digital thermometer and a plastic coating on the wires down to the small exposed junction at the end. Note that the digital thermometer is also a thermocouple, but it relies on an internal electronic reference rather than a second junction. Ceramic digital thermometer probe: This is similar to the standard digital thermometer probe, however, there is a white ceramic material near the junction and the dot on the plug is blue. The ceramic protects the plastic insulation. However it radiates enough heat to make measurements slightly less accurate than with the standard probe. AT TEMPERATURES ABOVE 100ºC, BE CAREFUL TO USE ONLY THE CERAMIC DIGITAL THERMOCOUPLE PROBE AS HIGH TEMPERATURES WILL DAMAGE THE PLASTIC INSULATION ON THE OTHER PROBES! A. Calibrating the temperature probes. 1. After getting acquainted with the multimeter, use hot water and cold ice water and check the calibration for the thermocouple at five or six points between 0ºC and 100ºC. You will note that the electrical response function is almost but not perfectly linear. Crushed ice and a hot plate are available. Be careful with the hot plate. It may be VERY hot from previous use! 2. Establish five or six points which relate R vs T for the thermistor between 0ºC and 100ºC. It is usually best if this is done simultaneously with number 1 above - be sure that you have the multimeter set for the proper type of measurement as you switch between the thermocouple and the thermistor. First plot R vs T for these points. Note that the relationship is very nonlinear. Now plot ln(r) vs 1/T or R vs 1/T as outlined above (remember that T must be in Kelvin). Use P-Lab or Excel to determine the calibration constants. When you are measuring the resistance of the thermistor for calibration purposes you should use the ohmmeter rather than the Wheatstone bridge. Only use the Wheatstone bridge when you are trying to measure very small changes in resistance. B. Using a thermistor to measure small temperature changes. To measure small temperature changes, a special Wheatstone bridge is provided. Connect the thermistor and the multimeter to the leads marked. Put the thermistor in the initial configuration and let it come to thermal equilibrium. Turn the dial of the variable resistor until the multimeter reads zero on its most sensitive voltage scale. After making the small temperature change, note the reading of the meter. As time permits, try several of the following (you should have time to try at least one): 1. Measure the rise in temperature due to the heat of mixing of 1 cc alcohol in 100 cc of water at room temperature. A special tube with a removable stopper is available for this measurement. a. Put 100 cc of water in a beaker. 6-5

6 b. Put 1cc of methyl alcohol in the special tube provided. This volume is indicated by the yellow line. Be sure the rubber stopper is in the end of the tube nearest the yellow mark. c. Place a glass stirring rod in the tube with the alcohol. d. Place the tube into the beaker without releasing the alcohol. e. Allow everything to come to the same temperature as the water. This will take several minutes. f. Carefully push the rubber stopper out of the tube. Avoid agitation which could add heat to the system. 2. Measure the temperature rise due to dropping one drop of hot water into 50 cc of water at room temperature. 3. Measure the change in the boiling temperature due to a small but measured amount of impurity. 4. Place a few chunks of ice in a beaker with water and without stirring measure the temperature at several different points in the water. Do not spend too much time on this measurement. You should see a serious problem of lack of stability involved in these measurements at the level of sensitivity of the thermistor. C. Measuring temperature extremes with thermocouples. 1. If time permits, use the ceramic digital thermometer probe to measure the temperature of the hot plate. You may try measuring this temperature as a function of different heat settings or you may try measuring the temperature at different locations on the plate. 6-6