The optical sensor is specially designed and reportedly sensitive to 2-3mm water level. Instead of the sensor being pointed vertically down, Pacific Sun design a horizontal optical sensor with a 90 degree prism;
This clever design prevents water droplets accumulated at the bottom of sensors from interfering with readings.

The sensor reads the water level every 5 seconds; should the water fail to achieve the desired level after 20 read cycles, the Sentry ATO will shut down the pump/solenoid while sounding an alarm to notify the aquarist the top-off reservoir or water supply is dry (or there is another problem).
The alarm can also be programed to sound if the water level is too high.