Using CJC Sources with the PXIe-4353

Updated Jun 15, 2018

Reported In

Hardware

  • PXIe-4353

Driver

  • NI-DAQmx

Issue Details

How is the CJC value applied when calculating the thermocouple measurement from the PXIe-4353 card? When given a user control after the constant value is selected, how do your modules read this? 

Solution

As per the Seebeck Effect, temperature differences across metal will produce a voltage. This voltage is measured by the PXIe-4353, and this effect shows that the voltage is directly proportional to the temperature at the hot junction minus that of the cold junction. This is shown by the following calculation:
 
V = 𝜶 (THJ - TCJ)
 
where V = voltage, 𝜶 = Seebeck coefficient for the metal used for the thermocouple, THJ is the hot junction temperature, and TCJ is the cold junction temperature.  

As such, the device will measure the voltage coming in, use the appropriate 𝜶 value based on the thermocouple type chosen and the cold junction temperature defined by the CJC Source to calculate the temperature observed (THJ). 

Additional Information

NI has three options for the CJC:
  • Built-in: uses the CJC terminal built into the device for TCJ 
  • Constant: uses a constant value provided for TCJ 
  • Channel: specifies a channel to be taking the cold junction temperature measurements for TCJ 

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