Type K Thermocouple

Discussion in 'General Electronics Chat' started by Thermocouple, Jun 26, 2014.

  1. Thermocouple

    Thread Starter New Member

    Jun 26, 2014
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    If I have a type thermocouple and insert copper connectors in the middle of it I know that would give me an extra two junctions where the copper meets the chromel/alumel and vice versa. What is the effect on the voltage if the first junction is heated, the second or if they are heated equally? I'm told that if they are heated equally then there is no overall affect. Can anybody explain to my why that mathematically checks out?
     
  2. MrCarlos

    Active Member

    Jan 2, 2010
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  3. MrAl

    Well-Known Member

    Jun 17, 2014
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    492
    Hi,

    The only requirement is that anything in between should be the same metal for each wire (not necessarily the same metal as the wire itself) and connected the same way for both wires.

    For the two wires, if the connectors can transfer the heat from one section of a wire to the next section and also conduct the current then it should be ok. It's not the junctions it's the voltage drop and thermal conductivity that is important. The junctions are just coincidental.

    For example, for one of the wires 4 feet long, if we cut the wire in half and insert a brass block say 1/16 inch thick between the two halves, it should work the same way as long as the heat can get from the first half wire to the brass and from the brass to the next half wire, and the current get get from the first half wire to the brass and from the brass to the next half wire.
     
  4. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    They would technically be called 1/2 junctions. The point is to keep them isothermal.

    I ran into a TC measurement devive from measurementcomputing where they hadn't a clue how to do it right. Most people don't.

    An air current would mess up the read value.

    To do it RIGHT, the 1/2 junctions at the meter should be massive and isothermal. On Fluke thermometer did it right.

    In terms of your application, Extension wire for the same thermocouple can be used to extend a thermocouples reach. Use conectors designed for the proper thermocouple as well.

    The exact effect your looking for will depend on lots of variables and there are way too many thermocouple types. S, R and T have very different changes in temperature vs voltage.

    It's just easy to do it the right way.

    The cold junction compensation on an instrument may be quirky and the cause of most of the errors.
     
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