the control means comprising a comparator for comparing the measured pressure with the reference value to control the opening of the inlet obturator in this way the repetitive sequence can be arranged so that the inlet obturator is closed while the pressure p 1 in the transfer chamber is above the reference value p c and opens as soon as p 1 drops below p c the difference or margin between the reference value p c and the pressure p 0 in the reservoir is defined as a function of the volume of the transfer chamber and of the maximum incremental mass u03b4m of liquid which may be admitted by the inlet preferably and also in accordance with the invention the control means can also be responsive means generating a signal controlling the time for which the inlet obturator is open the evaporation of liquid while the two obturators are closed causes the pressure to rise in the transfer chamber above the reference value and the outlet obturator then opens the value of the time control signal and thus the length of time before the inlet obturator closes is defined as a function of the volume of the transfer chamber the pressure p 0 in the reservoir the temperature t 0 in the reservoir and the physical characteristics of the fluid used and in certain cases this signal can be constant other features and advantages of the invention will appear from the following description by way of non limitative example with reference to the accompanying drawing which is a schematic diagram of apparatus according to a particular embodiment of the invention this apparatus includes a closed storage reservoir 1 in which a cryogenic fluid is contained in liquid gas phase equilibrium such for example as liquid hydrogen or liquid.