The Johnson Ultima bilge pump switch uses new technology which can detect disruptions caused by water and fluids. The detector cells used never come into contact with the water and can not be corroded by this or any other foreign debris. The Johnson Ultima bilge pump switch automatically activates at a fluid level of 50mm (2) and deactivates at 20mm (3/4"). The electronics incorporate an 8 second delay to ensure that water level is true. The Johnson Ultima bilge pump switch is a two wire device and is fully submersible."