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Question regarding thermocouples and a voltage terminal block

Discussion in 'Electronic Design' started by kd6532, Dec 13, 2006.

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  1. kd6532

    kd6532 Guest

    Right now some peers and myself are designing a pv module testing
    system, in which, one of the things we will be testing is temperature.
    To do so we will be using four thermocouples on each solar panel, and
    we wanted to join the four into a voltage terminal block to reduce the
    amount of wires we will be running to our multi tracer. The problem is
    we are having trouble finding a terminal block that will establish a
    CLEAN connection and allow us to connect the four wires in and have
    just one coming out for both the copper and constantan leads. Is their
    a better way to approach this? We found one promising terminal block at
    Phoenix Conacts that is specifically constructed for thermocouples
    composed of copper/constantan, but four leads cannot go into one of the
    inputs, and the tech. support basically told us to try another route.
    Any suggestions are welcomed, and thanks for your time.

    Kris
     
  2. John  Larkin

    John Larkin Guest

    Are you going to parallel the tc's into one signal pair? If so, *any*
    terminal block will work fine. Just find one big enough to squish all
    five wires together. Or crimp them into a butt splice, any wires in
    either or both ends (3 in one end, two in the other maybe), doesn't
    matter.

    John
     
  3. kd6532

    kd6532 Guest

    Interesting. I am wondering, since tc's basically convey the
    temperature as the potential difference between the two conductors,
    using a different metal at the junction would not alter this potential
    that we will be measuring across the copper and the constantan. And
    just out of curiousity, using a metal doesn't typically change the pot.
    difference does it?

    Thanks again,
    Kris
     
  4. Geo

    Geo Guest

    google for :-/ thermocouple "cold junction"

    Geo
     
  5. John  Larkin

    John Larkin Guest


    As long as the junctions are isothermal, they can't generate any
    potential. As a practical matter, scrunching some number of wires
    under a screw, or crimping them into a butt splice, will be
    isothermal.

    And oh, we like to bottom-post here.

    John
     
  6. I don't understand- are you paralleling the T/Cs? If so, for type T,
    just solder the leads together and be done with it.



    Best regards,
    Spehro Pefhany
     
  7. John Perry

    John Perry Guest

    ....Well, that's a bit too simplified, I believe.

    Different conductors in contact generate a potential difference.

    Differences in temperature influence this potential difference in
    particular ways depending upon the specific conductors connected.

    The key is to know the temperature of all other connections (called
    junctions in thermocouple language) to different conductors so that you
    can infer the temperature of the test junction based on the known
    temperatures and the known junction relations.

    So, you can use just two different metals, join them at one junction,
    and connect them to a measuring device. BUT, this device has to be at a
    specific temperature if there's any other metal involved -- for
    instance, your voltmeter certainly has only copper in its circuitry, so
    where it connects to the constantan wire, you have a second junction.
    You have to know the temperature of that junction before you can infer
    the test junction's temperature from the thermocouple relation.

    This means you have to either control the voltmeter's terminal
    temperature, or measure it in some independent way. For decades, the
    usual method was to chill the second junction in ice water (thus the
    standard term cold junction). Later, it was controlled at a higher
    temperature. Now, we simply measure the temperature of the second
    junction and calculate the test junction's temperature from the
    temperature difference between the two junctions.

    But the key is to know the temperature and the materials of every
    junction in the circuit.
    And we ought to edit irrelevant text from our messages.

    John Perry
     
  8. John  Larkin

    John Larkin Guest

    The OP wasn't entirely clear, but it sounded to me that all 5 wires at
    each junction were the same tc alloy. That being true, the temp of
    those junctions doesn't matter, since each junction is isothermal and
    mostly all the same stuff.

    If that's not true, well, the temps do matter. As usual, the exact
    situation is unclear. A diagram would help.

    John
     
  9. Glen Walpert

    Glen Walpert Guest

    ....Well, that's just plain wrong. Different conductors in contact do
    not generate any potential difference unless a poor connection is
    made, and that effect has nothing to do with proper thermocouple
    operation. The voltage in a properly assembled thermocouple is
    generated entirely by the temperature gradient along the conductors.
    The junctions serve only to complete the circuit so that the
    difference in voltage produced by the same temperature gradient along
    two different wire materials can be measured. Of course the junction
    temperatures are the temperatures of the ends of the two different
    wires and so determine the total temperature gradient (and therefore
    voltage) along the length of both wires, so the junction temperatures
    are used in all thermocouple calculations even though the junctions do
    not generate any voltage.

    So in order to make a proper temperature measurement with a
    thermocouple the connections to the copper voltmeter terminals must be
    at the same temperature, in order that the temperature gradient along
    both thermocouple wires is the same, and one end (either one) must be
    at a known temperature so that the other temperature can be
    calculated.

    http://www.electronics-cooling.com/Resources/EC_Articles/JAN97/jan97_01.htm
     
  10. John Perry

    John Perry Guest

    Yes, I'm familiar with all that theoretical verbage. And, as you point
    out later, it's effectively indistinguishable from the practical reality
    I spelled out. Which is much easier to introduce to one who doesn't
    already know it all.

    jp
     
  11. cloudnine

    cloudnine Guest

    The TC's are being paralleled together.

    Do you think by soldering the leads together of the type T
    thermocouples would give as accurate a measurement as they could with a
    terminal block of the correct metals?

    Attempting to get as accurate as possible paralleling those 4 TC's into
    the input and having 1 output is our main objective.

    Thanks
     
  12. cloudnine

    cloudnine Guest

    The TC's are being paralleled together.

    Do you think by soldering the leads together of the type T
    thermocouples would give as accurate a measurement as they could with a
    terminal block of the correct metals?

    Attempting to get as accurate as possible paralleling those 4 TC's into
    the input and having 1 output is our main objective.

    Thanks
     
  13. The solder adds no error, as long as all of it is at the
    same temperature (no thermal gradient across the soldered
    section, since temperature gradient is responsible for the
    thermocouple voltage).

    I assume you are paralleling thermocouples to average many
    temperature measurements. If so, keep in mind that the
    resistance of each of those thermocouples plays an important
    part in how each one contributes to the average. Lower
    resistance couples will contribute a proportionately larger
    part of the average result.
     
  14. cloudnine

    cloudnine Guest

    If all metals were placed in a box at the same temperature, is there
    any reason to believe the soldier section would have a thermal
    gradient?

    Thanks,

    Pete
     
  15. Not unless one side of the box were exposed to heat and one
    side exposed to cold. Wrap the box in thermal insulation
    and the internal gradients will be very low.
     
  16. John  Larkin

    John Larkin Guest

    If you are paralleling identical-alloy leads, scrunching them together
    in a clamp-type terminal block or under one screw, the terminal block
    material *doesn't matter*. And a crimped butt splice would be just as
    good. Or twist, solder, and tape. All good to micro-kelvins. The
    biggest source of error will be alloy differences between the t/c
    leads and the extension wire, so keep both of the 5-wire junctions
    close together; but that's way second-order, still a minute error.

    Besides, thermocouples aren't super accurate to start with, and I
    can't imagine the solar cell measurement has to be ultra-precise. So
    just do it.

    John
     
  17. John  Larkin

    John Larkin Guest

    Really, none of that sort of thing is necessary. Connect the 5 wires
    together at one point, any way you can, and it will be a negligable
    contribution to system error. This is *not* an issue.

    What kind of signal conditioning will you be doing on the other end?
    That error will probably swamp the junction thing by, literally,
    1000:1. Most commercial t/c acquisition stuff has really mediocre
    reference junction compensation.

    John
     
  18. Glen Walpert

    Glen Walpert Guest

    I can't agree with that view at all. The verbiage "thermocouple
    voltage produced by the junction" is complete nonsense; how can it
    possibly help anyone's understanding? It does not take much
    investigation to actually understand the physical basis for the
    function of thermocouples, which in my opinion is far better than
    relying on a completely fictitious explanation even if that fiction
    can be used to make correct calculations.
     
  19. Sure. As someone else said, you're actually going to be getting a
    weighted average of the temperatures, based on the thermocouple
    resistances, but if they're all similar materials and similar lengths
    it will be good enough for your purposes.

    Just don't do anything silly like putting the solder blobs near a heat
    source. You can tack them down by putting them into a terminal strip
    or whatever. You certainly do not need T/C material in the strips.

    Best regards,
    Spehro Pefhany
     
  20. YD

    YD Guest

    Late at night, by candle light, John Larkin
    At best the actual reading will be the average of all four TCs. No way
    to tell which one is out of whack if the readings look suspicious.

    TCs are common in industrial applications and eg. Devicenet devices
    can have several inputs and transmit the readings to a central
    receiver over a single wire. Try taking it up in a controls and
    instrumentation forum. Go the whole hog and get a PLC and some SCADA
    for a few kilobucks.

    - YD.
     
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