Sensors at Work
IF YOU’VE INSTALLED OR MAINTAINED irrigation systems for any length of time, you’ve seen a number of electronic gizmos come and go. Some have proved useful; some, not so much.
You may have tried some of these new devices and been impressed by them. Or, you may have been disappointed at times when they didn’t work the way they were supposed to.
However, some of these new electronic innovations have been around long enough now that most of the kinks have been worked out of them. And, for many irrigation professionals, they’re not mere gadgets, but good, steady, reliable tools they can count on to perform well every day. Such is the case with rain sensors. It’s also true of the latest advancement in sensing devices: soil moisture sensors.
“Sensors have been around for several decades. Over the years, they have been improved and made more accurate,” says Troy Lezy, marketing manager at Hunter Industries.
“They are easy to install, making them an excellent add-on that helps increase efficiency and adds profit opportunities for contractors.”
More and more contractors are discovering these little wonders. “While the technology has been around for awhile, there does seem to be more increased interest in it lately,” says Peter Lackner, product manager for Toro Irrigation in Riverside, California.
“There are more water restrictions and water bans going into effect across the country, so homeowners and property managers are looking for ways to save water,” says Brian Mueller, marketing manager of new product development for Rain Bird. “Therefore, contractors are looking for water-efficient products that they can sell to them—new systems or attachments to existing systems.”
Some smart controllers come with rain sensors; others have ports allowing them to be added on later. Installed on the eaves of a building, a rain sensor prevents overwatering by simply shutting off irrigation when it detects precipitation.
Most rain sensors have little absorbent discs inside of them. When these discs get saturated, they send a signal to the controller to shut off the system. When the discs dry out, they signal that the schedule can resume. Another type of rain sensor is called a “tipping bucket.” Just as the name implies, it contains a little cup that fills up and tips over when filled with water; when it does, irrigation stops.
In the city of Frisco, in central Texas, the triple threat of rapid growth, plus drought, plus lots of new home construction was quickly depleting local water sources. “Automatic sprinkler systems were installed on every one of those new homes; that’s just the nature of the competitive homebuilding industry here,” says Gary R. Hartwell, P.E., Frisco’s director of public works. “That’s our biggest issue—the excessive use of these automatic sprinkler systems.”
“What generally happens is, right before a house is finished, they have to turn on the sprinkler system to water the new sod,” Hartwell explains. “Well, the homeowners move in, and they’re more concerned about hanging pictures and unpacking boxes, so that controller keeps overwatering for some time.”
Back in 2006, Hartwell realized that the city was losing its battle against water waste. “That’s when I said, we really need to explore smart controllers. I’ve used several brands of all different types,” he says.
He’s tested ET-based controllers that gather data from either onsite or off-site weather stations, and ones that used soil moisture probes. Having thoroughly tested all the methods, he feels that when used properly, they all work well, but that the soil moisture sensors work the best.
Soil moisture sensors work in a number of different ways. Buried about eight to ten inches below the ground, most of these units work by measuring electrical resistance within the soil. Water is a conductor, so the wetter the soil, the lower the resistance.
Hartwell’s testing resulted in a list of approved irrigation devices that residents of Frisco can choose from to cut their water bills. Several soil moisture sensors by a number of different manufacturers are listed, along with various brands of smart controllers. Though he can’t offer any hard numbers, he says the city’s residents are using much less water than before the program was instituted.
As town manager at a planned community in Naples, Florida, Steven L. Anderson is in charge of irrigation, not just for the landscape and turf areas, but for the subsurface drip system used under the town’s clay tennis courts. He’s found that soil moisture sensors help him keep those courts in optimum playing condition, a big priority for the residents there.
“Clay has to have a certain moisture content to hold itself in place,” explains Anderson. “If it’s too wet or too dry, it’s dangerous. We have to make sure we have the best playing surface all year ’round.” And with the help of soil moisture sensors, that’s been much easier to achieve.
A clay court isn’t a landscape. But soil moisture sensors worked well in this sensitive application; you can imagine how well they work with turf and plant material.
Water savers Do soil moisture and rain sensors really save water? Anderson thinks so. “The soil moisture sensors definitely saved us water,” he says. In Florida, rain sensors are required on irrigation systems. “The law actually states, ‘any rain shutoff device,’” says Judith Benson, an irrigation consultant and contractor, and owner and president of Orlando, Floridabased Clear Water PSI. “We can use soil moisture sensors in place of rain sensors and still meet the code.”
As for her personal preference, “I happen to like soil moisture sensors better. We’ve been installing them for more than ten years now. We’ve used several different brands, and worked with a couple of manufacturers, even helping with one company’s prototypes. We got really fabulous results—60 to 70 percent water savings. Huge, huge results, because we were using multiple sensors, one in each zone, on a goodsized property.”
“Using single sensors on averagesize lots down here, we still exceeded 30 percent in savings,” adds Benson. “Some of those jumped up to 60 percent just with a single sensor.”
“Rain sensors and soil moisture sensors work hand in hand,” says Mueller. “The savings typically associated with a rain sensor is about 30 percent, versus an irrigation controller that is just a timer, doing schedule-based irrigation. A soil moisture sensor gives you about 40 to 45 percent water savings against a schedule-based controller.”
“So, if you install both a rain sensor and a soil moisture sensor on the same controller, you can save up to 70 percent,” Mueller concludes. “The water savings is cumulative.”
Wired or wireless?
Rain sensors are usually wireless, talking to the controller via radio frequencies. By contrast, many soil moisture sensors are hardwired. One benefit of a hardwired system is that it allows several moisture probes to be hooked up to one controller.
However, the most exciting new development in soil moisture-sensing technology is in the wireless models that are now available. Many of these soil moisture probes can be used with any brand of controller, making them a very flexible, and affordable choice.
“With wired systems,” says Benson, “the cost of each unit is not that much, but when they’re wired, the labor costs do go up.” Even so, “the majority of the units we have out there are wired. It is nice, though, to have a wireless option.”
Making them work
As with any irrigation system, no matter what type of control you’re using, there is a learning curve; the same is true of the seemingly simple soil moisture sensors. “It took a little while to master using them properly, but once we figured out how to make them work, they’ve been great,” says Graham McGregor, owner of Hurr- Vasa Landscape, Inc., in Ft. Collins, Colorado. “They definitely save water.” Like Hartwell, he experimented at his own house first.
“The unit was super-easy to install,” he continues, “but the biggest thing was figuring out where to place it in the yard. If you put it in a low spot, where it’s wet, then the sensor is always wet. You’ll get some dry spots in the areas that are drier. The challenge was finding that middle-of-the-road area in the yard to represent the average across the board. Once I figured that part of it out, it worked great.”
Benson prefers to use multiple soil moisture sensors. “That’s the ideal situation. On a really small lot, a quarter of an acre or so, you can get by with a single one. You put it in a ‘median’ area, one that’s not too dry or too wet, not in full sun but not in full shade, either. That way, you can get a reasonable result with a single sensor.”
“But it’s not the optimum. We try to get a client to let us put in a minimum of two sensors: one in a wet area, one in a dry area. We get a better representation of the landscape that way, and a better water savings overall. It averages out, giving the wet zone less and the dry zone more. I tell the property owners that they work kind of like thermostats, but for water.”
Maintenance and tweaking
Benson occasionally goes back and digs up her soil moisture sensors, but not because of any malfunction. “When we’re initially installing these, the root zone of the turf is very shallow; that’s where the water is. As we reduce water consumption, we actually lengthen the turf’s root zone—the roots go deeper into the soil.”
“If we’ve got someone under a contract, we’ll go back a year or two later, dig up the sensor, and put it down lower, where the root zone is now. There’s no capital expense for the client, and they still get additional savings. We can get another 15 to 20 percent water savings on top of the original number.”
An earlier generation of soil moisture sensors tended to suffer from corrosion after a short time in the ground. That doesn’t happen with today’s products. They’re pretty much “bury and forget” devices, with the exception of wireless units, which will need a new battery every three years or so. Some of these units even work off of solar power.
Rain sensors are a different story, at least in Florida, where Benson works. “We’ve done several studies on rain sensors,” she says. “We’ve found that, on average, rain sensors need to be repaired within one to three years. Not replaced, but repaired.”
She explains, “Standard rain sensors fail quickly in our area. It’s not so much the design as the fact that we’re in a harsh environment, going from really wet to really dry for long periods of time. The little absorbent discs used in many brands of rain sensors tend to lose their absorbency over time.” In Florida, the problem is so well-known that it’s been written into law. “State law says that on every service call, the irrigation contractor is supposed to verify that the rain sensor is working. If it’s not, we’re supposed to address that with the property owner before we tackle any other repairs.”
Rain and soil moisture sensors are here to stay. So is the need for more efficient irrigation. If you’re not already acquainted with these hightech helpers, there’s no better time than the present.