How is cold-junction compensation for thermocouple temperature achieved?
Thread Content
As is well known, a thermocouple generates a thermoelectrical potential when the materials of its two thermal electrodes are different and when the temperatures at the two junctions differ. E_temp = E_field (the potential measured in the control room) – E_control_room (the potential corresponding to the temperature in the control room). I assume that there is a temperature sensing element in the control room, whose value is used in the calculations to determine the temperature; however, my colleague said I was wrong. He claimed that compensation wires are available, but that these wires merely extend the function of the thermocouple. Cold-junction compensation is not carried out in this case. I would appreciate your guidance on this matter
Cold-end compensation essentially involves extending the thermocouple, taking one end of it to a relatively stable environment and then performing calculations. If it is placed in the field where the temperature is variable, calculations cannot be carried out, and the results obtained will not be accurate.
Your colleague is wrong; the function of a compensation wire is to extend the cold end of the thermocouple. There are four methods for thermocouple temperature compensation: (instrument mechanical zero adjustment method), (cold junction temperature correction method), (ice bath method), and (bridge compensation method). rather than being achieved with compensation wires.
The wire should be extended in order to bring the reference terminal to a location where the temperature remains relatively stable. If compensation is needed, the method mentioned above should be used; these days, thermistors are often employed for such compensation. The thermistor changes in response to the temperature of the reference terminal, thereby offsetting the impact of changes in the reference terminal’s temperature on the temperature measurement.
The compensation wire serves to extend the measurement circuit; it does not provide actual compensation. It functions by bringing the cold terminal to a relatively constant temperature (theoretically 0 degrees). If the temperature is not 0 degrees, then it is necessary to compensate for that thermoelectric potential. Manually, this is done by referring to a calibration table. In modern DCS systems, I/O cards typically use the bridge compensation method, which is the approach mentioned by the colleague above.
Attach a thermistor to the compensation module on our DCS. . It’s obvious. . I don’t know how it’s calculated internally. . . They are all integrated. . Can’t see the program
Since thermocouples measure relatively high temperatures, compensation may not be necessary. Thermistors generally require compensation; compensation wires are used to lead them indoors for temperature compensation.