Abstract: A thermal barrier for a Dewar vessel combines an insulative vapor plug and a vapor barrier. The plug is sized so as to define an open space between it and the neck portion of the Dewar vessel to allow venting of vaporous cryogen from the inner vessel of the Dewar vessel through a Dewar opening. The vapor barrier provides an interference between the plug and the neck portion that disrupts venting of vaporous cryogen but does not form an airtight seal that would block venting and cause unacceptable build-up of pressure within the inner vessel. Multiple vapor barriers, especially four or more, provide multiple interferences that create multiple chambers between the plug and the neck portion. Each interference disrupts migration of vaporous cryogen as an incremental increase (e.g., 2 psig or less) in vapor pressure of each chamber causes the chamber to breach and then another incremental increase in vapor pressure of the liquid cryogen in the vaporous state is required to breach each successive chamber.

Abstract: A thermal barrier for a Dewar vessel combines an insulative vapor plug and a vapor barrier. The plug is sized so as to define an open space between it and the neck portion of the Dewar vessel to allow venting of vaporous cryogen from the inner vessel of the Dewar vessel through a Dewar opening. The vapor barrier provides an interference between the plug and the neck portion that disrupts venting of vaporous cryogen but does not form an airtight seal that would block venting and cause unacceptable build-up of pressure within the inner vessel. Multiple vapor barriers, especially four or more, provide multiple interferences that create multiple chambers between the plug and the neck portion. Each interference disrupts migration of vaporous cryogen as an incremental increase (e.g., 2 psig or less) in vapor pressure of each chamber causes the chamber to breach and then another incremental increase in vapor pressure of the liquid cryogen in the vaporous state is required to breach each successive chamber.