Compact size - the entire optical sensor unit ONLY 0.8 inches diameter. The unit works on rimmed and rimless tanks, and as well as sump tanks. The dual precision optical sensors are 0.1 cm apart, which enables it to monitor the level fluctuation precisely (less than 0.6cm difference) New and Improved Features - Upgraded ETFT (Extra Time Filling Technology), AFT (Anti-fluctuation Technology), and QST 2.0 (Quick Security Technology). With all these new upgrades combined, our ATOs are far superior and safer. ETFT recognizes your tank size after it first initialized. Working alongside the ETFT, the AFT will be able to decide how long it takes to refill your tank without over-filling it when the QST alarm is triggered. QST 2.0 provides real-time protection against overfilling and pump dry run.  The controller will give a visual alert and stop the pump immediately when it senses a refilling time more than 6 times longer than the time calculated by ETFT.  QST can detect a malfunction of the sensor/controller or even an empty reservoir.In version 2.0 we incorporate ETFT feature and not relying on first filling time from version 1. Flooding barely happens when following the instructions carefully.