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 !!! Great idea when power conservation is needed and as an added benefit, no float switch is needed!. All that plus the added benefits of our standard bilge pump . Check valve included with hidden air vent to prevent airlocking. True electromagnetic suppression EMC . A built-in thermal cell that protects the pump and the vessel when things get too hot. These are the smallest automatic pumps available on the market!!!. Features . Completely automatic with manual override No Float Switch Needed!. Submersible and Ignition Protected. High e 500GPH Electronic Sensing Bilge Pump 12V. In the next generation sensing bilge pumps, th Manufactured by Rule. Part Number 25SA