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increasing length of thermistor probe?

Discussion in 'Electronic Design' started by Albert Porch III, Feb 19, 2007.

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  1. I have a small digital thermometer that has a 12 foot length probe with
    thermistor at end. Since the area where I need to see temperature readings
    from outdoors is much greater than this, I have been wondering about
    increasing the probe length to between 50-100 feet. Can this be done and
    what affect would it have on the accuracy?

  2. The accuracy is likely to be affected. A better answer
    can be given upon more details provided. What is the
    resistence of the sensor, is it a two wire or a four
    wire measurement?

  3. Rich Grise

    Rich Grise Guest

    I've used thermocouples with "long" leads - like 4 feet; but I don't
    really know how they would work with really long leads, like 100' - but
    it seems to me that if you use the right wire (and it is, in fact, a
    thermocouple), in a twisted pair, it shouldn't make that much difference;
    but I'm no authority there - I'll need somebody to corroroborate this or
    correct me.

  4. joseph2k

    joseph2k Guest

    It is not too bad for medium to high R value thermistors (a few kOhm to
    several 10's of kOhm). The resistance of the wire will produce an offset
    component and the resistance combines with the temperature coeffecient of
    the wire to produce a scaling error. This can be calibrated out.
    Thermocouples using thermocouple wire can be remoted to 100' as well. Good
    "cold" junction compensation is required though.
  5. Phil Allison

    Phil Allison Guest

    "Albert Porch III"

    ** Only way to find out is to try it.

    Some RG59 co-ax would be the go as an extension.

    See if you can measure the probe's resistance - if more than 100 ohms your
    are fine to try.

    ....... Phil

  6. Actually most of the inexpensive thermistor thermometers use the
    thermistor resistance to charge and discharge a capacitor (comparing
    that with a precision reference resistor) and there will be some
    effect from cable capacitance, particularly at low (measured)

    Thermocouples can be run to essentially unlimited length, and there is
    no special need for cold junction compensation based on lenght.. the
    important factor is the input current and impedance of the
    thermocouple meter, which places an upper limit on the loop resistance
    of the sensor for a given level of accuracy.

    Best regards,
    Spehro Pefhany
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