Abstract: An external pressure building circuit for a portable cryogenic liquid cylinder which includes a tubular member disposed outside of the cylinder. The tubular member has an upper portion and a lower portion. The upper portion of the tubular member is in communication with the gas containing portion of the cryogenic liquid cylinder; the lower portion is in communication with the liquid containing portion of the cryogenic liquid cylinder. The tubular member between about 30 and 60 inches long and includes a plurality of fins attached to the tubular member. The fins act as a heat transfer device. In operation a quantity of cryogenic liquid enters the lower portion of the tubular member and is heated. Once the liquid is converted to a gas, it is returned to the gas containing portion of the cylinder. By converting a sufficient quantity of cryogenic liquid to gas, a pressure of greater than 400 p.s.i. can be maintained at a high continuous rate of flow, e.g., greater than 2000 cfh.