The Rule-Mate® 1500 GPH pump consists of a powerful Rule® bilge pump and a highly intelligent circuit board in a single housing. The pump activated when the water reaches 2-1/2 and automatically shuts off when the pump senses no water against the impeller, reducing the remaining water to approximately 5/8. Rule-Mate IntelligenceRule-Mate Bilge pumps have built-in programming that can sense when the pump is running but not pumping water. This is a very unique feature to bilge pumps because it protects the pump and the vessel, especially the vessels batteries. If debris fouls the sensor or the pump becomes air-bound, the pump will stop running and begin to check for water every 2-1/2 minutes. Once the debris is removed, the pump will revert to its normal operation, turning on when the water reaches 2-1/2 and shutting off at 5/8.