Long Term Expanding-Disk Rain Sensor Accuracy

Rain sensors are devices that connect to automatic irrigation systems to interrupt scheduled irrigation events as a result of rainfall. The accuracy of expanding-disk rain sensors over nearly five years was evaluated at a research site in north central Florida. Seven rain sensor model/rainfall set point combinations were evaluated. The seven configurations are as follows: Wireless Rain-Clik (WL) rain sensor, Mini-Clik rain sensors with rainfall set point of, 3, 6, 6, and 13 mm (3MC, 6MC, Hunter, and 13MC), Irritrol RFS 1000 at a 6-mm set point (Irritrol), and Toro TWRS at a 6-mm set point (Toro). The MC and WL configurations were evaluated for 1,182 days with four replicates. The Hunter, Irritrol, and Toro configurations were monitored 1,150 days with eight replicates. Data were collected for each configuration to determine the total time in interrupt mode. This information was compared with measured rainfall data on site. On average, the WL, 3MC, 6MC, 13MC, Hunter, Irritrol, and Toro required 3.2, 1.9, 1.6, 6.6, 3.8, 4.3, and 5.8 mm of rainfall for interrupt mode, respectively. Accuracy ranged from 27 to 97%. Rain sensor accuracy had percentile point changes from $-23$ to 25 over the study period, where a negative value indicated a decrease in accuracy. In general, rain sensors at 6-mm thresholds or less become more accurate after 560 days of installation, because they became less sensitive over time. However, rain sensors with newer expanding-disk hygroscopic material seemed to result in more accurate response and were less erratic than sensors in the field in most cases for over a year. Hygroscopic material that had been installed longer had more erratic behaviors such as going into interrupt mode with little or no rainfall. For the best accuracy and most consistent behavior, Hunter Mini-Clik and Hunter Wireless Rain-Clik rain sensors should be replaced after 1 year while Irritrol RSF 1000 and Toro TWRS rain sensors do not need to be replaced for at least three years. Rain sensors could increase water savings to homeowners and have environmental benefits but should not be used in applications requiring consistent accuracy greater than 70%.