What's the risk of rain shutoffs?
So you are designing or installing an irrigation system. Have you included a rain shutoff? If you work in an area of the country where it rains only a couple of times a year, you may think you don't need a rain shutoff. For the most part, that may be true. But most grounds managers probably should use one. In fact, in some areas, law requires you to install one. But, even in low-rainfall areas, water conservation is important.
Rain shutoffs-also known as rain sensors or rain detectors-come in a variety of types and configurations. Basically, their type of use places them into one of two categories: the quick-response shutoff and the preset-amount shutoff.
* The quick-response shutoff. This type of unit senses rain almost immediately when it begins to fall and turns off the irrigation system if it is running. This type of shutoff is appropriate if you are concerned about your company's public image or with runoff during rainfall. Consider, for example, that you manage a high-profile irrigation system in an area where people are sensitive to inappropriate use of water. Rain begins falling in the middle of an irrigation cycle-and your phone starts ringing. "What's the point of irrigating in the middle of a rainstorm?" callers ask. The callers probably know little about soil-water deficits or any runoff at your site. It doesn't matter, though, because the appearance exists that you are wasting water. If this scenario describes your situation, then this type of rain shutoff is for you. Besides, in the event you might get runoff, you will save water with these units-resulting in a value to you and to society.
* The preset-amount shutoff. Perhaps you irrigate your site mostly at night and rarely-if ever-get irate phone calls about inappropriate watering. You, then, may assume you don't need a rain shutoff. Wrong. In your case, you need the second type of sensor with the preset shutoff. This type doesn't turn off your irrigation system until your site receives adequate rain fall. The shutoff then keeps the system off until your site actually registers a plant-water need. The disadvantage to preset shutoffs is that irrigation and rain can occur together with possible runoff. But, after your site reaches the preset rain amount, the shutoff automatically turns off your system and saves water.
A couple of disadvantages to the units: Some hold water, which can ruin a unit during freezing weather. Also, the units can give false readings or shut down a system at the wrong time if debris collects in their water-storage areas. So you've got to be vigilant about cleaning out the units and protecting them during freezing weather.
How the units work To make an informed choice about which type of unit is appropriate for your site, you should know something about how shutoffs work. Preset-amount shutoffs operate via several methods: microswitches, probes or rain switches.
*Microswitches. Some devices have a water-storage container with a mechanism that turns the system off when water reaches a preset depth. With these types of rain shutoffs, the irrigation system remains off until a set amount of water evaporates from the storage container. These units typically use a ceramic wick to assist water evaporation. They operate on a weight principle: the storage container and its water weight push down on a microswitch mounted below the cup. Because freezing temperatures can break the cup, many of these units are hinge-mounted so they will tilt during cold periods.
*Probes. Another type of unit accumulates rain in a reservoir and has probes to sense the water's level. Adjustments on these units typically allow users to set shutoff when rainfall reaches as little as 0.125 inch. These systems typically reset after water evaporates from the collection dish.
*Disks. A third method uses disks or wafers that absorb water and swell to activate a switch, which opens the electrical circuit. As the disks dry, they shrink and close the circuit again. Adjustments allow you to vary activation levels. With proper adjustment, manufacturers say the disks dry out at a rate that parallels turf-water use.
Quick-response shutoffs work differently. These units sense water droplets on the surface of a sensor, which immediately triggers the irrigation system to shut down. Within an hour or two after rain stops, the sensor dries off. The irrigation system is then ready to continue irrigating according to its schedule. These units experience no delay based on rainfall amount.
Making the shutoff connection You can connect a rain shutoff to your system in two ways. Probably the most-used technique is to cut into and install the shutoff in the low-voltage common line. When rain activates the shutoff, the shutoff prevents the common line's current from reaching the valves and they, then, don't operate. With this method, you don't affect the controller, and it will continue to send signals to turn valves on and off as before-even if the valves aren't actually receiving the signals. You can install a bypass switch so you can operate your irrigation system even if the shutoff is activated.
The alternate method for installing shutoffs is to use a controller that has a specialized circuit for the sensors. With this method, the controller uses the signal from the shutoff to determine whether to irrigate. Remember that not all controllers are so equipped, and those that are may not be compatible with all shutoff units.
Summing it up Few would argue that the use of shutoffs is appropriate and economical. Even in low rainfall areas such as California, the units help meet the need for water conservation. For anyone purchasing a shutoff device, look for units with proven dependability and low maintenance. Also consider the use of vandalism shields and conduit connections. If you are bidding on irrigation installations, consider the lead of some installers, who use the installation of a free shutoff as a sales "closer."
In any case, most irrigation systems benefit from the installation of a rain shutoff. You truly have little to lose with their inclusion-except, perhaps, for some wasted water and money.
Turf-industry personnel have found numerous ways of fouling up simple shutoff installations. However, you can learn from their mistakes. For example, Victor Santana of Santana Lawn Sprinklers (Miami) says he's encountered systems where someone used the rain shutoff to deactivate the master valves but not the zone valves. With wiring in this configuration, he explains, zone valves can come on when the master valve is closed, allowing the zone valves to overheat and fail.
Joe Jacobsen has faced an even worse problem on some sites. Jacobsen, owner of Jacobsen Irrigation in Ormond Beach, Fla., says many irrigation systems in his area use pumps. He says he's found irrigation systems with rain shutoffs installed to open their systems' valve common wires without also switching off the common wires to the pumps. In these setups, the pumps can come on while the zone valves are closed. (Perhaps the installer of these systems also sells replacement pumps!)
Most installers interviewed here endorsed using disk-styled shutoffs because of their dependability and minimum maintenance requirements. (See below at left for specifics on disk-styled shutoffs.) Over the years, David Park of W.P. Law Inc. (Lexington, S.C.) has specified all three shutoff types (mentioned in the main article) on different systems he's designed, all with successful results. Nevertheless, he now specifies the disk type in most situations.
Brian Vinchesi of Irrigation Consulting and Engineering (Pepperell, Mass.) also specifies primarily the disk type. He says it reduces problems with winterization in the Northeastern United States, where he works.
Most installers interviewed here acknowledged they use shutoffs to meet the public's perception of water savings. However when asked about using the quick-shutoff device, those same people also expressed concern about how quick-shutoffs can cut off an irrigation system too quickly, causing a site to miss needed irrigation. This is because quick-shutoffs shut off an irrigation system as soon as it starts to rain. If the rain turns out to be light, the site is deprived of its scheduled irrigation setting. Preset shutoffs don't have this problem because it takes a heavier rainfall to trigger them.
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