The Fig. 922 Combination Vent/Overfill Alarm is a fully mechanical, high intensity audible alarm that alerts you when your tank is near full while also allowing your tank to breathe during filling and dispensing operations. The unit is equipped with a whistle which incorporates a 2″ or 3″ full port pressure/vacuum vent. the pressure poppet setting is 6 oz./in or 8 oz./in with a vacuum relief setting of 1 oz./in. The unit can be set to activate at 90% fill height by adjusting the cable length to the float device.