Irrigation Sensors
Rain sensors for irrigation systems are available in both wireless and hard-wired versions, most employing hygroscopic disks that swell in the presence of rain and shrink back down again as they dry out — an electrical switch is in turn depressed or released by the hygroscopic disk stack, and the rate of drying is typically adjusted by controlling the ventilation reaching the stack. However, some electrical type sensors are also marketed that use tipping bucket or conductance type probes to measure rainfall. Wireless and wired versions both use similar mechanisms to temporarily suspend watering by the irrigation controller — specifically they are connected to the irrigation controller's sensor terminals, or are installed in series with the solenoid valve common circuit such that they prevent the opening of any valves when rain has been sensed.
Some irrigation rain sensors also contain a freeze sensor to keep the system from operating in freezing temperatures. Typically freeze sensors are employed in regions where irrigation systems are not "blown-out" for the winter. However, in areas where there is occasionally a chance of overnight frosts, such as Florida, New Jersey, Minnesota, and Connecticut new lawn sprinkler systems are required to use a rain sensor.
A new trend in "smart" sensors (like the Virtual Rain Sensor) are using recent weather observation data (rainfall, sunlight, temperature, humidity, and pressure) to calculate evapotranspiration of water. Used in conjunction with forecast rain and temperatures data these can direct home automation hardware at variable intervals and durations to optimize water savings.
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