Method for measuring the level of liquid cryogen in a cryogen vessel
In a method for measuring the level of liquid cryogen in a cryogen vessel, a current pulse is applied to a cryogen level probe includes an ignition current pulse part of sufficient magnitude to ensure that a resistive front propagates down the probe to below the liquid cryogen surface; a recovery current pulse part allowing that part of the superconducting wire below the surface of the liquid cryogen to return to a superconducting state; and a measurement current pulse part of sufficient magnitude and duration to allow an accurate measurement of the superconducting wire's resistance to be measured.
1. A method for measuring a level of a 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 a magnitude of current to a first magnitude for a first duration, to form the ignition pulse part;
the ignition pulse part having the 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.
2. The method according to claim 1 , further comprising the step of monitoring a voltage appearing across the superconducting wire as a result of step of passing electric current through the superconducting wire, and detecting a selected voltage to provide an indication that propagation of the resistive front has begun.
3. The method according to claim 2 wherein, if the selected voltage is not reached within a predetermined time, aborting the measurement of the resistance, and indicating a fault.