Multiple thermocouple sensors on a metal bar.

Discussion in 'General Electronics Chat' started by cages, May 26, 2011.

  1. cages

    Thread Starter New Member

    Jul 3, 2010
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    Hello
    I am trying to measure the temperature gradient(6 points) on a heated metal bar using thermocouples and a data logger.
    The plan is to use bare ended thermocouples so the actual thermocouple tips are touching the metal bar.
    Thermocouples sensors generate a voltage that the logger converts to a temperature reading. I am not sure if the sensors will interfere with each other as they are all connected to a conducting surface?

    Could anyone please explain the potential problems if there are any.

    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
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    Make the mass of the metal bar far in excess of the mass of the thermocouples to make the interference negligible. Place the thermocouples at least 10 diameters (of the thermocouple) apart. Tie them on with small diameter wire. Shouldn't be difficult considering that thermocouples are rather tiny.
     
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
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    90
    I think he means electrical interference because each TC is electrically connected to the metal bar.
    The datalogger spec sheet should say if the inputs are isolated from each other or if it works with grounded TC's.
    If worse comes to worse, hook two of them up to the logger, touch them together and see what happens.
     
    cages likes this.
  4. cages

    Thread Starter New Member

    Jul 3, 2010
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    Thanks you for the suggestions, it is electrical noise I worried about. I will have a look at the data-sheet and touch tow thermocouple tips together to see if it causes any change in the readings.

    cages
     
  5. #12

    Expert

    Nov 30, 2010
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    Some teflon tape from the plumbing department might work. It's an excellent insulator, survives very high temperatures, and it's thin and low mass, but it punctures easily. If you can be careful enough to avoid punctures, it will insulate the thermocouples.
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    What temperatures are we talking about?

    Is it 1000 degF?

    More?

    Less?

    You can use non-conductive thermal pads to isolate the thermocouples from each other quite easily, but you must use thermal pads that are designed to handle the expected temperatures of the metal bar.
     
  7. cages

    Thread Starter New Member

    Jul 3, 2010
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    0
    The temperature to be measured should be between 100°C to 200Deg°C.
    I have done some measurements and the electrical conductivity between the thermocouples through the bar does not seem to effect the readings.

    The issue I am having is that it seems the thermocouples need to have a clean contact with the bar.
    I can tape two thermocouples to the bar about 1cm apart and get a variation of 4°C . Applying bit of pressure to the tape holding the thermocouples in place brings the readings to less than 1°C of each other.
    I am planning on trying a thermal paste on the tips of the thermocouples to see if that will improve the thermal conductivity between the bar and the thermocouple.

    Thanks.
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
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    Use thermal grease.
     
  9. someonesdad

    Senior Member

    Jul 7, 2009
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    Personally, I would tap some small screw holes and use a small screw to hold the thermocouple junction to the metal bar. My guess is you won't have to insulate the junctions from each other. What you want is good thermal contact that doesn't move. Even better, use a torch or arc (TIG works great) to weld the thermocouples to the metal bar. Note you don't even have to have the two thermocouple wires touching -- they can be a mm or so apart and things should still work fine (as long as there's no temperature gradient in the bar).

    If you use the tapped hole method and you do find that you need to electrically isolate the TC junctions from the bar, you can use thin dielectric washers (nylon is probably the easiest to find). Places like Omega sell thermocouples read-made in a variety of mounting methods.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    When I want to get an accurate measurement, I immerse the thermocouple. This means I drill into the metal for about an inch a hole just big enough for the thermocouple wire.

    Baring that, I make sure the ball and 1" of wire are touching the metals surface. Too many people touch the ball and don't think about the heat sinking the rest of the wire provides.
     
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