Saturday, December 24, 2011

Moisture Meter 'Repair' and Maintenance

Someone brought me this moisture meter with the complaint that it wasn't working.

There's not much to these things. All this one consists of is a little DC millivolt meter and a probe. Here's a view of the back of it opened up.

The meter's full scale deflection value is 330 mV -- a very small amount of electrical energy will deflect the needle.

The key to the meter's operation is the probe. The shaft-tube and tip are made of dissimilar metals, electrically insulated from one another.

The shaft-tube is connected to one terminal of the meter; the tip is connected to the other terminal. That's all there is to it.

Here's a view of the end of the probe.

I don't know what the tip is made of, but it's prone to oxidize, and tip oxidation is what impairs the operation of these units. Note how dull the tip's surface is in the photograph. I cleaned it up with sandpaper, and the meter is back in working order.

Theory of Operation

I've always wanted to write the heading, "Theory of Operation", so I could swear never to do it again. That phrase, "theory of operation", makes me gag every time I see it in a machine service manual; it's so pretentious and patronizing -- as if the writer is delivering 'special' knowledge from the podium of a university lecture hall. As far as I'm concerned, there's nothing 'theoretical' about it; the thing operates. If we need a heading for an explanation of how a thing works, what say we just use, "How it Works"; or, "Characteristics". Or, if we want to appear all techno-corporate, "Functionality". I'll leave 'theory of operation' to all the technical writers who haven't a clue about what they're writing about, and there are more than a few of those out there.

How it Works

When the probe is inserted in moist soil, the soil and the two metal elements of the probe mimic the operation of one of these.

The combination comprises a rudimentary primary voltaic cell. (A 'primary' cell is one that can't be recharged. Rechargables are 'secondary' cells.) The shaft-tube is the positive electrode, the tip is the negative electrode and the moist soil is the electrolyte. (Soil that plants can grow in is not 'dirt'; it's quite complex stuff with many ingredients and micro-organisms.) Within some limit, the moister the soil, the more energetic is the resulting cell, and the higher is the meter's reading.


Never leave the meter in soil as a 'monitor'. A characteristic of primary cells is that one electrode sacrifices itself to the cell's operation; in this case, the tip of the probe. A probe tip left in soil will erode.[1]

After taking a reading, wipe the probe clean. A smear of WD-40 on the tip wouldn't hurt at all. When the tip looks dull from oxidation, brighten it up with abrasive paper.

And there we are -- nothing 'theoretical' about it.

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[1] Note the pitted appearance of the probe's tip in the photo. This is not the first time this moisture meter has been in the workshop. It was once brought to me after having been left for quite a spell in soil. The tip looked awful. Its appearance was clear evidence of the electrode 'sacrifice' phenomenon that's typical of primary voltaic cell operation. Scraping and abrading the tip got it working again.

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