PLC's internal compensation of cold junction

[mikas_m]

Hello people,

I wonder if internal cold junction compensation in Ai card of PLCs (Siemens, AB etc.) means that when using thermocouple, there is a constant offset to the measured value, or not. In other words, do these analog cards have built in temperature sensor to compensate temperature of cold junction or this compensation is done with constant value?
Also, for example I have find that many of temperature transmitters that are used for thermocouples have option to choose between internal and external temperature compensation. I know that external compensation with PT100 for example is more accurate, but don't know if internal uses constant value or there is a temperature sensore inside the transmitter.

What is your experience?

 

[danw]

Modern day (for the last 20 years or so) cold junciton (CJ) compensation has been done with a CJ component on or near the AI card which should measure the temperature of the terminal connection block, although frequently it is on the card, not the terminal blcok.

I know that some calibrations done either in the lab or in the field still use external measurement, usually with a ice bath, for accuracy's sake. A more typical approach is to use a certified thermometer to read the temperature near the CJ component.

I am unaware of any AI cards that "fix" the Cj at a constant value.

I assume that there are some devices that let one specify a fixed CJ value, but I'm not sure why - that goes back to a long ways when people would bury a Cj reference jucntion, say, 2m below ground becauzs they knew that the ground temperature would maintain a constant 13.1° tenperature as a constant, in an era before inexpensive CJ circuits.

Dan