PT100 induction

Discussion in 'General Electronics Chat' started by roro36, Jul 19, 2011.

  1. roro36

    Thread Starter Member

    Sep 7, 2010
    Good day all,

    I have a project which requires the temperature be meaaured of the water in an induction furnace. With thermocouples, the induction at 2meters away is 30mV which obviously throws the thermocouple reading way off. If I switch to the PT100 sensor, will this be affected in the same way, or will the 3 wire system of the PT100 be able to deal with the induction created by the machine?
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    I used them for measuring temperature of inductors and and power converter transformers, on heatsinks of IGBT bridges without any problems.
  3. someonesdad

    Senior Member

    Jul 7, 2009
    Maybe you didn't apply the thermocouple correctly -- I suggest you consult a reference book or a supplier for their use in various environments (one source would be Omega). Here are some thoughts:

    I'd try to ground the thermocouple junction with a screw to a metal grounded surface. Since you're measuring the temperature of water, this might require some thought as to the thermal design.

    Make sure the thermocouple leads are twisted together. Putting a grounded shield around them would also likely help.

    The induction furnace should be inducing an AC signal. I would think a simple RC low-pass filter could filter the induced voltages out pretty easily. Use a more sophisticated filter if necessary.

    I can't see why you wouldn't have the same induction problems with a 3 wire or 4 wire RTD (I'd use a 4 wire RTD if I could). You'll have to rely on the CMRR of the measurement circuit. Use twisted wires and shielding here too.

    Carefully evaluate your whole measurement setup for ground loops and good measurement practices (e.g., should you be using guarding?). Use a scope and look for AC noise; like in the movie Top Secret, "Find it and kill it" (one of my favorite sight gags).

    If you're still stymied, you might have to look at using synchronous detection (i.e., use lock-in amplifier techniques, which should easily be able to deal with this measurement problem).

    You don't say what accuracy and precision you need. Maybe a cheap infrared detector like my little $10 Harbor Freight IR thermometer would be suitable if put in a Faraday cage of mu metal to insulate it from the induction fields. I've always wanted to use a video camera and some software to do simple OCR on a digital display to get the readings into a computer... :p Consult a measurement textbook for other ideas on how to measure the temperature. Get creative -- for example, you might be able to characterize the thermal expansion of some material in your application, then measure a length to infer the temperature.