A rain gauge is used to measure rainfall and/or output from an irrigation system. By checking a rain gauge after each rainfall, you can determine how much it has rained and then reduce your irrigation system’s watering time accordingly. A rain gauge (or small wide-mouth can) can also be used to determine how long to run your sprinklers. Putting multiple gauges or cans in a zone and then running your system will tell you how much water your sprinklers are delivering over what period of time per zone. Only 1/2 - to 3/4-inch of water per irrigation zone is needed for a healthy lawn.
A rain sensor is used to prevent an irrigation system from operating during or immediately after a rainfall. Rain causes the sensor to keep the irrigation system turned off during or after sufficient rainfall is measured. Most rain sensors have settings that allow some sort of rain measurement to take place. Settings can usually be adjusted so that a light shower will not effect the system operation. Sensors should be set to shut off the irrigation system when ½- to ¾- inch of rain is received.
Sensors should be mounted high off of the ground in a location that allows the device to collect rainfall without any obstructions from overhangs or trees. Place sensors out of the path of sprinkler spray and never place in full sun or full shade.
Florida law requires any automatic lawn sprinkler system installed after May 1, 1991 to have a rain sensor or switch that will override the irrigation system when adequate rainfall occurs. FAC 40C-2.042(2)(a)(5)