Making sense

For the technologically challenged, rain sensors might seem like modern-day marvels. How exactly do they know when to shut off an irrigation system when it rains?

According to Brian Mueller, marketing and new product development executive at Rain Bird Corp., there are three basic types of rain sensors, all defined by the method they capture rainfall and determine whether or not the amount of rainfall is sufficient to suspend irrigation.

The first kind uses a catch basin. Two metallic probes extend out of the sensor, and if they extend into the water (because the cup has captured a certain amount of water), the circuit gets closed, the relay is opened and the system shuts down. The second kind is a wired sensor, and all the major irrigation manufacturers use the same technology. As rain falls on hygroscopic discs or wafers, they absorb it and expand, pushing down a switch. As the discs dry, they contract and release the switch to allow irrigation to resume.

The third kind is a wireless version that operates in a range of 300-800 feet. A controller interface mounted next to the controller is connected via a wiring harness. The controller then speaks through a radio frequency to a battery-powered sensor on the property. “There is no mechanical linkage between the two, which provides more flexibility in installation and is more aesthetically pleasing to the homeowner,” Mueller says. “Plus, it’s much quicker to install because you don’t have to run the wire from a catch basin sensor back to the controller.”

These three types of irrigation sensors don’t necessarily make controllers more “smart,” as the term is used today. That is reserved for sensors that rely on local weather data to adjust watering cycles.

“Solar radiation would be if it’s sunny right now, you’ll get more evaporation than you would if it was completely overcast,” says John Wascher, controllers product manager for Hunter. Smart sensors make small adjustments to a preset schedule on the controller based on weather conditions at the site.

“Over the course of the year, when you look at the small adjustments it makes, you save 25 to 30 percent on your water bill,” Wascher says.

“It allows something inherently complex to be installed at the homeowner’s house by a contractor or homeowner,” he says. Some states require a rain shutoff device be installed on all new systems. So whether you’re mandated by law to install a rain sensor or not, it comes down to two choices: the low-cost solution (wired sensor) or the wireless sensor at roughly three times the cost of the wired sensor. But Mueller cautions that contractors shouldn’t just base their decision on price.

“With the wired sensor, you have to make the physical connection from the sensor to the controller, which could be 25 feet or longer,” he says.

“In installing it and making it aesthetically pleasing to the homeowner, sometimes you try to hide in the overhang of the house or run it through the attic, which is very time consuming. If time and efficiency is more important to the contractor, then a wireless sensor that can be installed in 10 minutes might be the answer.”

The author is a freelancer based in Cleveland.