Melting thermocouples

I’m trying to temperature map a depyrogenation oven and I’m having a little trouble with the thermocouples. I’m running the oven at 210 celcius and it is melting all the thermocouples together. They are meant to be able to handle at least 250 celcius. What I think is happening is this, the validation port is a metal tube that runs through the wall and touches one of the elements. I think that the elements are heating the tube to greater then the 250 while trying to heat the air in the oven to 210 and thus meltng the probes in the port. The strange thing is that previously people have been able to use this with no problems of melting thermocouples. Unfortunatly they are no longer at this company so I can’t ask them how they did it. As far as I’m aware they are the same thermocouples so I don’t think they are the problem, at least as far as using the wrong type goes.

Anyway what I’m asking is if anyone can see a solution to this? Can anyone think of a way to stop them melting together? Or suggest some sort of an insulation that can withstand those temperatures and not shed particulates?

I have had a similar problem in the the past and you need to use thermocouples that have high temperature insulation, special thermocouples are available specifically for this purpose. It looks like the thermocouples you are using are rated for the temperature but not the outer insulation.

Scott Mc Bean

Most thermocouples use PTFE insulation. This insulation (depending on the grade) will start to degrade at around 200 degrees centigrade, higher grades of PTFE will tolerate temperatures of up to 270 deg C.
What you must do when you get into these elevated temperature zones, is change to mineral insulated (MI) thermocouple probes. MI cables are made in varying lengths and you can buy a length that ensures only the MI cable is in the high temperature zone. Out of the high temperature zone you use matching extension cable.
The MI probes are safe to 1100+ degrees centigrade.
If you log on to:-

http://www.tc.co.uk/
they will send you a complete manual on making and using MI’s.

Regards
Alex Kennedy