Control system for liquefied gas container

In an assembly including a container containing therein liquefied gas including a gas phase and a liquid phase, a superconductive quantum interference device immersed in the liquid phase and maintained thereby at a precise low temperature enabling said device to be employed to measure weak intensity magnetic fields, and a refrigerator including a recondenser for condensing the gas phase and thereby maintaining said device in the liquid phase at said precise low temperature, whereby it is necessary to stop operation of said refrigeration and said recondenser when said device is being employed to measure a weak intensity magnetic field, during which time the temperature of said device tends to rise above said precise low temperature, the improvement comprising a control system for controlling the gas and liquid phases in said container to ensure that said device is maintained at said precise low temperature in spite of stopping operation of said refrigerator and said recondenser, said system comprising:

• pressure sensing means for sensing gas phase pressure in said container;
• an on-off valve connected to said container for discharging therefrom the gas phase;
• a source of gas of the same composition as the liquefied container in said container;
• a flow control valve means between said source and said container for selectively supplying at a controllable flow rate gas from said source to said container;
• and
• control means, operably coupled to said pressure sensing means, said on-off valve and said flow control valve means and responsive to an output from said pressure sensing means, for, when the pressure of the gas phase in said container is greater than a predetermined positive first value, opening said on-off valve to thereby discharge gas phase from said container and thus to prevent the temperature of said device from rising above said precise low temperature, and for, when the pressure of the gas phase in said container is negative, relative to atmospheric pressure, and of an absolute value greater than a predetermined second value, opening said flow control valve means by a predetermined degree of opening for a predetermined period of time to thereby supply gas from said source to said container at a controlled flow rate and thus to prevent the temperature of said device from dropping below said precise low temperature.