This unit is a 2" X 2" Vent alarm. It is designed to create an audible "whistle" as the oil tank is being filled. When tank is filled the "whistle" stops telling the delivery is complete. The unit fits a 2" bung hole in the tank and a 2" vent pipe which leads out to where the delivery adapter. is located. Many heating oil compas will not make a delivery unless a "vent alarm" is present to avoid any spills or damage to the tank. It can be used for indoor tanks and outdoor, but NOT underground tanks.