Transistor

1738-IT2IM12 thermocouple wiring - compensating cable

4 posts in this topic

I'm testing installation of a custom-built machine. The equipment is at room temperature of about 19°C for a couple of weeks but the PLC is reading 25 to 29°C from the J-type thermocouple inputs. I suspect the machine builder has used copper cable to wire the thermocouples back to the I/O point. He has also used what appear to be regular M12 connectors to make the connection.

5ac4dfda323c0_J-typeinput.png.5f16a3be7e

Configuration of the input module does not appear to be correct. We're in 50Hz-land.

I can't find any wiring instructions in the A-B documentation but I suspect that we should at least be using compensating cable and proper J-type connectors near the heater zones.

Any pointers?

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Posted (edited)

If it really has copper wires, an error like the one you describe is feasible, it is mainly due to the fact that the Analog Input Module is usually hotter than outside. if the cabinet door is opened probably  the temperature will drop a few degrees. 

For me to use or not thermocouple connectors is a minor problem and should not cause much error, but of course it is better to change them for the correct ones.

It is important to use the correct compensation cable and with the correct polarity !, there are many standards with varied colors and an error is easy to occur.

 

Hope it will help.

Edited by Lfe

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Problems like this are why I usually use RTD's instead of thermocouples.  

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Not to mention the cold junction offset values !    That is meant to be used as a substitute for the CJC devices themselves;  you're supposed to put the estimated temperature at the module in there, and un-check the CJC box.   

Your temperature isn't off by 10 degrees;  it's off by 40.

You need proper thermocouple extension cable if this system is going to get anywhere near accurate measurements.

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