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Rain sensors are available in three different designs, including water weight, electrical conductivity, and expansion disk types. This device detects rainfall events and disconnects power to irrigation systems to save water; both homeowners and businesses can use a rain sensor so that plants are not excessively watered and runoff is minimized. Additionally, each rain sensor type can be purchased in a hard wired or wireless version, depending on the consumer's desires and budget.
The water weight rain sensor uses a small dish attached to the irrigation system's main controller. As the dish fills with rainwater to a specific set point, the rain sensor will trigger the controller to shut the water valve off. The irrigation system will remain inoperable until the water in the dish naturally evaporates; this evaporation coincides with the soil's drying time. Once the water dissipates within the dish, the irrigation system will automatically turn back on for normal watering schedules.
Electrical conductivity uses the same rainwater collection dish; but, the irrigation system is triggered into an off position when two electrodes contact the collected water level. These electrodes are an open circuit during dry weather. Once water reaches the electrodes' preset level in the dish, the circuit becomes closed and effectively shuts the irrigation system down. The irrigation system returns to its normal operations once the electrodes do not sense any more water.
One of the most popular rain sensor types in the expansion disk model. A cork disk resides within a protective housing; as rain falls, the disk absorbs some of the water. As a result, the disk enlarges from the water absorption and triggers the irrigation system to shut down. Similar to the collection dish, the disk must evaporate most of its water before the system will restart its operations.
Most irrigation systems used to control the watering schedule are hard wired to an electric power source. Consumers can purchase a hard wired rain sensor and connect it to the power system. Newer wireless sensors are available that use radio waves from a nearby receiver to control the irrigation system; however, these devices are much more expensive than a basic hard wired type.
Many irrigation professionals prefer the expansion disk type of rain sensor. Both collection dish techniques have the drawback of collecting debris. These dishes must be maintained and cleaned periodically because debris accumulation can easily trigger a water weight sensor type to shut the system down.