Product review (5 stars): how it works
Seems there's a lot if questions the seller doesn't know. A 2 x 1-1/4 vent alarm screws into a 2 in Female opening on the tank and needs a 1-1/4 vent pipe.
The alarm makes a whistling noise when the tank is being filled once the tank gets full the whistle stops and the person filling the tank knows to stop.