This Neptune Systems Optical Sensor plugs into your Neptune Systems FMM and, using reflected infrared light, will signal your Apex when it changes state from being wet to dry or dry to wet. This sensor can be used for constructing your own ATO (auto-top-off), as a high-level warning sensor in your sump, water storage level sensor, and much more. The sensors can be mounted into custom brackets or through a hole in a vessel. Included are a fastening nut and silicone washer. These sensors are sold individually.