1. Whether resistance of thermocouple wires is relevant for temperature measurement? Or whether mv across thermocouple is measured by null method making resistance of wires irrelevant?

2. What is the purpose of extension wire? Can we use thermocouple wire instead of extension wire?

 

The input impedance of most thermocouple signal consitioning circuits is so high that the resistance of the wire is rarely a factor. The wire is as thick as it is primarily for mechanical robustness.

Any time you transition from one kind of metal wire to another you are creating a potential source of error because the transition is itself a thermocouple. It is best to use the same two thermocouple wires all the way back to the signal conditioner.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

Sometimes resistance is relevant and sometimes it's not. It depends on the impedance of the measuring instrument. Sometimes they pad or put some linearizing network across the thermocouple. Most modern types will tolerate resistance within reason. Thermal effects of long runs can creep into your error budget. And the reason for extension thermocouple wire is economy. It is often wire that doesn't fit the published curves, but is sufficiently close in composition that it doesn't produce additional thermocouples at connections. You can use measurement grade all the way if your job is not cost sensitive. Extension wire often lets you afford a larger gauge to lower the resistance.

There's a lot of subtle issues in getting really accurate thermocouple measurements over a range of ambient temperatures. With calibration and attention to the details, TC measurements can be very accurate and repeatable. Many are not.

Regards
cww

 

Hi, to answer your second question;
The primary purpose of extension wire was that a) it was cheaper than the thermocouple wire itself and
b) had the same characteristics.

No additional thermoelectric juction would be created by joining of extension wire and thermocouple wire.