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Temperature due to resistance calculator?

  1. Aug 21, 2010 #1

    ISX

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    Think this question is more of a physics than math question. Anyways I am trying to figure out how to make a calculator to figure out what temperature something is at based on the ohm reading I get. I have several readings to go by but I think doing it exponentially is the wrong way to go about it since the readings I calculate are not very accurate based on the quadratic regression method I have been doing.

    Wikipedia shows the formula on this page (http://en.wikipedia.org/wiki/Callendar-Van_Dusen_equation) but I don't know how to figure out any of the coefficients or anything.

    Thanks for any help I can get!
     
  2. jcsd
  3. Aug 21, 2010 #2
    By:
    Do you mean using a thermistor, or just by plugging into any random piece of electronics?
    The answer depends sensitively on your situation.
     
  4. Aug 21, 2010 #3

    ISX

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    Yeah just a little thermistor.

    Guess it would be this equation then. http://en.wikipedia.org/wiki/Steinhart–Hart_equation

    Temperature range of interest would be -40 to 212F. I just need help on the coefficient stuff.
     
  5. Aug 21, 2010 #4
    Unfortunately I think your best (if not only) bet is to measure some values at known temperatures and fit those to the equation. You have 3 constants, so you need at least 3 measurements, more would be better. The boiling and freezing points of water would be the easiest to do. If you don't have/want-to-use a thermometer, you could use the temperature of boiling and freezing of both pure and salt water for 4 total points.
     
  6. Aug 22, 2010 #5

    ISX

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    I do have many measurements. Here are a few. kOhms is first number Temp (F) is the second.

    73.00 5.0
    65.00 6.6
    56.00 13.0
    39.00 28.0
    25.80 43.0
    24.57 46.2
    18.96 53.7
     
  7. Aug 22, 2010 #6
    I think you should use a thermistor because the temperature varies greatly with the heat-dissipation of the resistors.
     
  8. Aug 22, 2010 #7

    ISX

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    It is a thermistor..
     
  9. Aug 22, 2010 #8

    Q_Goest

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    Science Advisor
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    Hi ISX
    Do you have Excel or some other spreadsheet program? Just put the numbers you have for temperature and resistance and have the program do the curve fit to a polynomial for you.

    For the values you provided, the chart is rather limited and I wouldn't suggest using it for values outside the tested limits, but attached is what you'd get. I'd suggest getting some more measurements, especially at the limits of your intended range (ie: test in boiling water, 212 F, and at -40 F).

    Another alternative is to get an equation from the thermistor manufacturer.
     
    Last edited: Aug 23, 2010
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