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Calculating temperature using NTC thermistor

  1. Feb 5, 2008 #1
    Hello,
    i was doing a practical experiment using NTC thermistor. I recorded the resistance at different voltages (1v, 2v, 3v, etc) and to find the temperature i need to use the temperature coefficient formula "The Steinhart-Hart equation":

    R(T) = R(T0) * [ 1 + a(T - T0) ]

    where
    T0 = reference temperature (deg Celsius)
    T = temperature of interest (deg Celsius)
    R(T0) = resistance at reference temperature (ohm)
    R(T) = resistance at temperature of interest (ohm)
    a = temperature coefficient of resistivity (1/deg Celsius)

    i am confused to how calculate the temperature.
    i know: T0 = room temperature (20 degrees C)
    a = for copper wire is 0.004041
    T = is what im trying to find
    R(T0) = ?? my thermistor says "resistance value to be 2kΩ" is this the value of R(T0)?
    R(T) = this is the resistance value i get when passing current through copper wire

    in some books i've seen the values of T to be in kelvin and sometimes in celcius. which is the correct way in this case? and instead of resistance some books replace it with "rho" for resistivity.
    please help

    does anyone know where i can find some example worked questions to understand this.

    Slim
     
  2. jcsd
  3. Feb 5, 2008 #2

    stewartcs

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    [tex] R(T) = R(T_0) \cdot [1 + a(T - T_0)] [/tex]

    Where,

    [tex]R(T)[/tex] = resistance at temperature of interest
    [tex]R(T_0)[/tex] = resistance at reference temperature
    [tex]a[/tex] = temperature coefficient of resistivity
    [tex]T[/tex] = temperature of interest
    [tex]T_0[/tex] = reference temperature

    This equation is for predicting the resistance at another temperature based on a resistance at a reference temperature. The resultant resistance is due to the temperature increase at the temperature of interest.

    Example:

    [tex] R(55) = R(20) \cdot [1 + 0.004041(55 - 20) [/tex]

    [tex] = 223.75 \cdot 1.14143 [/tex]

    [tex] = 255.396[/tex] [tex] \Omega [/tex]

    The temperature should be in Celsius.

    I'm not 100% sure if there is a way to relate your measured voltages to temperature using this equation so I'll let an EE chime in.

    CS
     
  4. Mar 2, 2008 #3
    I suggest you to take a look at the following application notes:
    Analog Devices AN 709 (right now can be found here http://www.analog.com/UploadedFiles/Application_Notes/2001119207465975025AN709_0.pdf )
    Microchip AN685 - http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en011704

    Anyway, I dont understand what you are trying to achieve.
    The formula you posted is NOT the Steinhart-Hart formula. The proper one is here http://en.wikipedia.org/wiki/Steinhart-Hart_equation

    If you want to measure temperature using the NTC thermistor, check the app. notes above. Basically, you connect a resistor and NTC thermistor in series and measure a voltage drop on one of these components.
    It is easy to get the NTC resistance then. If you know coefficients of Steinhart-Hart equation of your particular NTC, it is easy to work out the temperature of the NTC.

    good luck
    meereck
     
  5. Mar 2, 2008 #4

    dlgoff

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