the regulations require that such a tank be vapor tight except during manual gauging procedures this invention provides for a total combination which can provide the necessary seal to maintain vapor seal conditions in accordance with regulations such that only during manual gauging would any substantial vapors be permitted to escape through a gauging well referring to the drawings it will be observed that there is a storage tank 11 that holds a quantity of volatile product 12 therein there is a floating roof 15 which rests on the product 12 and which is in contact with the product 12 adjacent to a gauging well 16 gauging well 16 is firmly attached to the storage tank 11 at the bottom and it has a plurality of perforation slots 19 the slots are spaced apart along the length of the gauging well 16 there is a vapor tight seal 20 that is mounted on the roof 15 at the lower surface thereof which rests on the product 12 the seal 20 surrounds the outside of the gauging well 16 and of course the seal moves with the roof 15 as the level of the product changes in order to provide a vapor seal for the inside of the gauging well 6 which inside seal will act at all times except when a gauging operation is taking place there is a vapor seal structure which is illustrated in detail.