Rule 500GPH Electronic Sensing Bilge Pump - 12V [25SA]500GPH Electronic Sensing Bilge Pump 12V In the next generation sensing bilge pumps, the 500GPH S series pump works on the principle of impeller resistance. The pump will turn on every 2 1 2 minutes for about one second to sense for water. If water is present, the pump will sense resistance and continue to run until all water is removed. After that, it will resume checking for water every 2 1 2 minutes. Water is kept in check between 1 8" and 3 8"!!!