Often, when meeting a client for the first time, I will be asked if the rain sensor really works. There is this persistent rumor that rain sensors do not work or that they occasionally work. This is absolutely not true. Let me explain how they work, the different types of rain sensors, and why this rumor persists.
A rain sensor is a device that connects into the sprinkler controller or field wiring of an irrigation system and interrupts the signal between the controller and the zone valves. The most popular device that most manufacturers use is a small micro switch in their sensor that is in the normally closed position - contact is always made. When the rain sensor gets wet from rainfall the spongy discs inside get wet and like a sponge they expand and exert pressure on the micro switch forcing it to open the contact. This expansion (due to being wet) opens the switch and prevents the signal from the controller to reach the zone valves. Many of the rain sensors can be calibrated to shut the system down with as little as an 1/8th inch of rainfall. I can also set the rate at which the rain sensor dries out. This helps in case you want the system to come on short or long period after the rainfall. If I want the rain sensor to dry out quickly then I leave the vent wide open and if I want to have the sensor dry out slowly then I keep the vent closed. Sandy soils and sunny locations = open and heavy clay soils and shady areas = closed. I can also partially open and partially close based upon the need. Lately, I have used the Hunter Rain Click because it will automatically shut the system down as soon as it gets wet. A typical complaint that we receive from our clients is that they see their sprinkler system on in the rain. After checking on hundreds of these complaints over 20+ years I have found very few sensors to not be operating properly because "they just don't work". There are several reasons that I have found that may cause a rain sensor not to work. I have listed them in order of highest frequency;
1. The rain sensor has not received enough rainfall to cause the system to shut down. If it rains a ¼" and your rain sensor is calibrated to shut down when it rains a ½" or more then the sprinkler system will keep schedule and operate as programmed.
2. The sprinkler system started its cycle before it started raining and the rain sensor has not received enough rainfall to shut the system off.
3. The calibration on the rain sensor is set too high and needs to be adjusted to a lower setting. On conventional landscapes with a typical rain sensor, I have found that ¼" setting works best.
4. The controller rain switch is on by-pass rather than in the active position.
5. The rain sensor is damaged. I have found the rain sensor is damaged by construction on the house, falling trees and branches, squirrels, birds, insects, and kids.
6. The rain sensor is improperly installed. It was improperly wired, or not mounted high enough and out of the way of trees, eve's of homes, or on the roof, or too low where the sprinklers can hit it (my favorite).
7. The rain sensor is old and worn out and needs to be replaced! Hey, they don't last forever! Rain sensors can last up to 10 years depending on the conditions. Be prepared to replace. A sensor can cost anywhere from $100 to $500 to replace depending on the type, the mounting, and the wiring. Regardless of the cost the sensors pay for themselves and will save you money in many ways.
Rain sensors are an integral component of the irrigation control system. They will save thousands (hundreds of thousands) of gallons of precious water over their lifetime. They will save you hundreds and even thousands of dollars on your water bill. Rain Sensors prevent overwatering that can damage your lawn and landscape and are inexpensive to own and operate. My recommendation is that every system should have a rain sensing device, if you do not have one than have one installed today! If your sensor is 10+ years old then change it out as soon as possible.
Richard Silverman
