The device comprises in the tank above the liquid:P Pa pipe for supplying the tank with superfluid helium;P Pa member for removing the superfluid helium by spontaneously producing on its surface a so-called Rollin film, which is essentially located at the horizontal level to which it is desired to regulate the free surface of the helium in the tank and whereof the removing perimeter, which is a variable as a function of the altitude, increases over a height equal to the tolerance in the evolution of the level from its base to a maximum value at its upper junction point with a superleak associated therewith and which is then traversed by the thus removed superfluid helium; PPa heated chamber into which issues the superleak and which is linked, on the one hand, with an outlet having a controlled flow rate to the atmosphere of the tank and, on the other hand, to a chamber having flexible deformable walls and a variable volume, said walls causing, during their extension or retraction, the displacement of a rod, whose position is consequently characteristic of the helium level in the tank; PPmeans for making the entry of helium in the tank supply pipe dependent on the position of the rod.
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