How a Sprinkler System Water Sensor Works
Automatic sprinkler systems operate off a timer, but do not usually have a way to alter the watering pattern if a unexpected rain arrives, meaning you may be watering your lawn at the same time as a downpour. Sprinkler system water sensors use intricate and varying technologies to determine rain levels. As a result, your automatic sprinklers do not come on in the rain and resume a normal watering pattern once the area dries.
Sensors with expansion disks use water pressure to determine it has rained. Often made of cork, rain falling on the disks causes them to expand. At a set point, the cork expansion triggers a pressure switch to shut the irrigation system off. As the cork dries after the rain, it depressurizes the switch to reactivate the sprinkler system and resume its normal schedule. Depending on the manufacturer, you can adjust the rainfall setting on the disks so even a light rain triggers a sprinkler shutdown.
One of the simplest sensor technologies uses water weight. Typically installed on a roof, a sensor cup is open to the elements. As rain falls, it accumulates in the cup. Once the cup becomes heavy enough, an electronic trigger is sent to the irrigation system controller, effectively shutting the sprinklers off. As the water evaporates from the cup, the sprinklers return to their normal state. A major drawback to this sensor system is the open cup. You must periodically check the cup to ensure it doesn't have debris in it -- the sprinkler system will shut off from the cup's weight if it's full of leaves.
Also using a cup, electrode sensors rely on conductivity to shut off your irrigation system. As water fills the cup, tiny salts within the water allow the electrodes to complete a circuit -- they are effectively conducting electricity through the water. By completing the circuit, the electrodes trigger the sprinkler system to shut off. Similar to the other sensors, water evaporation eventually stops the electrodes from forming the circuit and allows the sprinklers to work again.
Soil Moisture Alternative
An alternative water sensor uses the soil's moisture level instead of rain. You insert the sensor into the ground so it penetrates halfway down your garden's root system. As rain falls, the sensor detects the moisture in the ground, much like the electrode cup technology. If the soil is damp enough, the sensor turns the sprinkler system off and resumes operation once the soil dries.
Writing professionally since 2010, Amy Rodriguez cultivates successful cacti, succulents, bulbs, carnivorous plants and orchids at home. With an electronics degree and more than 10 years of experience, she applies her love of gadgets to the gardening world as she continues her education through college classes and gardening activities.