Objective: To calibrate a thermocouple and find the corresponding curve-fit correlation.

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1 MEE 390sp11 Laboratory Experiment #1: Application of Thermocouples for Sensing Instructor: Milivoje Kostic TA: Chris Edwards February 1, 2011 Part #1: Thermocouple Calibration Apparatus: Thermocouple, controllable bath, Bulb Thermometer, Multimeter, Ice bath Useful Links: K-type Thermocouple Table Specifications: Thermocouple: Omega make type K thermocouple Constant temperature bath: Haake A81, 115V / 60Hz / 1500 VA Max temperature 180 C, precision 0.1 C Bulb Thermometer: Max temperature 150 C, precision 0.5 C Liquid: Water circulated to maintain constant temperature. Multimeter: Hewlett-Packard type 3478 or HP 34401a digital multimeter, 1 mv resolution Ice bath: Constant temperature 0 C Objective: To calibrate a thermocouple and find the corresponding curve-fit correlation. Theory: Thermocouples are based on the Seebeck effect. The Seebeck effect states when two dissimilar metal wires are connected with each other in a loop to form two junctions and maintained at two different temperatures, a change in voltage or electromotive force (emf) will be generated and the electrons will flow through the loop circuit. Refer to figure 1 to see a visual representation. The voltage drop will be proportional to the difference in temperature between the junctions and the metals used. Nickel-Aluminum T Desired 1 2 T IceBath Nickel-Chromium Nickel-Chromium emf (E DMM ) Figure 1: Design of thermocouple of two materials and two junction points The higher the temperature difference, the higher the emf, as well as the change in voltage in the loop. The magnitude of the emf is in the order of a millivolt or even fractions of a 1

2 millivolt. We need a precise and sensitive multimeter which can read up to microvolts to do this experiment Figure 2: Haake controllable temperature bath with thermocouple junction inset Legend: 1 On/Off switch 2 Cooling cycle start switch 3 Cooling cycle indicator light 4 Warning indicator light 5 Heating cycle indicator light 6 selection knob 7 Digital temperature display 8 set-in button 9 limit knob 10 Bulb thermometer 11 Hot water thermocouple wire 12 Ice bath thermocouple wire 13 Ice bath 14 Digital multimeter 15 Thermocouple junction (hot or cold) Procedure: 1. Check the level of water in the bath. Connect the apparatus to the power supply. 2. Turn on the water bath by switching the main switch (1) as shown in the figure. 3. Connect the ends of the thermocouple to the digital multimeter (DMM) and set the multimeter to read in millivolts DC. 2

3 4. Keep the set-in button (8) depressed and set the temperature to desired level (30 C to start) by turning the knob (6) and observing the display (7). Release the button (8) after setting the temperature. In normal mode, the temperature shown on the digital display is the actual temperature of the bath (T Bath ) against which the thermocouple sensor is to be calibrated. 5. Place one junction of the thermocouple into the ice bath and place the other junction (11) in the hot water bath. Then wait for few minutes for it to reach the steady state (i.e. the reading on the DMM steadies down except the last digit). Be careful to hold (tape) the sensor wire away from the circulator's propeller! 6. Write down the bulb thermometer (10) reading. 7. Write down the DMM reading in millivolts (E DMM ). 8. Repeat the steps 4 through 7 in steps of 5 degrees from 30 C to 60 C. Observations: Bath, T B ( C) Bath, T B ( F) Bulb T TH ( C) MM reading, E DMM [mv] corresponding to E, T TC [ C] corresponding to E, T TC [ F] For the Laboratory Report: 1. Plot the measured bath temperatures values (T Bath ) on x-axis against the corresponding thermocouple values (E DMM in mv) on y-axis. 2. Find the slope, intercept and the correlation coefficient of the curve-fitted line by any method. If the correlation coefficient is not very close to one, curve fit with higher order polynomial. NOTE: The DMM voltage reading of the hot water bath will correspond to the actual temperature of the hot water. This happens because the ice bath voltage reading is assumed to be 0 mv. However, your reading will never be exactly zero due to many reasons, for example: 1. The thermocouple is not perfect (make a short-circuit to check its zero). 2. The measured circuitry may be picking up some "noise," since it acts as an antenna, etc. 3

4 Part #2: Dynamic Response of a Thermocouple Sensor Apparatus: Thermocouple, controllable bath, Multimeter, Ice bath, Stopwatch (or similar timer) Useful Links: Specifications: Thermocouple: Omega make type T thermocouple Constant temperature bath: Haake A81, 115V / 60Hz / 1500 VA Max temperature 180 C, precision 0.1 C Liquid: Water circulated to maintain constant temperature. Multimeter: Hewlett-Packard type 3478 or HP 34401a digital multimeter, 1 mv resolution Ice bath: Constant temperature 0 C Objective: To measure the dynamic response of a thermocouple sensor to find its time constant and 90% rise time. Theory: The dynamic response of a temperature sensor will depend on its design, material properties, and the nature of the heat transfer process during the measurements. The dynamic response of a sensor is schematically presented figure 3 for a step-change (increase) of the input temperature from T room to T bath. For a decreasing step-change input, instead of rising time, there will be the corresponding falling time. We need to take enough number of measurements (7-15) during the 90% rise time. If we want to record measurements manually with a multimeter, we will have to modify the thermocouple sensor by putting it in a container, thus making a new sensor with larger time constant. Figure 3: Dynamic response of a sensor Procedure: 1. Check the level of water in the bath. Connect the apparatus to the Power supply. 2. Turn on the water bath by switching the main switch (1) as shown in the figure. 4

5 3. Keep the set-in button (8) depressed and set the temperature to desired level (60 C) by turning the knob (6) and observing the display (7). Release the button (8) after setting the temperature. 4. Connect the ends of the thermocouple to the DMM and set the multimeter to read in millivolts DC. 5. Place one junction of the thermocouple in the ice bath and the other can be kept at rest away from all heat sinks and/or sources. Wait for the multimeter reading to stabilize (i.e. the reading on the DMM steadies down except the last digit). Record the reading in millivolts (E Room ). This will be your initial (Time = 0 sec) reading. 6. Next, remove the junction from the ice bath and allow the DMM to stabilize again before continuing. This should allow both junctions to be at room temperature. 7. Now place one junction into the hot bath and leave the other junction at room temperature and start the stopwatch. 8. Record the multimeter readings every 5 seconds until you have recorded values for 1 minute. R Room = mv Observations: Time (sec) DMM reading E DMM (mv) E Response = E Room + E DMM corresponding to E Response, T TC ( C) corresponding to E Response, T TC ( F) For the Laboratory Report: 1. Plot the data with the time on X-axis and the temperature readings (T Tc ) on Y-axis. Curve fit with the corresponding exponential function, and determine the time constant and the 90% rise time (see figure 3 for visual representations). Perform this for both Celsius and Fahrenheit temperatures. 2. Comment on the measurements and the results. 5

6 Part #3: Measuring Radial Heat Conduction Required for honors students, optional for non-honor students Apparatus: HT10X Heat Transfer Service Unit, HT12 Radial Heat Conduction Accessory Useful Links: Specifications: Heat Transfer Service Unit: Armfield HT10x, Max voltage input 24.3 V Radial Heat Conduction Accessory: Max flow rate 1.5 L/min Liquid: Water Objective: To measure the temperature distribution for steady state conduction of energy through the wall of a cylinder (radial energy flow) and demonstrate the effect of a change in heat flow. Theory: When the inner and outer surfaces of a thick walled cylinder are each at different, uniform temperatures, heat flows radially through the cylinder wall. The disk can be considered as a series of successive layers.from continuity considerations, the radial heat flow through each of the successive layers in the wall must be constant if the flow is steady. However, since the area of the successive layers increases with the radius, there is a temperature gradient in the radial direction. The radial specimen in the HT12 consists of a 3.2 mm thick disk with inside radius R i = 7 mm and outside radius R o = 50 mm. Six K type thermocouples are positioned at uniform intervals of 10mm from the center of the disk i.e. 7 mm, mm, 20 mm, 30 mm, 40 mm and 50 mm radius respectively. The conduction heat transfer rate can be quantified by: = 2 If algebraically adjusted, the conductivity is = ( ) ( ) = ( ) ( ) (1) (2) 6

7 Apparatus: Legend 1 On/Off Switch 2 Voltage Input Control Knob 3 Voltage/current/thermocouple display 4 Thermocouple temperature display Figure 4: HT10X Heat Transfer Service Unit 5,6 Thermocouple attachment locations 7 Quarter-turn inlet valve 8 Pressure regulator 9 Flow control valve Figure 5: HT12 Radial Heat Conduction Accessory 7

8 Equipment Set-up: 1. Locate the HT12 Radial Heat Conduction accessory alongside the HT10X Heat Transfer Service Unit on a suitable bench. 2. Connect the six thermocouples on the HT12 to the appropriate sockets on the front of the service unit. Ensure that the labels on the thermocouples leads (T1-T6) match the labels on the sockets. 3. Set the HEATER VOLTAGE potentiometer to minimum (counterclockwise) and the selector switch to MANUAL then connect the heater lead from the HT12 to the socket marked Output 3 at the rear of the service unit. 4. Ensure that a cold water supply is connected to the inlet of the pressure regulating valveon HT Ensure that the flexible cooling water outlet tube is directed to a suitable drain. 6. Ensure that the service unit is connected to an electrical supply. Procedure: 1. Switch on the front Main switch (if the panel meters do not illuminate check the RCD and circuit breakers at the rear of the service unit, all switches at the rear should be up). 2. Turn on the cooling water and adjust the flow control valve (not the pressure regulator) to give approximately 1.5 L/min. Fully open gives roughly 1.5 L/min. 3. Set the Heater Voltage to 12 Volts (adjust the heater voltage potentiometer to give a reading of 12 Volts on the top panel meter with theselector switch set to position V). 4. Allow the HT12 to stabilize (monitor the temperatures using the lower selector switch/meter). 5. When the temperatures are stable record T1, T2, T3, T4, T5, T6, V, I. 6. Repeat the procedure and note the readings for the above values for the Heater Voltage of 17 V, 21 V and 23 V. Observations: No. Heater Voltage (Volts) Heater Current (Amps) T 1 ( C) T 2 ( C) T 3 ( C) T 4 ( C) T 5 ( C) T 6 ( C) For the Laboratory Report: 1. For each set of readings plot a graph of temperature against radius (The slope should be linear and negative): 2. Comment on the change in T1 with respect to power. 3. Calculate change in at different points of the plot. 8