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Interfacing to a thermocouple

Discussion in 'Electronic Basics' started by Nick., Dec 28, 2005.

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  1. Nick.

    Nick. Guest

    I need to set my thinking straight on how to connect to a type-K
    thermocouple...

    The hot junction is straightforward. The "cold" end is terminated by a
    plug. This will connect to a PCB-mounted socket, which will connect to an
    AD595 cold-junction compensated amp. Obviously there are now several "cold
    junctions" (thermocouple to plug, plug to socket, socket to solder, solder
    to copper etc...)

    Am I correct in assuming that as long as these are maintained at the same
    temperature they won't skew the actual temperature measurement?

    Thanks,
    Nick
     
  2. Yes.
     
  3. Nick.

    Nick. Guest

    Blimey, that was quick (and to the point!)

    Thanks.
     
  4. Dan Hollands

    Dan Hollands Guest


    And the cold junction of the OpAmp must be at that same temperature

    Often the plugs and sockets are made out of thermocouple material which
    takes them out of the equation. Everything from the point were the
    thermocouple material connects to non thermocouple material must be at the
    same temperature. Careful arrangement of components to avoid heating or
    moving air is required to make a stable accurate device.

    Dan
    --
    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
    585-872-2606

    www.QuickScoreRace.com
     
  5. jibberjabber

    jibberjabber Guest

    A connector/junction/etc only needs to be isothermal across itself. It does
    not need to be the same temperature as all the other connectors (if that is
    what you were asking). Ex.: a chromel wire enters a copper connector then
    exits as a chromel wire. If the 'entrance' junction is the same temperature
    as the 'exit' junction then the errors cancel.

    If you have a series of junctions then the first junction (where the TC wire
    first connects to another metal) has to be the same temperature as the
    AD595. If not then you get an error equal to this difference.

    Page 5 of AN-274 (Analog Application Sheet) shows inserting a second TC with
    "reverse polarity" in series with the first TC to cancel the error. See the
    diagram for placement details.

    jj
     
  6. ayeshasaftain

    ayeshasaftain

    1
    0
    Jul 7, 2009
    Problem with K type thermocouple and ad595

    Hi ,
    I am making a data acqusation system with 20 K type thermocouples.my connection are are thmermocouple>maleconnector>female cooncteyor>sheldedwire of 15-20 cm >wire end soldered on pcb>muxdg407>ad595cq>muxDG406>adc ad574>89C52>pc .
    I have got the prolem of fluctions.when only one thermocouple is connected i get small data varation of 2-3 oC but if 2 thermocouples are connected the tmep get out of control to 200 to 500 .even at room temp.also when i put my thermocouple in hot water.
    I have earth the the body of thermocouple and grounded pin-1 of ad595.if i donot ground it my output goes to zero even if thermocouple body is earthed.
    plz if any one can suggest a solution. or tell me what and where the probem is.Thanks
     
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