3 Worth-it Reasons You Need A Rain Sensor

An automatic sprinkler timer does exactly what it is told. Set it for 6 a.m. on Tuesdays and Fridays, and it will water at 6 a.m. on Tuesdays and Fridays, whether the lawn is dry or has just had an inch of rain. A rain sensor gives the timer the one piece of information it lacks.

The sensor is a small device mounted outdoors and connected to the controller. After a set amount of rain it interrupts the circuit that opens the valves, so scheduled cycles are skipped. When the sensor dries out, the schedule resumes on its own.

1. It saves water and money every wet week

The savings depend on how often it rains during the watering season and how much each cycle applies, but they add up quickly. University of Florida researchers tested expanding-disk sensors and found that the lower the rain threshold, the more water was saved. Their extension guidance recommends setting the sensor to the smallest amount the device allows, usually an eighth of an inch, and no higher than a quarter inch. In that 2008 study the payback period was under a year.

Skipped cycles also mean less runoff carrying fertilizer into storm drains, less disease in turf that stays wet, and less wear on valves. Florida considers the benefit large enough to put it in law: anyone who installs an automatic landscape irrigation system there must fit and maintain a device that interrupts it during periods of sufficient moisture, and contractors who work on a system must check that the device works.

2. It is simple to add

Sensors can be added to an existing system. Most controllers have terminals marked for a sensor; on older ones, the sensor is wired in series with the common wire. Wireless models avoid running a cable and allow more freedom in where the sensor goes.

The sensor connects to the controller’s low-voltage valve circuit, not to household current. Unplug the controller before opening it, keep connections in a dry enclosure, and leave any change to the household wiring that powers an outdoor controller to a licensed electrician.

The most common type uses a stack of disks that swell as they absorb rain and press on a switch. A rotating cap sets how much rain it takes. Other types collect water in a small cup and react to its weight or to contact with electrodes.

3. It only works if it sees the rain

Placement decides whether a sensor is useful. Mount it where rain falls on it freely: not under eaves, trees or awnings, not where roof runoff pours over it, and out of the spray of the sprinklers themselves. A spot near the roofline on the side of the house is common. Avoid extremes of exposure too. A sensor in full afternoon sun dries quickly and lets watering resume early, while one in deep shade can hold the system off after the soil has dried.

Test it at least once a year. Turn on a zone, then spray the sensor with a hose. A disk sensor should stop the zone within a few minutes; cup types react almost at once. If nothing happens, check that the controller’s sensor switch is set to active, check the wiring or battery, and replace worn disks.

Timers that adjust themselves to the weather also benefit. A controller that calculates a quarter inch of demand will still water after a tenth of an inch of rain, which is correct by its arithmetic and hard to explain to anyone watching from the sidewalk.