Sensoray 518 - Manual (Page 47)

46
Verifying Thermocouple Calibration
Thermocouple applications sometimes require periodic validation of thermocouple calibration.
Two methods are commonly used: ice bath and millivolt calibrator.
Ice Bath Method
The ice-point or boiling-point method has the advantage of requiring simple and inexpensive
equipment: a beaker full of ice water or boiling water. The thermocouple hot-junction is
immersed into a stirred ice bath (ice cubes and water) or into boiling water. Time is given to
allow the thermocouple to reach thermal equilibrium. The operator then verifies that 518 output
data is consistent with the water temperature (0 degrees C for an ice bath, 100 degrees C for
boiling water).
Certain precautions must be taken when executing this procedure. Water can be absorbed into
the thermocouple insulation if the thermocouple hot end is inserted directly into the bath. Should
the water form a conductive path, the effective hot-junction will move away from the intended
location. This relocated hot-junction is unlikely to have the same temperature as the bath,
resulting in an incorrect temperature indication. To avoid this problem, a thermowell should be
used to keep the thermocouple from making direct contact with the water.
A simple and inexpensive thermowell can be constructed from a piece of quarter-inch diameter
copper tubing. Cut a length of tube that is too long to be completely submerged in the bath.
Crimp one end of the tube with a hammer to make it water-tight. Insert the thermocouple hot-
junction all the way into the open end of the tube. Now insert the crimped end of the thermowell
into the water bath.
Millivolt Calibrator Method
The millivolt calibrator is typically more accurate than a water bath if the thermocouple
operating point is much higher than 100 degrees Centigrade. This procedure is straight-forward
and accurate provided the user remembers this important fact: thermocouple voltage is generated
across the length of the thermocouple wire, NOT at the hot-junction. Since the 518 compensates
for the cold-junction being other than zero degrees C, error will result if a temperature
independent voltage (corresponding to a corrected thermocouple voltage) is used to verify
thermocouple calibration.
Here is a valid way to check thermocouple calibration using a millivolt calibrator:
1. Measure the temperature of the 7409TC termination board with a calibrated low
thermal mass temperature probe and meter.
2. Use the NBS millivolt-temperature look-up tables to determine the thermocouple
voltage corresponding to the 7409TC temperature. This is the thermocouple correction
voltage Vc.
3. Decide on a temperature Tsim to be simulated by the millivolt calibrator.