Method for measuring the level of liquid cryogen in a cryogen vessel

A method for measuring the level of the surface of liquid cryogen in a cryogen vessel, comprising the steps of:

• introducing a cryogen level probe comprising a superconducting wire running the length of the probe, substantially vertically, into a cryogen vessel containing liquid cryogen;
• passing electric current through the superconducting wire to propagate a resistive front down the superconducting wire to the surface of the liquid cryogen by applying a current pulse having a profile comprising a leading edge part, an ignition pulse part, a recovery pulse part and a measurement pulse part;
• the leading edge part increasing the magnitude of current to a first magnitude for a first duration, to form the ignition pulse part;
• the ignition pulse part having a magnitude and duration that produce a resistive front propagating down the superconducting wire to a level below the surface of the liquid cryogen;
• the recovery pulse following the ignition pulse part and comprising a current of a second magnitude less than the first magnitude, for a second duration that causes the superconducting wire below the surface of the liquid cryogen to return to its superconducting state;
• and
• measuring a resistance of the superconducting wire with the measurement pulse part following the recovery pulse part, said measurement pulse part comprising a current of a third magnitude, greater than the second magnitude but less than the first magnitude, for a third duration, the third duration and the third magnitude being selected for measuring the resistance of the superconducting wire while maintaining a position of the resistive front during the measurement of the resistance.