A few weeks back, I posted a question about measuring the level of moisture in the ground (so I could control my sprinkler system accordingly). "The Captain" sent me a schematic that did this using a Whetstone Bridge to measure the conductivity.
A friend then suggested that I do a simpler thing and just measure the resistance, and see what kind of values I got, and how it changed over time. This seemed reasonable, so I bought a couple of 12" galvanized spikes at Home Depot, hammered them into my wife's rose garden about 3' apart, and grabbed the trusty VMM.
Immediately after hammering the spikes into the soil, the resistance measurement was at about 1k ... and rising slowly but steadily. And then I reversed the probes and measured about 500 ohms, holding steady. So I waited a day. Measuring one direction I got 1.5Mohms, the other direction
500kohm. Today it was 2.2Mohms one way and 1.5Mohms the other way.What in the sam hill blazes is going on here? Some sort of weird chemical reaction that makes the resistance different in one direction than the other? Does the Whetstone bridge circuit somehow get around this?
Thanks all! Chris chris at sc3 dot net