How does my moisture meter work?

Factual Questions
1

I bought a moisture meter today. Great instrument, but I’m at a loss to understand how it does its thing. HowStuffWorks didn’t know and a quick websearch turned up nothing but a lot of moisture meters for sale.

My moisture meter consists of a long metal rod with a little dial on top. The tip of the rod is wider than the rod itself, and pointed. When I stick the rod into the dirt of a flowerpot, the needle instantly moves to the appropriate place on the scale (a 10-point scale divided into “dry”, “moist” and “wet”. It requires no power or batteries.

Now, you can’t get energy from nothing, and the meter needs some energy, albeit a miniscule amount, for the needle to move. Whence does it get the energy? How does it sense the moistness so quickly? I suspect the two questions are related.

2

At a guess, I’ll say it works in a similar manner to a humidity meter.

You take the wet bulb temperature (the soil’s temp) and the air temp, and compare the two. The humidity can be deduced from the difference once you have calibrated it. The exact function will be different in soil, that link I gave you was for air humidity only, but the principle should be the same.

The temperature gauge is probably a bimetalic strip with a strain-gauge. As the strip is heated, it bends - the strain gauge measures the amount of bending and then converts it into a temperature readout. Completely passive and doesn’t require power, but can be inaccurate.

3

Nope, not a humidity meter.

It’s nothing but a battery (an electrochemical cell) with salty dirt as the electrolyte. Look carefully at the rod and you’ll find that it’s made of two different metals with an insulator between them. In the ones I’ve seen, one of the metals seems to be zinc, the other brass.

The exploratorium science museum had a version of this, a “human battery” which was simply two metal plates wired to a microamp meter. The visitor places their hands on the two plates, and their salty body completes the circuit as well as behaving as the battery’s electrolyte.

4

In that case, it must run dry eventually, right? Is that why you’re advised not to leave it in the pot?

5

You’re advised to take the thing out because bad things will happen to it.

It depends on exactly what the electrodes are made of, and what’s in the soil.

One or other of the electrodes could erode away, and/or a non-conducting coating could form on one of the electrodes. It that sense, it would “run dry”.