Thermocouple

Dear all,

I am having a vacuum setup with a Ni-NiCr-Thermocouple inside, which consists
of two welded wires. The question is, if it is allowed to ground the
thermocouple in the welded point, i.e. to attach it to the metallic sample
holder, where the temperature should be measured (reference point outside at
room temperature).

When the thermocouple is isolated in the vacuum, i.e. not attached to the
ground, the readings are fine. But as soon as the thermocouple is grounded, the
readings become oscure. From my point of view, the grounding of the
thermocouple should have no influence, because a difference in voltage is
measured.

The only possibility, which I considered is, that the device which evaluates
the thermo-voltage is not on the same ground than the metallic sample holder...

So: Is it possible to connect the thermocouple to the ground (metallic sample
holder) without a loss of the thermo-voltage?

Anybody has an idea?

Btw: Isolation of the thermocouple is not possible, because temperature gets to
high or isolation material has low thermal conductivity.

Christian

 

You need to check the specifications of the instrumentation that reads
the thermocouple voltage. It should only be meassuring the difference.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header is ignored.
To contact me, please use the feedback form on the S.E.R FAQ Web sites.

 

Subject: Thermocouple

Two possibilities:

1) Your instrument is operating at a given potential relative to the sample
"ground", causing sneak current, or

2) You may have ground fault current flowing through the sample from another
source, and the AC is somehow messing up the input. AC instrument isolation is
sometimes very different from DC isolation.

I'd recommend a small isolation transformer for instrument power if you can
find one and the instrument isn't connected through RS-232, GPIB, printer port,
or whatever, to anything else. In which case you might have to put that on the
isolation transformer, too. Also, try a battery-powered thermocouple
temperature meter and see if that makes a difference. That will prove your
theory.

Good luck
Chris