Shorting (with copper) the inputs of a T/C Module

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fifty60

fifty60

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Can anyone explain why a temperature controller reads around 25 C when you take a piece of copper wire and short out the + and - T/C inputs?
 
fifty60 said:
Can anyone explain why a temperature controller reads around 25 C when you take a piece of copper wire and short out the + and - T/C inputs?
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A thermocouple generates a small voltage that is variable based on temperature.

Apparently whatever voltage is present between the + and - inputs when you short them out with a piece of copper wire corresponds to the voltage present at 25 deg C.
 
petersonra said:
A thermocouple generates a small voltage that is variable based on temperature.

Apparently whatever voltage is present between the + and - inputs when you short them out with a piece of copper wire corresponds to the voltage present at 25 deg C.
Click to expand...

Which is probably the zero-point calibration for the instrument. For J-type thermocouples, the actual millivolt output of the junction is 0.0 at 32F, per a reference at Omega Engineering's web site.
 
The effective junction point for the "remote" thermocouple has become the point where you connected the two wires together. An X to copper junction and a copper to Y junction at the same temperature in series will act just like an X to Y junction at that same temperature.
The TC input circuit will contain what is called a cold junction compensator which allows proper reading of the remote temperature even though the internal temperature of the input circuit varies.
 
A thermocouple produces a voltage only in reference to the other junctions in the circuit, if they are at a different temperature. The Omega tables are referenced to 0 degrees C, but could be referenced to any other temperature. It is just that 0 C is easy to get. When a complete circuit is made, current flows and energy is transferred. The hot junction losses heat, the cold junction gains heat. The instrument simply references to 25 C, so when the input is shorted, it believes the thermocouple is at that temperature.

I hope this isn't so poorly written as to confuse the issue.

When I used to work at the local college, we had a thermocouple demonstrator w/ a circuit made of 2 different materials, w/ about a 1/4 inch cross section. When one junction was heated more than the other, it could produce currents over 100 A.
 
gadfly56 said:
Which is probably the zero-point calibration for the instrument. For J-type thermocouples, the actual millivolt output of the junction is 0.0 at 32F, per a reference at Omega Engineering's web site.
Click to expand...
The millivolt output is 0.0 volts at 32F only if the cold junction temperature is also 32F or a junction compensator is used. The table voltage properly applies to the complete circuit (two junctions) not to a single junction in isolation.
 
GoldDigger said:
The millivolt output is 0.0 volts at 32F only if the cold junction temperature is also 32F or a junction compensator is used. The table voltage properly applies to the complete circuit (two junctions) not to a single junction in isolation.
Click to expand...

You're right of course. I didn't recognize they were assuming a 32F cold junction. :slaphead:
 
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