how to build CJC sensor for SCXI

[hema]

Hi,

I am using SCXI 1130 with SCXI 1377 terminal block. and NI DMM is NI 4070.

 My application has to read 64 temperature channels.

I came to know that the SCXI 1130 does not have a built-in CJC channel. When i give the scan string as cjtemp->com0; i get not valid channel for this SCXI error.

But i require CJC voltage to give as an input to the niSwitch Convert Thermocouple Readings.VI

I do not know the CJC voltage.

So do I have to make my own CJC sensor, connect that to the SCXI switch and get the voltage?

Please let me know how to build my own J type CJC sensor. Or any document that would guide would be helpful.

Thank you. Hema

[PBear]

Hi Hema,

When using a switch and DMM to do thermocouple measurements, there are a few switches that connect to terminal blocks that have a built in CJC channel. For example, the SCXI-1331 terminal block can interface onto the 1127 and 1128 switch modules.

Since your terminal block does not have a cold junction compensation channel (nor are their any available for the 1130 switch module), you will have to provide this or use a constant value in your program. A CJC sensor can be a thermistor and you will also need to know the resistance of that thermistor. For example, the thermistor in the 2627 terminal block is 189kOhm. If you double click on on the niSWITCH Convert Thermocouple Reading.VI you'll be able to see the logic behind what would be involved in connecting a thermistor to one of your switch channels and implementing it's voltage value. Also, you could find a constant value that would work for you just by looking at the math under the hood.

[hema]

Hi,

I arrived at the cjc constant CJC value after some trial and error. I manually fixed the constant value by checking if the program reads temperature value closest to room temperature. But i have a doubt whether this constant will work all the time.

You have mentioned that i can get cjc value from the math which is there in niSWITCH Convert Thermocouple Reading.VI , For the conversion, the CJC value is taken as an input.

This value i do not know.

So is it OK if i have a constant value roughly worked out by comparing the room temp with an external temp sensor.

Thank you.