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Using a measured temperature for thermocouple CJC

I'm making a thermocouple temperature measurement where the cold/reference junction is not on the DAQ terminals, but instead needs to be measured using another thermocouple.  I assume that the 'CJC Source' option for 'Channel' should be used for this situation.  However, there are no available channels to choose from in the 'CJC Channel' dialog -- even if the CJC thermocouple has been defined as the first DAQmx channel.  How do I get the first thermocouple measurement to show up as an option for CJC?

 

This issue is part of what Y. Verburg discusses, but it is not clear how Verburg adds the CJC thermocouple channel: https://forums.ni.com/t5/LabVIEW/NI-9213-CJC-Channel/td-p/3749095/page/2 

 

The problem is demonstrated in the attached copy of a LV continuous input thermocouple example (from LV2018).  I've just added a second input for a Type K thermcouple which would be the CJC for the Type B thermocouple.  I'm using a cDAQ with an NI 9213 module and LV 2018.

2023-03-31 LabVIEW TC CJC example.png

 

 

 

 

 

 

 

 

 

 

 

 

 

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If you you use an external external CJC , do you use equal material (copper?) wires from that CJC to the AI channel?

 

I would read the input voltage of the external channel and modify the Convert Thermocouple Reading.vi

TC convert.png

Replace or extend the Step 1 to input the CJC temperature or the CJC voltage+ TC type to calculate the CJC temp.

Only if you use the same type of TC one could simply add both voltages, however usually one would try to make a more precise CJC measurement since TCs already have a quite high uncertainty and the CJC uncertainty than is of same order.

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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Hi Henrik,

 

Thanks for the reply and suggestion.  Yes, I have an external cold junction, and have Cu/Cu wires from that to the AI channel.  So your proposal would work, but just adds additional conversion steps.  I thought I could get away with doing the V -> T conversion with the DAQmx Read operation if I could figure out a channel definition for the 'CJC channel' input.  From your response, I take it that is not possible, and instead I'll need to do these additional 'manual' conversion steps?

 

Daniel

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