The 3" Fig. 922 Combination Pressure/Vacuum/Vent/Overfill Alarm alerts you when your AST is near full. It also allows the tank to breathe during filling and dispensing operations. It is equipped with a high intensity, "whistle" alarm which can be set by adjusting the cable length to float height. It is fully mechanical, with no wires or batteries needed. Vacuum Setting: 1 oz./in2. Minimum Fill Rate: 20 GPM