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.

                EXPANSION DISKS

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.

                               RAIN CUP

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.

                  ELECTRODES

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.
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.