Reading thermocouple output directly from the male plug ends?

[unsaint32]

I know I can measure mV of a TC (thermocouple) output with my voltmeter at the analog input card terminals. But I cannot get any mV reading when I disconnect the TC plug from the wires that go to the analog input card and apply the voltmeter probes directly to the male ends of the TC. Why is that?

 

[OkiePC]

Assuming your meter is set correctly and you are making good contact, then it is likely you have an open thermocouple. But when you measure at the long length of wire, you are picking up induced noise. The other explanation for the 5mV reading of the lead wire is that you have a shorted lead wire somewhere along the length (thereby producing a 2nd thermocouple).

This would be a tricky situation for troubleshooting as your PLC would think nothing is wrong and that the temperature is simply not changing.

 

[danw]

If you attempt to convert to temperature you will have an error because you have no cold junction measurement.

The cold junction measurement is the difference between the temperature of the cold end where your voltmeter is and the freezing point of water. It can easily be 30 or 40 degrees F.

 

[danatomega]

A thermocouple produces a voltage based on the Seebeck effect where two dissimilar metals join. when using a meter only you have created two more junctions which can cancel the thermocouple's voltage.
For a more detailed explanation, see:
http://www.omega.com/temperature/z/pdf/z021-032.pdf

 

[danw]

If the thermocouple is not hotter or colder than the point/end you are measuring then there won't be any generated EMF to measure.

A thermocouple lying on the desk will produce 0.00mV because there is no temperature gradient across the thermocouple.

I'm inclined to think that disconnecting the plug and jack loosened a wire so you have an open circuit.

 

[shooter]

the reading is 0 mV when both temperatures are equal, otherwise you should have a reading.