Thermocouple Cold Junction Compensation Query

[Jericho93]

Hello,

I am working with control instrumentation for the first time and I am looking into Type K Thermocouples.

My question is this, I understand the need for cold junction compensation - am I correct in assuming that I can use a RTD to measure to measure the temperature at the cold reference junction, and that it is ok for the temperature at the terminals of the cold reference junction to go up and down (together), and that this won't have a negative impact on the readings from the hot end of the thermocouples.

Thanks a bunch

Jericho.

 

[IRstuff]

Are you referring to

http://cna.mikkeliamk.fi/Public/Slabra/Schneider/Modicon_M340/M340_AnalogIO_manual.pdf

There appears to be more information on pg 83-86

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[danw2]

Thanks for the link to the manual.

Seeing is believing. I did not think that there was a commercial thermocouple AI card that did not have internal CJ as a standard feature in this day and age. Yet, here it is.

What a pain to not only have to supply a CJ sensor for each 4 channel card but to have to position the sensor for a decent CJ reading.

I'd probably opt for small diameter 2 wire Pt100's. The connections are so close to where the measurement needs to be made that the lead wire error will be insignificant.

And it appears that one gets to use all four input channels for thermocouples, when using the 2 wire RTD option (across EXO+ and EXO- in the graphic below).