Thermocouple touching aluminium

[kitepassion]

Hi all,

I have done a PID controller for a home made oven. The oven is in pure aluminium. When the thermocouple is in the air ( not touching the aluminium ) I got the right value of temperature. But when it is touching the aluminimum I got high oscillation in the value acquired..

Why this is happening? How can I avoid it?


Thank you

 

[crutschow]

Is this a bare themocouple? If so there may be some sort of feedback between the oven and the thermocouple circuit. You may need to electrically isolate the thermocouple from the oven, perhaps with a non-conductive epoxy and/or a transistor thermal pad.

 

[kitepassion]

Yes it is a bare thermocouple..

Which epoxy can stand 600 Celsius?
Any suggestion regarding an isolate thermocouple that can stand this temperature?

 

[FvM]

With a respectively designed thermoelement amplifier, ground connection of a thermocouple won't be a problem. Or use ceramic insulation.

 

[KlausST]

Hi,

isolationg the thermocouple is one way.

Isolating the measurement device is another possible solution. I guess it´s powered by line voltage instead of battery.

If working from battery does not solve the problme, then it may be an EMV problem. If this is the case, then you need proper EMV/EMI filtering to the thermocouple.
Usually there is an amplifier (with high gain) with cold juction compensation, make sure it works in a stable region with clean power supply.

Hope this helps
Klaus

 

[kitepassion]

Hi FvM,

I agree I used MAX6675 as amplifier. The oven is not connected to the same ground potential. I mean the aluminium heater. Is it the issue??
The oven is completely made of aluminium.

 

[FvM]

MAX6675 grounds one thermocouple leg, so the thermocouple must be isolated from aluminium chassis and other metal parts touching it.

 

[kitepassion]

Do you have any solutions which do not ground any legs?
The question of the epoxy is still open.

 

[jpanhalt]

The question of the epoxy is still open.

High-temperature epoxies: https://www.mcmaster.com/#high-temperature-epoxies/=rw6dqw

As suggested, also consider ceramic adhesives and sealants, which while not "epoxy" will have equally high temperature ratings. LocTite (Henkel) is one brand worth looking at. You could also consider a ceramic sleeve or small-bore pipe.

John

 

[art666]

as you say it reads the right temp when not touching aluminium, you might try a p-clip with ptfe tubing /or/ tape to isolate the thermocouple.
I have seen this done in microwave ovens.

 

[kitepassion]

Thank you really to all of you!

If I want to measure the temperature of the metal instead?

I know... I am complicating a bit the things..

 

[FvM]

You can use a thermocouple amplifier with instrumentation amplifier input stage, or use a circuit with grounded input like MAX6675 with isolated power supply. It may be necessary to filter the input against common mode noise