Water Level Indicator

dear sir ..

i am thirumalai i want to some technical information of the water level sensor and how to working principle magnetic type of water level sensors.already make a wire type sensor using for sl100 transistor directly apply to 12volt supply for in bottom level of the tank . so this type based on the electro plating my copper type sensor is corosive . how to prevent the copper sensor so i want idea and neat skitch the drawing .. thanks ..