Irrigation System Rain Sensors

No homeowner wants to run an irrigation system when it’s raining. A rain sensor will prevent this from happening. 

A rain sensor is a small device set up in an open area that’s exposed to the rain, and it works by interrupting the watering cycle of the automatic irrigation controller once a predetermined amount of rain has fallen.

Did you know that in a wet year, a rain sensor can pay for itself in just a couple months?

The TCEQ (Texas Commission on Environmental Quality) made it a requirement in 2009 that every new lawn sprinkler system installed, or any repairs involving a new controller, must include a rain sensor or other technology that automatically shuts off irrigation during a rainfall.

Residents of Austin can even get a rebate from the city by adding a rain sensor to their irrigation system.

Rain Sensor Benefits

  • Conserves water by preventing irrigation during and after a rainfall.
  • Saves money. You can save up to 35 percent on water usage.
  • Convenient. Depending on the type of controller you’re using, manually shutting down your system every time it rains isn’t practical, especially if you’re at work.
  • Reduces wear and tear on the irrigation system.
  • Reduces plant diseases and weeds caused by overwatering.
  • Protects the soil in your lawn from being stripped of nutrients by excessive amounts of water.
  • Prevents surface runoff from making its way into the ground water system and polluting it with fertilizers and pesticides.
  • Provides important input even to climate-based smart controllers that use local weather data to adjust schedules. The on-site rain sensor gives more accurate rainfall feedback to the controller than local weather stations can.

Types of Rain Sensors

Rain sensors, also called rain shutoff devices, are available in four different types:

  1. Water collecting cup. This is one of the original types of rain sensors still in use today. Water collects in a cup or basin, and when a predetermined weight is reached, a switch is tripped, and the irrigation cycle is interrupted. Once the cup is emptied, or a portion of the water evaporates, the irrigation cycle can start again.
  2. Electrodes. This rain sensor uses two electrodes to detect the water level in a small dish. The electrodes can be adjusted to different distances from the bottom of the dish. When the water level reaches the electrodes, it completes a circuit and trips a switch, interrupting the irrigation cycle.
  3. Expansion or hygroscopic disks. The most popular type of rain sensor, this technology uses multiple stacked disks made of a synthetic material similar to cork that expand as they get wet. The expanding disks trigger a switch that interrupts the irrigation cycle when the chosen rainfall setting is reached. Once the disks dry out and shrink, the switch is released, and the system resumes its scheduled cycles.

Rainfall shutoff levels are adjustable (e.g. 1/8, 1/4, 1/2, 3/4 or 1 inch), the number of settings depending on the model. The rainfall setting for a particular site depends on such factors as the frequency and amount of rainfall for that location, soil type, humidity, and amount of direct sunlight on the sensor.

The drying-out time for the hygroscopic disks can be regulated by adjusting the sensor’s vent ring (turning the ring to open the vent exposes the disks to more air, and closing it exposes them to less air). In this way irrigation cycles can be delayed anywhere from 2 hours to 5 days, depending on the model. Actual dry-out times are determined by local weather conditions (e.g. humidity, wind, sunlight).

  1. Quick shutdown. Some rain sensor that shuts down the irrigation system very quickly after it starts to rain, within 2-5 minutes. Unlike other sensors, there are no preset quantities of measured rainfall required to activate it. For more humid and rainy areas, this is ideal.

Other rain sensor brands can quickly shut down within 2 minutes once the set rainfall amount has been reached. 

Wired and Wireless Rain Sensors

Rain sensors connect to irrigation controllers either with a direct wire or wirelessly. 

Wired rain sensors cost less, there are no batteries to replace, but are more expensive to install. A wire has to be run between the sensor and irrigation controller, so there’s a limit to the distance and location where you can mount the sensor (25-30 feet). It’s best to place the wire so it’s protected from weathering or other types of damage.

Wireless rain sensors are generally more expensive, but they have many features. There’s a greater choice of location with wireless, with some models having a communication range of up to 800 feet from the controller.

With some models, the settings for the sensor can be programmed through the receiver, which can also actively manage the controller. For example, you can:

  • Choose the rainfall setting that shuts the system down.
  • Override the sensor (e.g. for 72 hours).
  • Suspend irrigation (e.g. for 72 hours).

The receiver also displays such information as:

  • When watering is enabled or disabled.
  • When the sensor has been bypassed by the controller to allow watering.
  • The sensor-receiver pairing.
  • The signal strength.
  • The sensor battery level.

Maintenance

Some maintenance steps for rain sensors include:

  • A regular clean-out of debris. With cup sensors, the added weight of debris can cause an interruption of the irrigation cycle. Debris in sensors using electrodes can raise the water level, connecting the circuit that shuts down irrigation.
  • For hygroscopic sensors, also check for debris. Twist the cap open and clean out any leaves, cobwebs, insects, etc. that interfere with the normal functioning of the disks. Also check their condition. If they look moldy or misshapen, they should be replaced. Over time disks lose their ability to expand and contract.
  • Check the battery in wireless sensors and replace when needed.
  • Periodically test the sensors to make sure they’re working (see your owner’s manual) and are still in a vertical position to catch the rain.

Rain/Freeze Sensors

Rain sensors are often combined with a freeze sensor. For some cities in Texas, both are mandatory. Rain/freeze sensors are available in wired and wireless models. The freeze sensor suspends irrigation once the set freeze temperature is reached and resumes a regular schedule when the temperature rises above the freeze cut-off.

For wired rain/freeze sensors, the set point is usually 37°F. With wireless rain/freeze sensors, some models give you a choice of freeze temperature settings you can adjust at the receiver. The receiver will also show you when irrigation has been suspended if the temperature drops below the chosen freeze setting.

A fully functioning sprinkler system will help keep your lawn looking its best.  Call the licensed irrigators at Smart Earth Sprinklers at (512) 694-1147 or contact us online for the very best in sprinkler system repairs, maintenance, and upgrades.

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