Extension Grade vs Thermocouple Grade Wire

[fifty60]

I am trying to understand the difference between Extension Grade and Thermocouple Grade wire. I can only use Thermocouple grade wire at my sensing point, and I can use extenstion grade wire to extend T/C grade wire. I understand that much.

But when it comes to temperature ratings, I get confused. Ext. grade wire has standard limits of error up to 100 C, whereas T/C grade wire has limits of error that are much higher.

Here is my question: If I need a thermocouple that is to sense up to 200 C, can I use the T/C grade wire in the sensing area (where it is exposed to 200C) and then once outside of the sensing area land in a T/C biased terminal block and then make the rest of the run with Extension Grade wire?

The extension grade wire will not be exposed to temperatures above 100 C, but the signal it is carrying will come from a T/C grade wire that is above 100 C. Is that the correct use of extension grade thermocouple wire?

 

[Smart $]

Correct. Extension grade has a lower operating ambient temperature rating but will pass a signal coming from a higher temperature ambient.

 

[GoldDigger]

The thermocouple EMF is produced where two dissimilar metals or alloys meet. If you have a loop of wire made up of two thermocouple metals with a junction at each end the voltage around the loop will depend strongly on the difference between the two junction temperatures but also less strongly on the individual junction temperatures.
As long as the wire composition is identical from one junction to the other the intermediate temperatures will not matter at all.

But to get a very precise and repeatable voltage to temperature relationship you need to have the composition of the alloy or the purity of the metals very tightly controlled. This is expensive.

If you use lower quality wires, but still in the same family as the two thermocouple wires, then the loop voltage will also depend on the temperature of the four junction points where the "cheap" wires are spliced into the loop. The reason that you can get away with lower quality wire for this application is the assumption that the four junctions involved are at close to the same temperature, however that may compare to the hot and cold junction temperatures.
The layout of the signal wire helps guarantee that the two junctions at each end of the extension are very close to the same temperature, while putting an overall temperature range limit on the extension wire helps to keep the temperature of the two junction pairs close together.
Because the extension wire alloys are close to the composition of the thermocouple ends, the small differences in junction temperatures will have a small effect on the overall loop voltage.

 

[fifty60]

Smart$, thanks. GoldDigger, I had to read what you wrote 3 times, but it is very clear and to the point. Thanks.