The Neptune MOS replaces the previous OS-1-M and features an updated design that fits into tighter spaces and helps to prevent water incursion into the sensor. The MOS plugs into an Apex FMM port and uses reflected infrared light to 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 on any type of reservoir that has up to 1/4" thick walls.