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Thermocouple

Discussion in 'Electronic Design' started by Danny, Nov 26, 2003.

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  1. Danny

    Danny Guest

    Hi

    I am making a thermocouple from Copper/Constantan, i plan to twist the
    ends together at the appropriate junctions, and then solder them in
    place, a spot welder is a little bit out of my league, would the solder
    affect the performance of the thermocouple or not?


    Danny
     
  2. I read in sci.electronics.design that Danny <>
    Provided it doesn't melt, no. The thermal voltage is not developed at
    the junctions, contrary to popular belief, but in the wires themselves.
    Even though you may have two junctions, copper-solder and solder-
    constantan, they are both at the same temperature, so there is no
    spurious voltage.
     
  3. James Meyer

    James Meyer Guest

    This is 100% in line with my research into thermocouples.

    Jim
     
  4. JeffM

    JeffM Guest

    Even though you may have two junctions,
    This assumes that the copper-solder junction
    produces a voltage which is equal and opposite
    to that of the solder-constantan junction
    (facts not in evidence).
     
  5. <>) about 'Thermocouple',
    The junctions DO NOT produce any voltages, contrary to popular belief.
    Thermoelectric voltages are produced by the temperature gradients in the
    wires.
     
  6. If there is no temperature difference, then there is no voltage.
    Always.

    Best regards,
    Spehro Pefhany
     
  7. Junctions do not produce voltage. Changes in temperature along
    conductors is what produces thermocouple voltage. As long as the
    solder joint is small enough to be at essentially one temperature, it
    produces no error voltage.
     
  8. James Meyer

    James Meyer Guest

    The facts that *are* in evidence state that the voltage generated by a
    thermocouple is *not* generated at the junctions. The voltage is generated
    along the length of the conductors due to the temperature differences between
    the ends of the conductors.

    In point of fact, a junction is not required at all for the generation
    of a voltage. A conductor with a temperature difference between its ends will
    generate a voltage. No junction is required unless you want to make use of a
    current flow due to the voltage generated.

    A little research before you post will reduce the number of times you
    put your foot in your mouth.

    Jim
     
  9. Luhan Monat

    Luhan Monat Guest

    Wow, this is not what I learned in my university physics class! It all
    had to do with how the free electrons interacted at the junction. Also,
    I have never heard of voltage created by temperature gradients.

    Do you have some acedemic reference your position? I would like to
    check this out and settle the matter for myself.
     
  10. No, it's the free electrons in the wires, which have a temperature
    gradient along them. The distribution of the gradient in a homogenous
    wire is not particularly important (law of Magnus). A T/C measures the
    difference between those potentials in different metals.

    An ideal T/C would have a zero-width junction. Obviously no
    temperature delta across that with finite heat flow. Real T/Cs have
    almost no delta-T across or even near the alloyed region that is the
    junction.
    Look up Seebeck (coefficient), as well as Peltier and Thomson (that
    Lord Kelvin bloke).

    Best regards,
    Spehro Pefhany
     
  11. I read in sci.electronics.design that Luhan Monat
    No, I don't have any references; I don't have access to a university
    library. But I feel sure you will find the information on the Net with a
    Google search, with appropriate references.
     
  12. Do you have a name for the effect that causes this small gradient at
    the junction, so I could look it up?

    I have never heard of such a thing. I have used intrinsic junctions
    made by welding the two thermocouple wires to some third metal object,
    to measure the approximate average of temperature between the two
    contact points. The composition of the metal between the two
    thermocouple leads appears to have no effect on the temperature
    reading.
     
  13. Luhan Monat

    Luhan Monat Guest

    Yep, its the gradient in each wire plus a small contribution from the
    junction itself.
     
  14. Here is an introduction to thermoelectricity:
    http://www.chem.cornell.edu/fjd3/thermo/intro.html

    I see no mention of a voltage at the junction of two dissimilar
    materials.

    Besides, if such a junction could produce a voltage (that could create
    a continuous current) at a single temperature, it would violate the
    conservation of energy. It would produce continuous power with no
    absorption of energy from anywhere.
     
  15. There is a so-called 'contact-potential' which is a contribution to
    the electrochemical potential but other contributions (from a charge
    double-layer at the boundary) cancel it out so there is no net
    electrochemical potential difference across a metal-metal
    junction at equilibrium.

    Strictly speaking, a normal voltmeter measures electrochemical
    potential difference, not electrostatic potential difference.

    Charles.
     
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