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Thread: types of thermistors in water temperature sensors, and possible failure modes

  1. #1
    Join Date
    Dec 1969
    Location
    Oakland, CA
    Posts
    48

    Default types of thermistors in water temperature sensors, and possible failure modes

    Hi,

    one of my solar controllers started acting up and I am trying to isolate the problem. I measured the resistance of the sensor in the return pipe but it seems a bit too high at 27,500 ohm. If the sensor uses a 10k thermistor, this corresponds to a 5C water temperature (41F), but the water is more like 15C (60F) which should give a reading of about 16 kOhm.

    Is anybody familiar with these sensors? Do they all use 10k thermistors? And if the thermistor goes bad, can it go bad in that direction?

    More detail:

    In case you're wondering why I am using a solar controller in the middle of the winter: my last heating contractor evidently thought it was a good idea to use a solar controller for my hot tub. Without delving into too much detail (I use a single high-efficiency Viessman boiler for home heat, DHW, hot tub, and pool, with a heat exchanger), the hot tub temperature is regulated by the max temperature cutoff in the solar controller.

    In addition to not working right now, this setup is pretty impractical, so I am replacing the solar controller with a Raspberry Pi, a relay board, and an A/D converter board. To wire the sensor to the A/D converter, I need to know the range of resistance values in the range of temperatures of interest. Hence the original question.

    Thanks!
    30'x16' (irregular) indoor gunite/plaster pool, 10k gal,
    Autopilot DLG-220 with SC-48 cell, 3/4 HP recirc pump,
    solar panels, heat exchanger from 200 kBTU/hour Viessmann boiler

  2. #2
    Join Date
    May 2010
    Location
    Houston
    Posts
    32

    Default Re: types of thermistors in water temperature sensors, and possible failure modes

    If you can find the vendor and part number, the vendor should have the spec's for that part. My Jandy sensors range from 19.898 K ohms @50deg F to a low of 5.327 K ohms @104 deg F. The one I have show 15.314 K ohms @60 deg F. So it is fairly close to your reported number. I haven't seen many complete failures so it could be something else.

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