This unique and easy-to-install device makes a whistling sound which stops when your tank is full, preventing spills into the marine ecosystem. While the tank is being filled it produces a “whistling” sound, loud enough to be heard from any point in the boat over typical wind and background noises. As the tank approaches full, the sound turns to a “warble” and when completely full it immediately stops. 3.75"L (overall) 2"L (installed) x 2"W. Installed in vent line near tank.