Lab 1 Instructional Manual

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University of Texas *

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Apr 3, 2024

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ME139L Experimental Heat Transfer Background and Guidelines for Lab #1 Characteristics of Temperature Sensors Two submissions are needed for the lab report. Check report template for details. (Please submit a copy of your results to your TA before leaving the lab. Photos of this manual with data can work.) Objectives 1) Understand basic thermocouple circuits and how thermocouple emf (electromotive force, i.e. potential difference between two electrodes) is converted to temperature. 2) Understand the characteristics of resistance temperature sensors, specifically Resistance Temperature Detectors (RTDs) and thermistors, and how to determine temperature with these devices. 3) Begin to understand the potential sources of sensor error and how to determine the resolution and “accuracy” of a temperature measurement. Background Temperature is the fundamental variable of interest in all thermal systems. A number of different types of sensors are available for measuring temperature, ranging from simple analog/mechanical thermometers to electrical and optical transducers. The purpose of this lab is to learn about temperature sensors and their inherent advantages and disadvantages relative to a given application. PreLab preparation (individual graded work) Complete the Pre-Lab assignment. Lab work (team-based) Work through the lab procedures given below, take individual notes of experimental results and observations, and answer the questions as you go along. Remember that you will be required to write a report on these results. Report (same team as above) During the lab, record your answers, notes, and observations. Merge your individual answers, notes, and observations into a joint team report. You may also include additional sections containing photos, sketches, calculations, and explanations. Copies of each group member's individual in-lab notes are to be attached as an appendix to the team report. No individual credit will be assigned to members whose notes are not included. 1

Equipment checklist Identify and check off the following equipment needed for this lab. Table 1 List of equipment needed in Lab 1 Check Number Description 1 Keithley 2700 multimeter/data acquisition system (Keithley) 1 Omega HHM290 "Supermeter" (Omega SM) 2 Thermocouples, homemade, Type K, soldered junction 2 Thermocouples, homemade, Type K, twisted junction 1 Thermocouple probe, Type K grounded junction, 1/4" D, Omega KQSS- 14G-12 1 RTD probe (3-wire), Omega PR-13-2-100-1/8-12-E 1 Thermistor probe (2-wire), Omega 44004 1 Thermometer, dial type, Omega H-0-100C 1 Thermometer, glass type, Omega GT-736570 2 Thermos bottles, stainless steel 1 Thermos bottle, plastic 2 Test leads, banana/banana connectors (1 red, 1 black) 4 Alligator clips 1 K-type connector lead, standard-to-mini plug Signal conditioning/readout devices Any transducer that produces an electrical signal requires a signal conditioning and display unit. We will use two such systems, an Omega "Supermeter" (price $250) and a Keithley 2700 6 1/2 digit multimeter/data acquisition unit (price $2500). From their respective spec sheets, determine the resolution and accuracy of each of these instruments. Assume that you are measuring the temperature of water at nominally 50 o C with a K-type thermocouple, an RTD, and a thermistor. Use the thermocouple, RTD, and thermistor tables to determine what voltage and resistances correspond to 50 o C . Table 2 Resolution and accuracy for both meters Meter Resolution (mV) K-Type TC Accuracy (mV) K-Type TC Resolution (  RTD Accuracy (  RTD Resolution (  Thermistor Accuracy (  Thermistor Supermeter 0.01 mV 0.015 mV 0.01  0.39  0.1  2.7  Keithley 100 nV 0.0035 mV 0.001  0.018  0.001  0.087  * The same table as shown in prelab. You can use the values in this table directly, and there is no need to show the calculation procedure again. The temperature sensors that we will use are based on measuring changes in voltage (typically mV) and resistance (ohms) for a given temperature change (  T). For each of the sensor types listed below, calculate the resolution in  C for our two signal conditioners based on the approximate sensitivities shown below. Use the resolutions in mV or  determined in Table 2 as a basis for these calculations. 2

Table 3 Resolution in C for both meters Sensor type/sensitivity Resolution (  C) for Omega Supermeter Resolution (  C) for Keithley Thermocouple-Type K Platinum resistance RTD Thermistor Calculations: Preparation of temperature baths Prepare three temperature baths: 1. Ice/water mixture (stainless steel thermos) 2. Tap water (plastic thermos) 3. Hot water (stainless steel thermos) Measure the temperature of each bath with both the dial and glass thermometers and compare the results. Table 4 Measured bath temperatures using thermometers Bath Temperature (  C) dial thermometer Temperature (  C) glass thermometer Ice water 6 7 Tap water 25 (every marker is 2) 24 (every marker is 0.5) Hot water 92 92 Based on resolution and accuracy given below for these two thermometers, can you say that the two devices agree? Within what range of uncertainty? Resolution dial = 1 o C, Resolution glass = 0.5 o C,  T dial = 1% reading o C,  T glass = 1 o C 3

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Basic thermocouple circuit construction Should the copper and steel connectors used to form this circuit affect the output readings of the chromel-alumel thermocouples? Explain why or why not, and under what conditions. Basic thermocouple circuit tests You are to test the basic thermocouple circuit under three conditions: 1. Test 1: T1 = tap water, T2 = ice water 2. Test 2: T1 = hot water, T2 = tap water 3. Test 3: T1 = hot water, T2 = ice water For each of the three conditions, measure temperature of the water baths with the glass thermometers. Record the thermocouple emf with both the Keithley and the Omega SM for each test condition before proceeding to the next condition. Calculate the value of T1 based on the tables for Type K thermocouples using the Keithley measurements. Table 5 Measurements using glass thermometer and thermocouple Test T1 (  C) T2 (  C) emf (mV) Keithley emf (mV) Omega SM T1 (  C) thermocouple 1 23 6 .90 .0006V 2 73 21 2.17 .0020V 3 73 7 2.9 .0028V Calculations: 4 Connect the red (alumel) wires of the two homemade Type K soldered junction thermocouples using an alligator clip as shown in the picture to form a basic hot/cold junction thermocouple circuit. The yellow (chromel) wires should be connected to the readout instrument with the banana/banana test leads: the T1 (hot side) junction to the red lead (+ input) and the T2 (cold side) junction to the black lead (- input).

Does the construction of the junction make a difference in the readings? Repeat Test 3 using the twisted junction homemade thermocouples and compare your results with the soldered junction tests above. Table 6 Soldered vs twisted thermocouples Junction type T1 (  C) T2 (  C) emf (mV) Keithley T1 (  C) thermocouple Soldered 67 4 2.75 Twisted 67 4 2.7 Based on your observations, would you say that the method of construction of the junction affects the accuracy of thermocouple temperature measurements? What other factors besides temperature measurement accuracy might influence your choice of one junction type over another? Violating the "law of intermediate metals" With the circuit still in the Test 3 configuration (T1 = hot, T2 = ice), reconnect the junction between the yellow thermocouple wire and the red test lead with an alligator clip. With your hand, grip the junction between the alligator clip and the yellow wire for about 30 seconds and observe the emf reading on the Keithley meter. Release your grip and continue to observe it for several more minutes. Describe and explain your observations: 2.7 without hand 0.1 with hand it goes down when the hand is holding it 5

Direct thermocouple measurements using the Omega Supermeter The Omega Supermeter (and in fact, the Keithley as well) can accept direct thermocouple inputs and display the temperature directly in either  C or  F. Connect the 1/4" grounded junction Type K thermocouple probe to the T1 input on the top of the Supermeter and measure the temperatures of the three baths. Compare the Supermeter readings to those on the glass thermometers. Table 7 Measurements using glass thermometer and direct thermocouples Bath T(  C) thermometer T(  C) thermocouple with SM Ice water 4 3.6 Tap water 24 23.5 Hot water 60 59.1 Do the temperatures agree within the specified accuracy limits of the Supermeter and thermometers? Ref: Temperature measurement section in Omega Supermeter Spec.pdf How is it possible for the thermocouple to work without a reference junction? Is there, in fact, a reference junction in the circuit, and if so, where is it? How does the meter know what the temperature of the reference junction is and what does it do to translate this information into a temperature reading? 6

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When you have completed this section, you should carefully disassemble and replace the thermocouples in the Temp Kits. 7

Resistance thermometry (RTDs and thermistors) Using alligator clips and the red and black test leads, connect the thermistor to the Keithley Multimeter, and measure the resistance for each of the three baths. Use the thermistor calibration table to determine the temperatures and compare with the readings on the glass thermometers. Note: Omega SM can also measure the resistance, but it is optional in this part since Keithley is more accurate. Feel free to try Omega SM if you have enough time and the values should be comparable to what you have got using Keithley. Thermistor Table 8 Measurements using thermometer and thermistor Bath T(  C) thermometer Resistance (  ) Keithley T(  C) Keithley Ice water 1 6.72KOhm Tap water 24 2.2KOhm Hot water 56 0.66KOhm Connect the RTD sensor connections (red and +) to the two instruments and repeat the above experiment. RTD Table 9 Measurements using thermometer and RTD Bath T(  C) thermometer Resistance (  ) Keithley T(  C) Keithley Ice water 2 102Ohm Tap water 24 0.11KOhm Hot water 52 0.12KOhm Based on these results and the specifications for the two sensors, what can you conclude about comparative temperature sensitivity of thermistors vs. RTDs? Which of these two devices would you choose for measuring human body temperatures? Which would be more suitable for measuring oil temperature of an engine? 8 Two types of resistance temperature sensors will be used in this lab: a platinum-element resistance temperature detector (RTD) and a semiconductor thermistor. The RTD has three connections (red and + to the sensor and - to a dummy lead wire), while the thermistor has only two (both to the sensor).

Effect of lead wire resistance on temperature measurement with RTDs In the previous experiment, you noted that RTDs have a much lower resistance and temperature sensitivity than thermistors. Thus, the resistance of lead wires connecting the sensor to the measuring instrument can introduce significant errors. To demonstrate this point, use the Keithley multimeter to measure the total resistance of the black and red instrument lead wires (hook them end-to-end and plug the two ends into the meter input terminals). R INSTRUMENT LEADS = 0.02Ohm Now measure the total resistance of the instrument leads plus the internal RTD lead wires by connecting the instrument leads with alligator clips to the + and - outputs of the RTD (this bypasses the platinum resistance sensor at the tip of the probe and measures only the internal lead wire resistance). R INSTRUMENT LEADS+INTERNAL LEADWIRES = 1.46Ohm Therefore, taking the difference R INTERNAL LEADWIRES = 1.44Ohm Correct your resistance readings for the RTD experiment carried out above (Keithley data only) to account for the internal lead wire resistance plus the instrument leads and recalculate the temperatures. Table 10. RTD corrected for internal lead wire resistance Bath Original Resistance (  ) Keithley Original T(  C) Keithley Corrected Resistance (  ) Corrected T(  C) Leadwire  T error (  C) Tap water Lead wire error is a common concern in RTD measurements and various techniques are used to compensate for it. However, it doesn't seem to be a problem with thermistors. Can you explain why? Congratulations! You have completed Lab #1. Let’s make your first one a really good lab report! 9

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