Moisture Meter

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

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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.

plants?

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 results.

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. See

Barry Lennox>

Succes, Wim Ton

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