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EDN garbage?

Discussion in 'Electronic Basics' started by George Herold, Feb 5, 2013.

  1. I browsed the referedd to paper.. I didn't find anything about the
    gradients along the wire.... I assume it says something to the same
    effect somewhere.

    To be honest the thermo-electric effect has always been a bit
    mysterious to me. But I never heard of thermal gradients along the
    wire making a difference. I guess maybe if you try and draw too much
    current from the TC jucntion.


    Did it quantify things
    Yeah show me some numbers! Do a measurement.
    I saw none of that.

    Sorry for wasting both our times.

    George H.
     
  2. Yup, thanks
    Oh, now you tell me. :^)
    I use copper-constantan TC as 'mostly' non-magnetic sensors. (fairly
    fine gauge wire.) I'm not sure what other sensors would be non-
    magnetic.
    Maybe a little SMD diode? The TC's are very convenient though... just
    stuff the wire where you want to measure. And cheap OTS
    conttrollers.

    George H.
     
  3. whit3rd

    whit3rd Guest

    [calling the temperature gradients in wire an error source]

    Well, the gradients are NOT an error source, but you'll
    never convince him, because he can show you a counterexample.
    Take a length of soft iron wire (sacrifice a coathanger, if you have to).
    Connect the ends to an ammeter.
    You'll see zero circuit current. Now fire up a torch and heat the wire
    to make a hot spot. Still zero circuit current. Now slowly move the
    hot spot.

    When the hot spot in the wire goes one way, the current turns negative. Move
    it the other way, the current goes positive.

    The temperature gradient in the wire DOES make a thermocouple net voltage
    here, because your hot spot is a different allotrope (sometimes called Austenite)
    of iron crystal than the cold wire is. The Austenite-Martensite transition at
    the trailing edge of your moving hot spot occurs at lower temperature than
    the transition at the leading edge, because the transition has hysteresis. The
    iron wire is thus two different metals, with connections at different temperatures.

    So, if the thermocouple materials undergo phase transitions, the gradients
    in the connecting wire do have a net effect on the circuit. That's a good argument
    for knowing a LOT about the thermocouple wire materials before you measure
    with them.
     
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