Rain Sensors: Types, Uses and Maintenance
Posted on Dec. 11, 2023 / Irrigation / Subscribe 0
By: Alberto Ricordi
The rain sensors available in the market today have evolved over time. There are three main types of rain sensors: water collecting basin, electric conductivity, and hydroscopic disk. This article will describe each sensor type and their maintenance needs.
Water Collecting Basin
The water collection basin is one of the earliest types of irrigation rain sensors. It is based on a water collecting container that triggers a switch once it is loaded with a certain amount of water. It is the weight of the water that activates the switch. The problem with this design is that anything else that weighs as much as the pre-set amount of water will activate the switch, such as: a bird standing or nesting on the sensor, dead insects, fallen leaves and fruits, etc. This will cause unwanted interruptions in the watering cycle and landscape damage. On the other hand, wider basin types may allow wind to blow rainwater out of the container, which in turn delays the shut-off.
The water weight concept can be used for mist systems for plant propagation. It works on the principle of evaporation. The weight of mist on a metal screen makes it swing up or down, controlling the solenoid, and simulating the leaf surface getting wet and dry.
Figures 1 and 2. A sensor that works on the principle of evaporation. (Image source: Electronic Leaf.)
Conductivity
The second generation of rain sensors works on the principle of electrical conductivity. This method also uses the amount of water accumulated on a basin to trigger the sensor. However, instead of measuring water weight, this method measures water height. There are two electrodes set at a specific distance from the bottom of the cup. Once water accumulates to a certain level, it touches the electrodes and activates the sensor. This allows the electrodes to identify a pre-set amount of water, based on rainfall precipitation. Unfortunately, this system is also subject to interference from unwanted debris in the basin. The volume of debris accumulated in the basin will displace water, causing the water level to rise faster and trip the sensor prematurely during short rainfall, resulting in potential damage to the landscape from unwanted interrupted irrigation.
Figure 3. Rain sensor based on conductivity, with a small basin to collect rainfall and electrodes to activate the sensor. (Image source: Gotcher et al. 2014)
Hygroscopic Disk
This is currently the most common type of rain sensor. It eliminates the water reservoir component, therefore eliminating the risks of unwanted debris interfering with the rain gauge component of the rain sensor, making it low maintenance and more reliable when compared with basin sensor types. This type of sensor uses Hygroscopic disks, which are made of a synthetic material similar to cork and expand when wet. The expanded disks trigger the switch after a preset amount of rain falls into the disks’ receptacle. The sensor will disable the controller as long as the disks remain wet and expanded. Once the disks dry out, they will shrink back to their original size, deactivate the sensor, and resume scheduled irrigation.
The sensors will fail if the disks lose their shrinking capacity, are unable to return to their original size, or if there is debris between disks or in the switch mechanisms, such as animals nesting in the sensor. Therefore, the sensors should be inspected regularly to avoid unwanted interruption of the automatic irrigation.
Figure 4. Inside of an expanding disc rain sensor. Image source: Gotcher et al. 2014 (Photos courtesy of Hunter Industries).
Rain sensor “Bypass” at the controller
Controllers may have a “bypass” switch or option at the front panel. This allows the user to bypass the sensor signal and resume the irrigation schedule, regardless of the rain sensor reading. This is a useful function when the rain sensor is not operating properly, or if automatic irrigation is wanted regardless of the weather.
Product selection and proper installation
Always check your irrigation controller’s manual and specifications to ensure compatibility with the rain sensor and proper wiring.
Wireless sensors are a convenient option because they eliminate the need for wires between the sensor and the controller. Instead, the sensor communicates with a receiver wired to the controller (Figure 5). Make sure to check the reach of your rain sensor to ensure it is installed within range from the receiver. Wireless sensors require batteries, and therefore, need to be checked regularly to have batteries replaced as needed. Wired sensors do not require batteries.
Verify the mounting options and requirements. Each model has its own unique mounting hardware, such as rain gutter and flat surface mounts. Most manufacturers have installation manuals available online.
Common issues related to rain sensors are irrigation systems that keep running after a rain period, when the system is expected to be interrupted; or, an irrigation system that takes too long for the irrigation to resume after a rain event. This could be due to several reasons, such as:
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Improper location installation. Ensure the sensor is exposed to rainfall for proper function, and not blocked by any structure or vegetation above or adjacent to the sensor location.
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Strong wind may blow water away from the sensor and interfere with its capacity to identify rainfall.
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delay between rainfall and disk expansion to trigger the sensor, since the disks need to absorb water to expand, and that can take several minutes;
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lifespan of the sensor or disks. Previous research showed that different models presented varied levels of accuracy, depending on how long they had been installed in the landscape (Meeks et al, 2012a).
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Variation in disks’ drying time (Meeks et al, 2012b).
In summary, always make sure the rain sensor is compatible with the irrigation controller and suitable for the location and installation conditions, plan for regular cleaning and maintenance, and observe its behavior to adjust the controller program accordingly based on how long it takes for the sensor to dry out and resume irrigation after a rainfall event.
References:
Electronic Leaf: https://www.phytotronics.com/product/electronicleaf/
Gotcher, Malarie; Taghvaeian, Saleh; Moss, Justin Quetone Smart irrigation technology: Controllers and sensors , Oklahoma Cooperative Extension Service, HLA-6445, 2014
Meeks, Leah & Dukes, Michael & Migliaccio, Kati & Cardenas, Bernard. (2012a). Long Term Expanding-Disk Rain Sensor Accuracy. Journal of Irrigation and Drainage Engineering. 138. 16-20. 10.1061/(ASCE)IR.1943-4774.0000381.
Meeks, Leah & Dukes, Michael & Migliaccio, Kati & Cardenas, Bernard. (2012b). Expanding-Disk Rain Sensor Dry-Out and Potential Irrigation Savings. Journal of Irrigation and Drainage Engineering. 138. 972-977. 10.1061/(ASCE)IR.1943-4774.0000487.
Alberto Ricordi, Landscape and Ornamental Crops Asst. Extension Agent - Oahu County, Cooperative Extension, CTAHR, University of Hawaii at Manoa
Disclaimer: Mention of a trademark or proprietary name does not constitute an endorsement, guarantee, or warranty by the University of Hawaiʻi Cooperative Extension or its employees and does not imply recommendation to the exclusion of other suitable products.
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