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anyone know how to hook up old pyrometer

Discussion in 'Electronic Design' started by [email protected], Jun 26, 2007.

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  1. Guest

    I bought an old pyrometer off ebay. its a sim-ply-trol. it can be seen

    It says "iron-constantan" and "ext. res 10 ohm" on the front. So I
    guess that means Type J. But what does the ext. res 10 ohm mean?

    Also, on the back, there are two screw terminals coming out of the
    meter, but one of them has a little plastic dohickey screwed to it
    that has some kind of wire wrapped around it which is attached to
    another, floating screw terminal, which is shorted to the OTHER panel
    mount terminal with a piece of wire. Is this so you can measure meter
    temperature without a thermocouple? And do I connect a real
    thermocouple directly to the screw terminals, or do I go through the

    And how does a mechanical device like this compensate for the cold
  2. The thermocouple should be 10 ohms (total loop resistance) for an
    accurate reading. The error if you're not at 10 ohms will depend on
    the coil resistance (umm.. if you try to measure it, use a digital and
    start at a high ohm range so as not to bend the needle accidentally).
    The meter has to measure the temperature at the terminals in order
    to read a thermocouple. Also the coil resistance is going to vary with
    temperature so there's probably another device inside to help
    compensate for that.
    I canna see it, but most likely the screw terminals.
    Something like a bimetal typically. Some don't bother, you just adjust
    the 'zero' screw so it reads 70°F or whatever at room temperature and
    hope for the best.

    Best regards,
    Spehro Pefhany
  3. Guest

    The thermocouple should be 10 ohms (total loop resistance) for an
    I measured the thermocouple resistance, its 6.4 ohms (DMM across
    thermocouple leads).

    I measured the meters resistance (DMM across direct screw terminals)
    25 ohms

    How do I figure out what the error should be?


  4. Put a few ohms metal film in series with the T/C (keep the leads
    short-- bend them into a 'U') and don't worry about it. Those things
    are VERY crude at the best of times.

    Best regards,
    Spehro Pefhany
  5. whit3rd

    whit3rd Guest

    Typically, it indicates that calibration will meet specs if the
    resistance of the thermocouple wire is 10 ohms or less.
    It's intended for hot furnaces (type J is good for 1600F) so ten
    one way or the other is not important. Probably no cold junction
  6. Guest

    10 ohms or less? Not exactly 10 ohms? So if my thermocouple measures
    6.4 ohms I am good to go? Also, this meter goes from 0 to 300C, does
    that mean its probably meant to be more accurate than a 1600F one?
  7. Exactly 10 ohms. Modern electronic indicators have a maximum
    resistance (usually at least 100 ohms).

    Best regards,
    Spehro Pefhany
  8. Rich Grise

    Rich Grise Guest

    Try it! Connect it the way that looks obvious, and get a thing of
    ice water and a thing of boiling water and see what readings you get.

    Good Luck!
  9. Guest

    I did. It seems to work okay for ice water, but boiling water went way
    beyond 125C on the meter. I measured the water with another known good
    K type thermocouple and digital meter and it was 99C.

    I guess this meter is F-ed. But what goes bad in a mechanical meter
    like this? Its all metal right? Looks like I wasted $50
  10. Rich Grise

    Rich Grise Guest

    I wouldn't say it's fried - it deflects, right? But the different reading
    might be what the little unknown thingie on the back is - a calibration
    thing or something.

    It's worth further investigation. :)

  11. - maybe it's connected wrong and that thing has to be in series

    - I'd expect an error of only about +11% with the resistance the
    way you have it (about 10'C on 80'C above room temperature) if
    the correct total resistance is 35 ohms.

    Best regards,
    Spehro Pefhany
  12. Jasen

    Jasen Guest

    maybe it needs all of the 10 ohms resistance (and you currently have 6?)
    try adding some resistance in series with the cold thermocouple.

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