Thermocouple outside temperature range - damage or just crappy readings?

[shadewind]

I got myself a K type thermocouple from Agilent for use with my Agilent U1242B DMM. I wanted to use it for measuring the temperature when doing hot air reworking. I tried it out and it seemed to work fine but then I read that it's only rated up to 200 degrees C. Before I knew this, I placed in front of the hot air gun at 350 C for while.

Will this have damaged the thermocouple or does surpassing the rating only give crappy readings? I suppose the insulation plastic might have melted slightly but it looks fine anyway and it still gives readings but I can't be sure if they're correct.

[IanB]

My understanding is it the cover material that has a limited temperature range rather than the thermocouple itself. Most likely if it looks fine it is fine.

[saturation]

Beyond the rating, the thermocouple calibration is not guaranteed, it won't damage the metal per se, but that depends on what type of wire the thermocouple is made off before it too is damaged.  The insulation on the thermocouple is rated usually + 50%, so it will melt at ~ > 300+C.

For a hot air gun rated to 500C I'd get an K type in eBay, they are fairly low tech and generally reliable.  You can also test its accuracy against a water bath of ice and boiling water.   I got one 800C for $12, its encased in steel mesh and has heavily metal/glass insulated wires.

[Kiriakos-GR]

All the low cost - Low temperature / K type thermocouples are almost indestructible 
The ones with plastic external tube cover, looks better in the eye but the plastic can melt.

The major concern about high temps it to not burn your self, the K type thermocouple is not in risk.

Check this link about accessories and K probes. 
https://www.eevblog.com/forum/index.php?topic=448.45

[Jon Chandler]

If the jacket melts allowing the conductors to short along the length somewhere, the results could be interesting.  This would effectively cause multiple junctions in parallel - I'm not certain what the result would be.

[ejeffrey]

You would just get a temperature reading that was somewhere between the higher and lower temperature junction.  Which junction wins depends on the resistance of the thermocouple wires and the contact resistance of the unwanted junction.

High temperature thermocouples have fiberglass or other high temperature dielectric to prevent this from happening.

[Kiriakos-GR]

If the jacket melts allowing the conductors to short

Those conductors are twisted wire and looks naked, and this is the amazing of it, naked wires touching its other and still works. 
I will post a picture later on.

[shadewind]

The cover is fine, it just started to look a bit "soft" if you know what I mean.

[Kiriakos-GR]

Do not worry, the only way to kill a thermocouple is knock him down with a hammer.