Hope Is Not a Method

A liquid cryogen pump in which liquid is maintained on both sides of the pump piston during operation. In the preferred embodiment, vapor, when present in the pump chamber, is passed from in front to behind the piston by providing a predetermined clearance between the piston and the cylinder in which it is reciprocated. The clearance is preferably large enough to pass enough liquid cryogen to maintain the desired head of liquid behind the piston. In the embodiment set forth, the pump is arranged so that it can be immersed in the liquid cryogen to be pumped with its pumping chamber and valves at or close to the bottom of the liquid cryogen supply. Heat transfer between the warm, upper end of the pump and its cold end is effectively restricted, and, preferably, the warm end is connected to the cold end of the pump by thin-walled tubular members formed of low thermal conductive materials to minimize heat transfer by conduction.