moisture meters for irrigation

A while ago, after putting in sprinklers around the house, instead of getting a standard sprinkler timer at Home Depot, I bought an RS-485 digital output module, some relays, and an RS232-RS485 converter. I wrote a *really* basic perl program to control the thing, and basically set it up so that the sprinklers come on now and again via cron; the only real ‘computerized’ thing I did was to have it check weather.com to see if it said “rain” and not water if it was supposed to rain.

Due to changing landscaping (some of our plants survive, but others died) and the fact that the drip system on one of the circuits has some persistent (and annoying) leaks, and the fact that when we got the Electro-Gun termite treatment we had to disconnect it, along with everything else electronic that connected to in-wall or in-attic wires (yeah, that kind of sucked at lot) we kind of stopped using it.

We’ve been doing more potted plants lately; we have a lovely dwarf orange tree, some mint and chives, and a couple of plumerias that refuse to die no matter how much we neglect them. Lately it’s been getting a little much to remember to water everything with the hose though, so I figured it was time to fix up the sprinklers and such again.

The sprinklers are back and hooked up (had to repair a pipe break from a dog that pulled too hard on a leash wrapped around the valves), and the drip systems go to all the potted plants now. The drip system still leaks in a couple of places, but I may try some X-Treme Tape on those spots.

One thing that I’ve started wondering about, though, has been how hard it would be to set up moisture meters to figure out when to turn on the sprinklers. The same place that sells the digital output interface also sells an RS-485 analog input module (and since RS-485 can be a ‘network’, I can use the same serial port on the same computer to monitor it too). I just have to figure out how to measure the moisture in the dirt around various plants.

Turns out the wine industry (and probably other parts of the agriculture industry) have been doing this for a while. There are a couple of places that sell moisture monitors; it’s basically electrical probes stuck in some kind of gypsum compound that’s connected to a probe to measure moisture content.

Upon further research, it looks like I may have to refigure this a bit. The standard “insert two nails in the ground and measure the resistance” won’t work because the salinity (and hence the resistance) of the soil may vary with the fertilizer content. “Insert two nails in gypsum (plaster of paris) and bury in the ground and measure the resistance” appears to work a bit better, but a) if you use DC instead of AC to measure you’ll get a galvanic effect and one of the probes will degrade, and b) it’s longer lasting than two nails in the ground, but still degrades after 3-5 years.

So I’ll have to use some kind of AC current to measure (maybe use the 24VAC sprinkler transformer for this plus a bunch of rectifiers), figure out how to quickly make a bunch of probes that are similar enough that I don’t have to spend a lot of time calibrating them, and then see if it’ll work at all for being a useful measure of “when to turn on the sprinklers”.

But on the plus side, it’s easier to water our plants again. Now to remember to fertilize…

3 thoughts on “moisture meters for irrigation”

  1. Yeah, except to measure resistance you need a small voltage (so you can measure the drop); apparently this is enough to make the ions move and the probes degrade.

    It’s not a huge deal; it gives me a chance to play with a more complicated circuit, which is never a bad thing. }:>

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