Blue-White Industries F-300S 1.5"-4.0"P F300 Sensor/Switch Kit detects if water flow rate is at desired level. A built-in contact closure switch can remotely turn on/off a chemical pump and/or other external devices when flow rate is within desired range. Additionally, this can be used as an alarm signal if flow rate is not within flow range target, potentially indicating a clogged filter, malfunctioning equipment, closed valve, etc.