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Electronic temp gauge....Broken?

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by rsuperdick, Nov 12, 2002.

  1. rsuperdick

    rsuperdick New Member

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    hi! I saw information on a website about installing a cheap temp gauge on my rc nitro car. I bought the household electronic temp gauge @ wal mart. The web site called for me to remove the metal tube around the wires and attach the wires to my cooling head. The two wires were attached or encased in what appeared to be a small glass bead. They broke off of the bead, and now the gauge doesn't work. If I touch the two wires together, the temp changes momentarily, then goes back to not working. Is it ruined? How does it work. It was only 15$ so Im not going to cry about it, but if I can make it work, that would be great! Please help!
     
  2. Phasor

    Phasor Member

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    It sounds like a thermocouple type temperature gauge, in which case the little glass bead was pretty vital!

    A thermocouple is two different metals joined together at a point (yours was joined inside the bead). Basically, when the junction is heated up, a small voltage is generated, and the greater the temperature the higher the voltage. By measuring the voltage, you can figure out the temperature.

    Since you have broken off the junction, you can either:
    1) Buy a new one. or
    2) Attempt to repair it by stripping back the ends off each wire, and soldering them together. Mind you, I don't know if the solder will stand up to the heat of a petrol engine!
     
  3. bogdanfirst

    bogdanfirst New Member

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    the effect that it uses something reverse the Peltier effect
    when current passes through 2 conductrs conected in series made of different type of material one of the conductors will heat and the other one will cool down, thus absorbing heat and it is transferred to the part that heates....this is basicly the Peltier effect....
    e termocuople uses something in revere: two type of conductors are jouned depending on the temperature a small voltage appears something like a few microvolts, maybe reach some milivolts......
    usually the two wires are sealed in a glass tube filled with some gas(i think some compound of nitrogen) so when u breaked the glass th gass is gone.....
    the idea is that without the gass the termocouple will work , but not very well.....
     
  4. dave

    Dave New Member

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  5. sparky098

    sparky098 New Member

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    I agree that it sounds like a thermocouple, and in my mind joining the two 'wires' by solder or by others means could effect the characteristics of the gauge. Myself, I'm not familiar with the Peltier effect, but do know that the theory behind a T/C is the Seebeck Effect. This states that when two dissimilar metals are joined together (most commonly nickel-chromium and nickel-aluminium - type K) to form a junction, the difference in temp between the hot and cold junction will induce a voltage (small - 4.096mV @100 deg C) proportional to the temperature.
    So if you add solder you're adding the properties of a different metal.
    Also I'm not aware of the need for 'gas-filled' T/Cs, this sounds like capilliary type temp indicators.

    I would say it's back to Walmart :wink:
     
  6. mechie

    mechie New Member

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    Thermocouples ?

    I reckon the sensor in question is most likely a glass-bead thermistor - there are several types out there, both positive temp coefficient and negative temp coefficient (resistance rises with temperature increase and falls with temperature increase, respectively).
    The resistance could be anywhere from around 50 ohms upto to a few hundred k, according to temperature and design.
    Try a few resistors as substitutes, find a value that makes the indicator read zero degrees C and then a value that causes a reading of 100 deg. C. These values will help narrow down the choice of thermistor -- or prove me a complete waffling idiot :oops:

    My only other guess would be that it is a 'Resistance Thermometer' - similar operation, same method of narrowing the selection of a replacement, different method of construction.

    On a different note ...
    Thermocouples are subjects of the Seebeck effect. Trying to solder two wires together (introducing an intermediate metal) will spoil the thermocouple 'calibration' because of the tainting that takes place on and close to the surface of the two metals (they are no longer pure).
    The Seebeck effect allows for "The Law of Intermediate Metals"; a third, fourth etc. metal as long as the additional junction temperatures are known and allowed for. In this case (a soldered joint) it can be argued that the new junctions (wire1-to-solder and soder-to wire2) are both at the same temperature and therefore cancel, giving no offset at all ! (there is still a pollution issue though).
    To form a junction the two wires should ideally be welded, a good tight twist would suffice though :!:
     

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