Input Calibration
286 UDC3500 Universal Digital Controller Product Manual 3/07
Table 5-7 Procedure to determine calibration voltages for Thermocouple
Differential input types other than the Factory Setting
Step Action
1
Obtain a copy of the ITS-90 Standard for the Thermocouple Type you will be using.
2
Find the thermoelectric voltage for the desired operating temperature.
3
Find the thermoelectric voltages for the temperatures –50°F and +150°F away from the
desired operating temperature.
4
The zero calibration voltage will be the thermoelectric voltage for the –50°F
temperature minus the thermoelectric voltage for the desired operating temperature.
This will be a negative voltage.
5
The span calibration voltage will be the thermoelectric voltage for the +150°F
temperature minus the thermoelectric voltage for the desired operating temperature.
This will be a positive voltage.
For example: Determine the calibration voltage values for a pair of J-type thermocouples
at an operating temperature of 450
°F (this is equivalent to the Factory setting).
• The ITS-90 standard for the J thermocouple shows that the thermoelectric
voltage for 450
°F is 12.568 millivolts.
• The –50°F point would be 400°F. The ITS-90 standard shows that the
thermoelectric voltage for 400
°F is 11.025 millivolts.
• The +150°F point would be 600°F. The ITS-90 standard shows that the
thermoelectric voltage for 600
°F is 17.188 millivolts.
• The zero calibration voltage is thus 11.025 minus 12.568 millivolts or –1.543
millivolts (this can be rounded off to –1.54 millivolts without significant loss of
accuracy).
• The span calibration voltage is thus 17.188 minus 12.568 millivolts or +4.62
millivolts.
• Use –1.54 millivolts for the Zero calibration value and +4.62 millivolts for the
Span calibration value.
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