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Use plug / jack with K Type thermocouple?

Discussion in 'General Electronics Discussion' started by Andy.wpg, Oct 19, 2015.

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  1. Andy.wpg

    Andy.wpg

    19
    1
    Apr 8, 2015
    I am building a project that uses two K Type thermocouples connected to AD595 ic's.

    In previous versions of this, I have soldered to pads designed as per the AD595 datasheet. The datasheet seems to suggest this is the best way to do it, since introducing other junctions into the circuit messes up the readings.

    How do companies do it with jacks on their products? For example, most remote BBQ thermometers I have seen have a jack that allows you to unplug the thermocouple. Makes it easier to clean, store, etc.

    Can I use a jack / plug arrangement with K Type thermocouples? Would it just mean my calibration routine would come up with a different value? Any disadvantages to this idea?

    Thanks for any and all replies

    Andy
     
  2. Martaine2005

    Martaine2005

    3,563
    969
    May 12, 2015
    Hi Andy,
    The Seebeck Effect.
    Your jack would be called the cold junction.
    Ironically, I was looking into getting another meter with temperature measurement.
    I couldn't quite grasp how it works.
    So I decided to research it and found a great video going into detail how it all works.
    This video from Dave Jones is a must watch!!!

    Martin
     
    Last edited: Oct 19, 2015
  3. Y2KEDDIE

    Y2KEDDIE

    259
    15
    Sep 23, 2012
    A very crude explanation, but:

    Most meters (measuring devices) use ISO thrermo connectors. These connectors make sure the (two) connections are at the same temperature.
    It is the combination of the thermocouple tip junction and the reference (Cold junctions) your device measures.
    Each junction is a voltage producing device, these voltages add up algebraicly to predetermined voltage at a specific temperature. to produce a specific voltage.

    As long as you know what materials your junctions are made of and their respective temperatures you can calculate the voltage.

    Accuracy is relative. If your measuring steak on the barbi, a few degrees + or- doesn't matter much. The jacks on your meter ( or soldered foil connections of your circuit board) are probably close enough in temperature.
     
  4. Y2KEDDIE

    Y2KEDDIE

    259
    15
    Sep 23, 2012
    Typically color coding of different material lead wires are such that the color red is always negative. (USA). the other lead is colored depending on the theromocouple type. (Yellow and Red for type K),(White and Red for type J) .

    Voltage produced is usually in millivolts and varies a few tenths of a milli volt per degree C. When mixing different junction materials and changing polarities one can see this can add or subtract from your reading. A small temperature difference at your Cold junction may not make a big difference, but its always good practice to match your junction connections.
     
  5. Y2KEDDIE

    Y2KEDDIE

    259
    15
    Sep 23, 2012
    The maintenance mechanic only cares if there is a big difference in temperatures, as opposed to the exact temperatures the performance engineer wants.

    If I unplug the cold junction connection and reverse polarity the temperature only changes a few degrees., and the change can go unnoticed.
     
  6. Bluejets

    Bluejets

    5,034
    1,051
    Oct 5, 2014
    At Sunwater we used to use din rail terminals which used the same material as the cable (iron and nickel i think it was)
    They came from Sprecher and Schuh but there would be others.
     
  7. Andy.wpg

    Andy.wpg

    19
    1
    Apr 8, 2015
    Thanks! Dave is great (I knew I went to his site a lot for a reason!) and the video really cleared it up for me too.
     
  8. Andy.wpg

    Andy.wpg

    19
    1
    Apr 8, 2015
    Thanks for the replies everyone. What I decided on was to order some male and female K Type connectors so I can at least unplug the cables for cleaning / storage. Hopefully, the combination of the correct connectors and the AD595's, this thing will be fairly accurate.

    Thanks again!
     
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