Expert IrrigationFlorida Statute 373.62 requires anyone operating an automatic landscape irrigation system to install, maintain and operate a device that shuts the system down when there has been sufficient rainfall. It is not optional and it is not new. What is remarkable is how many of the sensors we find in the field are no longer working.
Most of the ones we find are dead
The common type uses expanding disks that swell when wet and break the circuit to the valves. Those disks dry out, crumble and stop expanding, usually within a few years in this climate. The sensor still looks perfectly normal on the wall. Nothing on your controller tells you it has stopped working. The first sign is usually a water bill, or a neighbour asking why your sprinklers ran during a thunderstorm.
Mounted where rain never reaches it
The second failure is placement, and it is entirely avoidable. We regularly find sensors tucked under an eave, beneath a soffit, or shaded by a mature tree canopy that was a sapling when the system went in. A rain sensor under cover will never trigger no matter how hard it rains. It needs open sky above it and it needs to be out of the throw of your own sprinklers, which will otherwise trip it every cycle.
Maintenance most people never do
The disks want replacing at least every couple of years and the sensor wants testing annually. Testing is genuinely simple: start a zone manually, then depress the sensor plunger or wet the disks and confirm the zone shuts off. If it keeps running, either the sensor has failed or it was never wired to interrupt the circuit in the first place — which we do find, particularly where a controller has been replaced by someone who did not reconnect it.
What the evidence actually says
Being straight about this: under controlled conditions, expanding-disk rain sensors have shown water savings in the range of seventeen to thirty-four percent. But UF/IFAS also notes that in real-world practice they often have not reduced excess irrigation much — because they are installed in the wrong place, positioned badly, or long dead. A sensor that is actually maintained is a completely different proposition from one that is merely present on the wall.
Set the threshold low
Most units allow a threshold between a quarter inch and an inch. Set it to the lowest the unit allows, usually an eighth of an inch. In a climate where roughly seventy percent of annual rainfall arrives between mid-May and mid-October, a sensor set to an inch will sit there doing nothing through most of the storms that should have stopped your system.