Multiple point thermocouple

Discussion in 'General Electronics Chat' started by cmartinez, Jul 27, 2015.

  1. cmartinez

    Thread Starter AAC Fanatic!

    Jan 17, 2007
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    I understand that a thermocouple is an instrument that produces a voltage at the junction of two different metals that is dependent on temperature, and is later referenced, amplified, conditioned and measured by special circuitry.
    I've assembled and used circuitry for this purpose and have few doubts as to how it works.

    However, there's one idea that has long been on my mind lately and I'm wondering if it's possible.

    I'd like to build a single thermocouple capable of measuring temperature at multiple points. Say I consider building a "K" type thermocouple. For this purpose I'd use a single chromel wire, to which several alumel wires are welded at even spacing along its length.
    In the end the assembly would look something like this:
    Capture01.JPG

    Using the chromel wire as "common" would save on the amount of wiring involved.

    Is this possible? wouldn't the signals from the multiple thermocouples interfere with one another?
     
  2. MikeML

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    Oct 2, 2009
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    As long as you only sample one alumel at a time, and the measuring circuit has a high input impedance and a cold-junction reference, and the multiplexer has a very high off resistance for the non-sampled thermocouples, it should work just fine.

    I have six cylinder head thermocouples, and six exhaust gas temp thermocouples, and several miscellaneous thermocouples in my Cessna. They all come to a single measurement system. Only the positive sides are multiplexed. The negative sides are all connected to the same cold reference node inside the monitor instrument. There is no reason why the cold reference node couldn't be extended out of the instrument to save the weight of negative wires....
     
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  3. cmartinez

    Thread Starter AAC Fanatic!

    Jan 17, 2007
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    Great! Thanks for your very clear explanation. I'm going to start doing some experimenting... see how it goes.
    So you're recommending I use an analog multiplexer for that?
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The commercial aircraft engine monitor system uses a single-ended (not differential) analog multiplexer chip ahead of the plus input of a single instrumentation amp. The minus input to the instrumentation amp is offset by the cold junction reference generator chip (Analog Devices). It reads to about 0.25 deg F...
     
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