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We got a bunch of yellow thermocouple wire which the Omega web site says is type K extension grade wire. They also sell thermocouple grade wire which looks to be brown. What's the difference?

I was speculating that the thermocouple grade is super pure and gives voltages that are in agreement with what's expected and extension grade will need to be calibrated but probably won't be too far off.

Does anybody have an experience with this? How far off will it be?

Thank you for any ideas.

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Introduction

Thermocouple Wire that is used in a thermocouple from the point of sensing to the point of cold junction compensation (cjc end) where the signal is measured.

A thermocouple is a sensor for measuring temperature, that consists of two dissimilar metals that are joined together at the sensing end. Different thermocouple types (e. G. J, K, T, E, etc) use different mixtures of metals in the wire. At the cjc end, the millivolt value provided by the thermocouple represents the difference in temperature of the sensing end as compared to the cjc end (also called the reference end).

Cable Colours, you need to consult a cable colour coding guide like this one. R.J. Cables or this one T.C. colour codes

Yellow does infact refer to a K type thermocouple extension cable as opposed to Brown which is for a K type thermocouple compensating cable.

Cable types, THERMOCOUPLE EXTENSION & COMPENSATING CABLES R.J. Cables

Thermocouple extension & compensating cables, transmits, the cold junction of temperature sensor installed at the measuring point to the measuring instruments installed at remote location in control room.

Extension cables are made from the same metals used in thermocouples, but for compensating cable, materials of similar thermoelectric properties as of thermocouple in use, are employed to reduce the cost.

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From the table info the "extension" grades seem to be different alloys ( likely lower cost). For K type , the extension alloy is limited to 392 F ; that is lower than most applications for K . I welded and installed many K couples ( long ago) and they all were used above 500 + F. K thermocouples are Chromel ( A K A , Nichrome ) and Alumel ( 94- Ni , 2- Al, 3- Mn ). The table says the extension is " nickel aluminum " alloy, maybe that means it is Alumel. But I don't know how that affects the emf. So , I would stay with regular K unless your service temperature is below 392 F.

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  • $\begingroup$ Temperature range? Good point. The alloy can't be too different because if it were, it would function as a junction, affecting the indicated temperature. We use it in a tempering furnace and I noticed the insulation does not hold up much above maybe 400 F or so. I expect the max temp is listed 392 F because that converts to 200 C. $\endgroup$ Sep 7 '19 at 21:24

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