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Moisture Meter

Discussion in 'Electronic Design' started by jsmith, Jan 12, 2005.

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  1. jsmith

    jsmith Guest

    Anyone have a simple design approach for a moisture meter for house plants?
  2. Rich Grise

    Rich Grise Guest

    Well, you could go to the plant store and buy one for about five bucks,
    and reverse-engineer it.

    Good Luck!
  3. Robert Baer

    Robert Baer Guest

    Yup! Seen them in stores with garden stuff (Home Depot comes to mind).
    It is an ohm-meter; some look like a single probe that one pokes into
    the ground - it has two sections insulated from each other.
  4. I think they are actually two dissimilar metals and they are connected
    to a microammeter. The moisture is the electrolyte in a sort of
    battery- and the metals are consumed over time.

    Best regards,
    Spehro Pefhany
  5. Wim Ton

    Wim Ton Guest

    I used a commercial IC from National Semiconductors to measure the
    resistance with AC. Despite using AC, metal electrodes dissolved in a few
    months. This was soved by using carbon rods from old D size batteries.

    It has been discussed before, see:

    The standard low-cost way of doing this is with a "Gypsum block"
    normally a pair of wires cast in a gypsum block. Strip about 20mm of
    insulation off some wire, then cast them in a plaster of paris mix. A
    35mm film cannister makes a good mould. The theory behind this is that
    the gypsum takes up the same equilibrium moisture content as the
    surrounding soil. Just putting probes in the soil gives inconsistent
    results depending on soil pH, soil texture, fertilisers added, etc.

    You must use AC when measuring the probe impedance, otherwise
    electrolysis gives you very wierd results, although a number of newer
    commercial systems use short DC pulses of a few mSec with good

    There's many thousands of these things buried in orchards, etc. They
    have a typical life of 4-5 years.

    There's also a number of other techniques used, all much more complex.

    Barry Lennox>

    Succes, Wim Ton
  6. Robert Baer

    Robert Baer Guest

    That would be another way to impliment the function.
    The one i saw was an ohmmeter.
  7. Rich Grise

    Rich Grise Guest

    How often did you have to change its battery?

  8. Robert Baer

    Robert Baer Guest

    It used a solar cell as its power, did not see any battery inside.
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