Method for Measuring Liquid Cryogen Level Using a Level Probe

A method of measuring a level of liquid cryogen, comprising the steps of:

• providing a cryogen level probe comprising a length of superconductive material, means for applying a current to the superconductive material, a heater for heating the superconductive material and means for measuring the voltage across the superconductive material,
• dipping the liquid cryogen level probe in a liquid cryogen,
• applying a propagation current to the heater to heat a part of the superconductive material above the critical temperature, thereby causing a normal resistive front to propagate along the superconductive material to the level of the liquid cryogen;
• removing the propagation current from the heater;
• applying a measurement current to the superconductive material;
• measuring a voltage across the superconductive material thereby to determine the level of the liquid cryogen,
• characterised in that the method comprises the steps of
• monitoring the magnitude of the voltage across the superconductive material while the propagation current is being applied;
• and
• ceasing application of the propagation current in response to the detection of a voltage across the superconductive material which exceeds a certain threshold level.