NORTHERN ILLINOIS UNIVERSITY - Department of Mechanical Engineering
MEE 390 EXPERIMENTAL METHODS IN MECHANICAL ENGINEERING
©1990-1997 M. Kostic

Lab: Thermocouple Calibration

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

Apparatus:
Thermocouple;
Temperature controllable bath;
Digital Thermometer;
Multimeter.

Useful Links:
Introduction to Thermocouples
Temperature Meas. or [Eqs]
Thermocouple Color Codes
Technical Reference [K-Tble]

 Specifications:
Thermocouple: Omega make, Type: J, K, or T thermocouples
Constant temperature bath: Haake A81, 115V / 60Hz / 1500 VA
Max temperature 180° C, precision 0.1° C
Liquid : Water or Antifreeze circulated to maintain constant temperature.
Digital Thermometer: Omega HH23 (K, J, T type); 5 digit LCD, 0.2° C repeatability (Similar product)
Multimeter: Hewlett-Packard type 3478 or HP 34401A digital multimeter, 1 m V resolution;


Theory: The thermocouples are based on the Seebeck effect, see Sec.8.5-Thermoelectric Temperature measurement, Text p.343-362 or similar reference. The Seebeck effect states when two dissimilar metal wire are connected with each other in a loop to form two junctions, maintained at two different temperatures, a voltage potential or electromotive force (E=emf) will be generated and the current will flow through the loop circuit. The current will be proportional to the difference in temperature between the junctions and the metals used. The higher the temperature difference, the higher is the electromotive force (emf) and the current flow in the loop. The magnitude of the emf is in the order of few millivolts. We need a precise and sensitive multimeter (MM) which can read up to microvolts to do this experiment.

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. Keep the set-in switch (8) depressed and set the temperature to desired level (30 deg C to start) by turning the knob(6) and observing the display(7). Release the switch(8) after setting the temperature. In normal mode, the temperature shown on the digital display is the actual temperature of the bath (TB) against which the thermocouple sensor is to be calibrated.

4. Connect the ends of the thermocouple to the digital multimeter (MM) and set the multimeter to read in millivolts DC.

5. Dip one junction of the thermocouple in the thermo-bath liquid and wait for few minutes for it to reach the steady state (i.e. the reading on the MM steadies down except the last digit). Be careful to hold (tape) the sensor wire away from the circulator's propeller!

6. Note down the digital MM reading in millivolts (EMM) and repeat the steps 3 through 6 in steps of 5 degrees from 30 to 60 degrees Celsius.

7. As mentioned in the theory, the multimeter reading corresponds to the difference in temperature between the surroundings (room) and the bath. To calibrate the thermocouple, we have to take the room temperature into consideration to get the absolute value of temperature measured. Find the equivalent millivolt value for the room temperature from the corresponding Thermocouple table (ERM). Then, add that millivolt value, corresponding to the room temperature, to every multimeter reading (EMM). Tabulate the values.

Observations (Type-K thermocouple used):
Room temperature : 20.5° C = 66.2° F; corresponding mV reading from tables ERM = 0.818 mV

Bath Temperature, TB [° C]

Bath Temperature,
TB [° F]

MM reading,
EMM [mV]

Total emf,
E = EMM+ ERM
[mV]

Temperature corresponding to E,
TTC [° C]

Temperature corresp. to E,
TTC [° F]

30

  

0.361

1.179

29.4

  

35

  

0.579

1.397

34.8

  

40

  

0.788

1.606

39.9

  

45

  

0.995

1.813

44.9

  

50

  

1.202

2.020

49.9

  

55

  

1.407

2.225

54.9

  

60

  

1.613

2.431

59.8

  


Plot the measured bath temperatures values (TB) on y-axis against the corresponding thermocouple emf (millivolt) values (E) on x-axis. 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. Calculate the 95% uncertainty in temperature measurement of the calibrated thermocouple sensor if the obtained curve-fit correlation is used. Worked out Problem 5.30 should help you with the assignment.

NOTE:
The multimeter voltage reading should be zero when you measure room temperate without the reference junction (the thermocouple connected directly to the meter) Then the reference temperature is the same as measured temperature, both equal to the room temperature, therefore the temperature difference and generated voltage should be zeros. However, your reading will never be exactly zero due to many reasons:

  1. if the tip of the thermocouple is wet it may be sub-cooled due to evaporation;
  2. the room temperature is non-uniform;
  3. he meter is not perfect (make a short-circuit to check its zero),
  4. the measured circuitry may be picking up some "noise," since it acts as an antenna; etc.