Only small users of liquid helium — or those blessed with more money than they know what to do with — can afford to let helium out into the atmosphere. Where liquid helium is bought out, it is a matter of returning the helium gas in cylinders to the supplier. There may be an economic advantage in returning the gas at a low level of impurity, and in this case what is said below about purification will be relevant. The owner of a liquefier will want to recover the helium gas evaporated from the cryostats supplied from the liquefier, purify it to an extent acceptable to the liquefier, and store it under pressure at room temperature. In the case of a closed refrigeration system, storage of the charge of gas at room temperature when the plant is not running is likely to be the only problem.