V2 FMM Optical Level Sensor w/ Magnetic Mount - Neptune Systems
These are Version 2. The new upgraded more water resistant version with the cord near the edge of the sensor.
Whether for your ATO reservoir, to use as a sump-high alert, or for your long term water storage such as a brute container, these optical sensors plugin into the FMM. In standalone use when used in your ATO reservoir (plugged into port-4), the FMM will stop your PMUP and sound an alarm when the sensor is above the water level. If you plug it into port-3, and mount it high in your sump, if your water level ever goes that high, an alarm will sound and your PMUP will stop operating.