ME 368 Laboratory 4 Thermocouples
Laboratory 4
Thermocouples
Equipment needed:
· myDAQ connected to computer running LabVIEW
· Traceable® thermometer
· 2 lengths each of copper wire and 1 length of constantan wire (these look just like ordinary wires, except one appears copper and the other appears silver)
· 1/8” diameter stainless-sheathed thermocouple and associated extension-wire jack
· Ice water bath in red plastic cooler made from distilled water and non-distilled ice
· Room temperature water bath in white Styrofoam cup bath made from tap water
· 2 Thermo Scientific 280 series water baths filled with distilled water
· Boiling water bath made using distilled water in a hot pot
Goals and Objectives
· Gain exposure to thermocouples and the thermocouple effect
· Understand differences between thermocouples and thermistors, including:
1. One is a temperature-dependent voltage-generating device, the other is a temperature-sensitive resistance
2. One has highly nonlinear temperature behavior, the other has approximately linear temperature behavior
· Understand linear regressions and related statistics
1.1 Thermocouple operation
Thermocouples are formed whenever two dissimilar metals are in contact. Some metal pairs are more sensitive to temperature than others, but all generate a small voltage based on the temperature difference between the points on the metals where the voltage is being measured and the junction between the metals. A great deal more background on thermocouples is available online and in various text books and references and you may want to review some such as:
· Wikipedia (http://en.wikipedia.org/wiki/Thermocouple, http://en.wikipedia.org/wiki/Thermoelectric_effect),
· Omega Engineering (http://www.omega.com/thermocouples.html) and
· efunda
(http://www.efunda.com/DesignStandards/sensors/thermocouples/thmcple_theory.cfm).
It is important to remember that it is the temperature gradient in the metal that causes the electrical potential to arise. If you were to place a copper wire between two leads of a volt meter, and the entire wire was at the same temperature, you would not be able to measure a potential difference between them. Should you place a flame on the wire midway between the two voltmeter leads, you would generate a thermoelectric potential between the midpoint of the wire and each of the wire ends. However, the two potentials would cancel each other out, and the volt meter would again indicate that there was no potential difference in the wire.
Instead of using a uniform wire made of copper, let’s imagine we splice a pure copper wire to a copper-nickel alloy (55% Cu, 45% Ni, say) wire to form a wire composed of 2 segments. Now imagine we heat the splice joint with a flame. Since each wire will generate a different thermoelectric voltage, when a volt meter is connected to the ends of 2-segment wire (one connection to the pure copper wire, far from the joint, and another connection to the copper-nickel alloy wire, far from the joint) it will measure an electrical potential difference. The magnitude of the electric potential that is generated is related to the difference of the Seebeck Coefficients of the two materials. The Seebeck Coefficient of copper is approx. 6 mV/°C and that of the copper-nickel alloy (called constantan) is approx. -35 mV/°C. Thus, the junction of the two should give an electrical potential of about 41 mV for every °C of temperature difference between the junction and the ends of the wire. Note that these Seebeck Coefficient values are actually temperature dependent, so calibrations are made of careful measurements of the junction voltage over a very wide range of temperatures instead of using this constant value.
The copper-constantan combination is very common and is called a type-T thermocouple. Internationally accepted convention states that pairs of wires leading to a type-T thermocouple should be blue or marked with blue. There are several other common thermocouple wire combinations and these are discussed in the references listed above.
From the description above, it seems that measuring a temperature with a type-T thermocouple should be pretty straight forward. However, we ignored a couple very important issues. First, to use a volt meter, we usually use copper wires to connect between the measurement circuitry and the device to be measured. Thus, when the copper wire from the volt meter is connected with the constantan wire, another type-T thermocouple is formed, and its potential will be summed with the potential of the thermocouple we are trying to measure. In addition, the voltages are small so noise must be properly managed. In this lab, we will investigate temperature measurement with thermocouples and how these issues may be overcome.
lab completion a (2 pts): If a resolution of 0.1 °C is desired in temperature measurements made by a type-T thermocouple, what voltage resolution is required in the DAQ system?
lab completion b (2 points): Compare your answer to lab completion a to a) the voltage resolution Q of channel ai0 of the myDAQ setup for the +- 2 V range and to b) the noise level of 2.5 µV observed in lecture when the +- 2 V range was sampled from ai0 at 200kS/s and sample-compressed by a factor of 6000 (see L8_Ex1.vi). Draw conclusions.
1.2 Connecting a homemade thermocouple
· Jumper ai0- to the nearest AGND terminal. Use the screwdriver at your station to connect one of the copper wires to ai0+. Connect a constantan wire to ai0-.
· Twist the free ends of the copper and constantan wires together to form a thermocouple.
· Setup ai0 to monitor the +- 2V range at 200 kS/s with a sample-compression factor of 20,000 (1 data point every 0.1s), displaying the results on a waveform chart.
· Run your code and observe the chart.
· Place the thermocouple between your fingers and observe the voltage increase.
· Allow the thermocouple to settle on the room temperature and determine the average voltage measured when settled. An easy way to do this is to right click on the chart and select Export, Export Data to Excel, and then use the average function within Excel.
· Look up the voltage on the voltage chart for Type T thermocouples (see, e.g., type_T_thermocouple_chart.pdf on the course website in the equipment folder). What temperature is represented by that voltage?
1.3 Checking the accuracy of a temperature difference
We will now begin the process of correcting the voltage measurement so that the measured voltage corresponds to the correct temperature according the international standard thermocouple voltage tables.
· Place the thermocouple in an ice water bath, stirring it very gently
· Record the average voltage along with the reference temperature
· Repeat the above 2 steps for a room temperature bath
· Subtract the ice bath temperature voltage from the room temperature water voltage
· Look up the temperature value associated with the voltage difference in the Type T thermocouple chart.
lab completion c (2 points): Does your temperature based on the voltage difference seem reasonable?
1.4 Using an ice bath reference
It would generally be annoying to have to always subtract off the ice bath temperature voltage for your thermocouple, and sometimes it would be impossible to remove a thermocouple from its installation to check the ice bath voltage. However, we can use a property of thermocouples that allows us to add their voltages in series to automatically subtract the voltage using two thermocouples.
· Disconnect the constantan wire from the myDAQ; the copper wire should remain in its terminal.
· Place the second copper wire in terminal ai0-, leaving the jumper intact.
· Twist the free constantan end to the free copper end. Your circuit now should be connected as follows: AGND – jumper – ai0- – copper wire – T thermocouple junction – constantan wire – T thermocouple junction – copper wire – ai0+.
· Now, place the thermocouple with the copper wire connected to terminal ai0+ in an ice bath and place the thermocouple connected to terminal ai0- in a room water bath.
· Record the voltage measurement and look it up in the table.
lab completion d (2 points): How does this temperature compare with the temperature of the room water bath measured using the reference thermometer?
Note that this type of thermocouple arrangement is called a thermopile. Although they are useful, we will no longer use the thermopile in this lab.
1.5 Using the type-T thermocouple built into LabVIEW
LabVIEW is able to automatically convert measured voltages to temperatures (it has access to built-in chart data so that you do not need to cross reference voltages to a chart to determine temperatures)
· In the same code you’ve been working with, double-click on the DAQ Assistant to bring up the configuration window
· Highlight the “voltage” measurement channel and click the red X to delete it
· Click the blue plus to “Add channel”
· Select “Temperature”, then thermocouple and channel ai0
· In the configuration window that comes up, set the maximum temperature to 200 °C and the minimum temperature to -20 °C.
· Select “T” as the thermocouple type and make sure that “constant” is selected for the CJC (Cold Junction Compensation) Source.
· Connect only one hand-twisted thermocouple to channel ai0 as you did with the first part of this exercise. Look under the DAQ assistant, connection diagram tab, to be sure you have hooked the thermocouple up correctly (although this diagram does not show the jumper you need between ai0- and AGND)
· Measure an average ice bath temperature with this approach. The measurement may be a bit off initially. You can improve it by adjusting the CJC value within the DAQ Assistant configuration window. Repeat until the average is very near 0 °C. Measure the room temperature water and make sure it agrees (within 1.5 °C) with the reference thermometer value.
· Finally, unplug your homemade thermocouple and attach the commercial thermocouple. Check the ice water temperature and adjust the CJC value if needed.
lab completion e (2 points): What value of CJC did you settle upon? When you now measure ice and room-temp water with the commercial thermocouple, what is the average difference between the measured and reference temperatures?
mini report 1: Determine the temperature precision interval [K] (P = 95%) of your thermocouple in the room temperature bath, and compare it with the thermistor precision interval [K] in the same bath at the same rate from the previous lab (mini report 5 in that lab).
· Using the commercial thermocouple, measure all 5 reference baths as you did in the previous lab. Include sufficient averaging so that each temperature you measure in each bath has a negligible precision interval. Negligible here means negligible compared to anticipated systematic errors; a precision interval of 0.05 K or better (P = 95%) should be sufficient. Record one data point (with a negligible precision interval) in each bath.
mini report 2: This is the preparation-for-report-writing component of this lab. Your goal is to prepare a high-quality scientific plot. You will have to use Excel or other (non-LabVIEW) software. Plot your 5 T_measured vs.T_reference datapoints along with a linear fit and the associated the x-from-y estimate error bands. Example x-from-y error bands are shown as curves “c” in Figure 8.5 Dunn, and are computed using Equation 8.31 with n = 1 (one data point at each temperature on your plot) and ν = N – 2. Use your x-from-y error bands to predict the temperature uncertainty of your thermocouple for the 50 degrees C bath, following the approach associated with the vertical arrows shown in Figure 8.5
mini report 3: Use the highest -order polynomial fit possible to fit your data. This can be done in Excel using the “trendline” functionality. Describe which fit, the high-order polynomial or the linear fit, is most appropriate for use as a calibration function for your thermocouple and why. Note: you may need to plot your polynomial as x vs y to get the appropriate function.
Based on your linear fit, program a calibration (a.k.a. correction) function into LabVIEW so that LabVIEW displays the calibrated thermocouple result as well as the original thermocouple result. Test the calibrated thermocouple in all 5 water baths.
mini report 4: Quantify how much better or worse the accuracy of the calibrated thermocouple results are, compared to the original thermocouple results.
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