The Learned Monkey – How Sprinkler System Rain Detectors Work

A new segment that The Monkey will share that hope to provide some useful insights based on first hand experiences. Perhaps not. The Monkey is not ashamed either way.

If you have an irrigation system in Florida, you probably (or most assuredly should) have a device that detects when it is raining (or has recently rained) and then, if that is the case, turns your sprinklers off. I have one of those on my sprinkler system, but it had not worked for a couple of years. And frankly, in the summer around here, it rains enough that you can almost turn off the system for weeks on end and all will be well.

But come the Winter, it was time to figure out why this thing had stopped working. In other words, regardless of the rain, the sprinklers would come on.

Now, before looking into this, I thought there was some high-tech computer sensor that detected moistness and would open a circuit, thus closing the valves and shutting off the sprinklers. Maybe it measured direct humidity?

No, no, and no. Here’s how it works. The rain gauge contains a little reservoir that, at the bottom, is a kill switch (button). If you depress the switch, the power to the irrigation valves is turned off. Above the switch is a little confined area that is open on the sides and most of the top, but running up through the middle is a little plastic rod. On that rod there are little discs (that look like little cardboard washers) stacked on top of each other to the top of the confined area.

Here’s the science: water getting in through the top of the open-air capsule soaks the cardboard washers, causing them to absorb the water and expand. Since they are on a pole (or rod), they can only expand up and down. Upwards the rod ends on a little plastic cover, so they can’t expand that way. Thus they expand downward, depressing the kill switch and turning off the sprinklers. As they dry out and shrink, the switch is released.

That, my friends, is it. What happens is that these washers get moldy and cannot absorb water. A replacement rain detector is in order. If you don’t want to go to the trouble of replacing the entire detector, buy an inexpensive one and use the washers in it for your existing one.

Now, I’m certain there are more high tech versions, but this is the one I have.

Questions? Please do post.


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