Connect with us

Thermocouple

Discussion in 'Electronic Repair' started by Christian, Aug 30, 2004.

Scroll to continue with content
  1. Christian

    Christian Guest

    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
     
  2. 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.
     
  3. CFoley1064

    CFoley1064 Guest

    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
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-