A method is provided for filling an enclosure with an ignitable mixture of gases. The enclosure has an outer wall comprising an exterior surface, an interior surface, and a fill aperture extending therethrough. The method comprises delivering a predetermined amount of pressurized combustible gas into the enclosure through the fill aperture, delivering a first predetermined amount of pressurized inert gas into the enclosure through the fill aperture at a first pressurization rate, delivering a second predetermined amount of pressurized inert gas into the enclosure through the fill aperture at a second pressurization rate, welding a sealing member to the outer wall of the enclosure by a welding process to form a weld operable to seal the fill aperture and to permit the enclosure to store pressurized gas therein. The first pressurization rate allows the first predetermined amount of pressurized inert gas to mix with the predetermined amount of pressurized combustible gas to form an ignitable mixture. The second pressurization rate is lower than the first pressurization rate. The second pressurization rate allows stratification to form within the enclosure between a layer of the ignitable mixture and a layer of the second predetermined amount of pressurized inert gas. The layer of the second predetermined amount of pressurized inert gas is disposed between the layer of the ignitable mixture and the fill aperture.