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ENGR_201_Lab5_TC_201035_rev2 - ENGR 201 Evaluation and...

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ENGR 201 Evaluation and Presentation of Experimental Data I Spring 2011 (201035) Original: Dr. Scoles Rev: Dr Miller; Dr Chmielewski 5/10/11 page 1 of 12 Capturing Temperature Measurements with a Thermocouple Goals Build a subVI for voltage to temperature conversion Measure, plot and record temperature measurements from a Type K thermocouple (TC) Correct the measured voltages with a calibration curve Find the time constants of the TC cooling curves Equipment/Software LabVIEW software NI USB-6009 data acquisition unit Type K thermocouple, Omega KTSS-HH Nickel-10% chromium (+) vs. Nickel-5% aluminum and silicon (-) EPED Cold Junction Compensation (Type K) circuit Power resistor, 100 Ω , 25 Watt Hewlett Packard E3631A DC power supply GraphData.vi file from your voltage measurement exercise Spreadsheet software Lab notebook Reading or Viewing Read - Section 8.5 in Figliola and Beasley Review - Week 6 lecture notes View - Create a SubVI in LabVIEW video, E. Doering, Rose Hulman University, http://cnx.org/content/m14767/latest/ (5 minutes) Reference - NI USB-6009 User Guide, Laboratory Items folder, course Bb Vista site. Has tables of terminals and their functions. You will need this to choose terminals for differential voltage channels. Introduction A thermocouple (TC) can be used to measure temperature over wide ranges in a variety of measurement environments and with fine spatial resolution. The sensing operation of the TC is based on the Seebeck effect: when two dissimilar metals are joined at both ends to form an open loop, an open circuit voltage is developed (Figure 1). The voltage is proportional to the difference in temperature at the two junctions. The measured voltage is on the order of tens of millivolts. To extract the temperature at the measuring junction (T1) from the measured voltage, we will want to keep the reference junction (T2) at a fixed, known temperature.
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5/10/11 page 2 of 12 Figure 1. Two junctions, T1 and T2, formed by joining wire types A and B. 1 The ice/water bath at 0 °C (Figure 2) has become the standard for the reference temperature, and published thermocouple voltage vs temperature tables are based on this value. This method of providing the reference junction temperature is impractical in field- and lab-measurement situations, so alternatives have been developed. Figure 2. Thermocouple loop with the reference junction at 0 °C. Rather than using ice, two methods can be used to do cold-junction compensation – software and hardware. The temperature of the reference junction can be measured directly using a semiconductor sensor or thermistor. This would occupy two channels on your DAQ system – one for the T1 voltage and the other for T2. The T2 sensor can be chosen to provide a very accurate measurement in a narrow temperature span centered on the expected junction temperature. The measured T2 and the measured sensing junction voltage can be used in a calculation to remove the effect of the reference junction voltage and extract the temperature of T1.
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