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Resistance of thermocouple

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sheraz.pervaiz

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Dear all,

I have a query about the resistance of thermocouple. If we connect the positive terminal of the ohm-meter to the positive lead of the thermocouple and negative terminal on the negative lead, then the resistance is about 2.3 Ω and when we reverse the polarity its resistance increases to 3.3 Ω.

Why is there is an increase in the thermocouple resistance when we reverse the polarity?

Regards,
Sheraz Pervaiz
 

Just consider, that a ohmmeter can measure resistance exactly, if no external voltage sources are present in the circuit. Obviously, that isn't the case with a thermocouple.
 

LvM is absolutely right. The thermocouple produces some voltage and it will change the ohmmeter's reading. But you may do the measurements when the temperature is about 0K:D. To be serious there is some possibility to get different readings even with 0K. Often thermocouple wires are twisted instead of to be welded. In this case there is so called "cold" contact. This means bad contact. Any bad contact is non-linear with some semiconductor properties and easily can show different resistance for different current directions. If you are not sure how good is your thermocouple there is easy procedure to check. Put it into boiling water and write the readings. Then put it into melting ice and write this new number. If the numbers differ more than spec (usually not more than +/- 1 degC away from 100degC and 0degC) it is good chance that your thermocouple have a problem. If you need advise how to weld thermocouple in case of bad contact please let me know.

Best regards,
RF-OM
 

the junction has a potential difference of its own and ohmmeter applies a voltage and using the current reading displays the resistance.... in this case there is another potential source due to the junction which doesnt interchange its polarity when u interchange that at the measuring points...
 

An additional remark. The mean value of both readings gives the true thermocouple resistance.
 

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