Immersion circulator and thermocouples

Discussion in 'The Projects Forum' started by crazyhorse19, Jan 17, 2013.

  1. crazyhorse19

    Thread Starter New Member

    Jan 17, 2013
    2
    0
    Hi

    I have a old immersion circulator (model fisher polystat 24) that is broken in that everything works apart from the temperature setting.
    the unit heats the water but does not stop at the set temperature it just keeps heating it until it boils (the light to indicate the unit has not reached the required temperature also remains on)

    i am wondering if it may be either a broken thermocouple, or something more complicated. but i am confused as to what the elements on the unit are...
    to me it looks like there could be two thermocouples, or is one a water level sensor or something else (the two metal poles in the picture)?
    the wires from both go into two different circuit boards


    (for heating there are three pre-set levels for laborotory application at 25, 30, 37 and then a variable setting, where you can turn the dial from 20-100C, and then a fine tuning dial where you can adjust +/- 1C)

    just wanted some second opinions before i go and spend money on getting a new thermcouple or getting it repaired

    sorry for the newbie question and thanks in advanced
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,576
    231
    Have you tried to measure the voltage across the thermocouples...with one lead disconnected from the PCB. Place them in ice water and then in hot water. If they are OK, you should see a very low voltage...millivolts... that changes with temperature.

    Ken
     
    Last edited: Jan 17, 2013
  3. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,061
    Fisher offers pretty good service, in my experience. Can they not supply you a schematic?
     
  4. crazyhorse19

    Thread Starter New Member

    Jan 17, 2013
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    0
    thanks didnt really think to contact fisher since i got it second hand but i guess it cant hurt to get in touch with them
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I'm gonna place my bets that the SSR (solid state relay) providing the power to the heater element is fried.. and typically when they die they fail "closed" not open causing this exact problem.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Since the light indicating the desired temperature has been reached never goes off, I doubt that the SSR is the problem.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    doh !#$.. Missed that part..
     
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