Thermocouple wire gauge/diameter/size at high temperatures

[stephencox]

I have to ask this question.  Maybe my Google skills are just failing me, because it seems obvious, but I can't find the answer

I understand about problems with high resistance in thermocouples that use too much thin wire, but why are there lower high temperature limits given for high gauge wires?

What happens if I use these thermocouples at such temperatures?  Is the concern just that the wire will burn up quickly, or is there some reason that the temperature readings will be inaccurate?

[stephencox]

Ok, I called and spoke to someone at Omega.  According to them, the concern is for the wire burning up at high temperature, so as long as the junction is still functioning, it should be accurate to the nominal maximum of the wire type.

[johnsold]

I cannot answer your questions completely but will throw out a few thoughts.

1. Metallurgy is complicated. Over wide temperature ranges you can have changes is crystal structure, phase changes in alloys, grain growth, and probably other effects. Some of those might affect the output of a thermocouple, especially at high temperatures and small diameters.

2. Mechanical expansion and the strength of the material might result in sagging or breaking.

3. The manufacturing process - drawing into wire - may result in imperfections (due to inclusions, grain boundaries, and so on) which create unacceptable errors at high temperatures.

Lynn

[Buechsenschuetz]

All metals loose much of their mechanical strength far below the melting point. Maybe this is the reason why thicker wires are used for high-temperature thermocouples, the thicker wires will withstand more mechanical stress at high temperatures.

[Henrik_Volkers]

And there another point: Oxidaton/Obladation and reactions with the isolation material will change the Seebeck coefficient -> sensivity of the TC.  You cant avoid it, just try to minimize it.

This is a function of temperature and the ratio of surface to material -> smaller diameter faster change in sensivity.

So you can use a smaller TC wire for a shorter time 😉 (until recalibration/replacement)

[BradW]

How high is high temperature?  Base metal TCs are specified not by their alloy content but by the resulting Seebeck coefficient.  Some, like K type, have certain elevated temperatures which cause migration of the metals across the junction if you hold at those temps or repeatedly cycle to those temps in an oxidizing environment. That causes a Seebeck shift which throws the calibration off.   Bigger wire of course slows down that process.  Omega's Type N was formulated to resist that behavior.  I bumped into this with a very high temperature heater.  Type N was my solution.  If you can afford it, noble metal TCs (platinum) might be a preferred way to have fine gauge TCs at high temps, but they are $$ if you have a lot of them (I had dozens).

[stephencox]

High temperature in this case means high relative to the stated maximum of the wire gauge, but lower than the maximum of the wire type.  For the K type thermocouples, that means the range of 700-1250C.  I am aware of the green-rot problems that supposedly occur in vacuum or reducing environments at these temperatures (K types are stable in oxidizing environments), but I have not yet observed them myself for the short duration experiments that I do.

For the more intense experiments I use C type thermocouples to extend the temperature ceiling and vacuum stability of the thermocouple.  As far as I know, I wouldn't gain anything from switching to noble metal thermocouples.  Oxidation is not a concern, the precision of the type C is fine, and the high service temperature can be handy.  I am, however, not an expert, so I would love to be corrected if I am wrong.