Lab: Capturing Temperature Measurements with a Thermocouple

Procedure

1. With the Hewlett Packard E3631A DC power supply off connect two alligator leads from the power resistor terminals to the + and COM terminals (under ±25V label) as shown in Figure 5).

Figure 4. TC inserted into the core of the bower resistor (not to scale)

1. Start your temperature measurement VI. Within the NI software, set the thermocouple type to, ‘K' and set the units to ‘C'. Enable data logging in the NI software, collecting 1 sample/second.

2. Record the starting temperature as the ambient temperature, T_∞, in the analysis discussion that follows.

3. Insert the Omega thermocouple into the center of the power resistor. The thermocouple should not touch the sides of the resistor, it must float at the center of the radius.

4. Set the power supply voltage to 16 V.

a. Turn on power supply by pressing the "Power" button. Press the "Output On/Off" button. Voltages are shown on the left half of the supply display, and currents on the right half. If a digit on the voltage side is not flashing, press the "Voltage/Current" button. Use the "Adjust" knob to set the voltage.

5. Observe the TC temperature as the resistor warms up to its maximum temperature, typically between 50 and 70°C (122 and 158°F). Collect data until dT/dt=0.0167, or one degree/minute, we will treat that as steady-state.

6. Remove the TC from the power resistor, hold it vertically without waiving it around and continue to measure the temperature until it returns to a value close to the ambient level. This is measuring the free convective cooling response of the thermocouple.

7. Stop your VI and save the data to a file.

8. Repeat this heating and cooling cycle two more times, saving the data into a new file each time.

9. Make sure you have three good cooling curves saved before you leave the lab.

These curves should generally look alike.

10. Turn off the power supply.

a. Press "Output On/Off" on the supply, and turn off "Power".

Your Report

Prepare a written report following the guidelines in our grading rubric. This report is due one week after your lab.

Required Graphical Results

Each of the three cooling curves should be included in the report

Some Discussion Points That Must Be Covered

• From your readings and lecture, what are some of the advantages and disadvantages of using the Cold Junction Compensation circuit for temperature measurement?

• Why would you choose a differential input channel thermocouple for this application rather than a single-ended channel?

• Explain your reasoning behind the setup of your voltage measurement task.

• What are some of the sources of measurement error in this experiment? What is the Omega Type K thermocouple temperature accuracy? Can you use propagation of error to estimate the error in the temperature readings?

• How well did the exponential cooling model fit the temperature data?

Required Printouts

• One page hardcopy of final temperature measurement front panel showing measured data for the three trials

• Excel, Labview, or Matlab analysis of your temperature measurements. Make sure all tables and figures are properly labeled in the body of your lab report or the appendix section.

• If you use Labview: one page hardcopy of final temperature measurement block diagram including the subVI. The diagram should have text documentation explaining the VI's function and the team member names.