Robust dynamical method and device for detecting the level of a liquid using resistance temperature detectors

A method of operation of a device for determining a position of a gas/liquid interface of a liquefied gas, helium, superfluid helium, neon, hydrogen, nitrogen or oxygen in a cryogenic tank, the device having at least one self-heated resistance temperature detector mounted inside the cryogenic tank with a support made from a material with low thermal conductivity in comparison to a thermal conductivity of surrounding media, the detector being connected to a current or voltage pulse generator, the device further having means to read out a temperature of the detector in dependence on an electric resistance of that detector, the method comprising the steps of:

• a) applying at least one current or voltage heating pulse from the pulse generator to the detector, a power and duration of that heating pulse being sufficient for overheating the detector at an end of the heating pulse to a temperature Theated above a temperature of a detector environment Tenv plus a temperature resolution Δ(Tenv) of the detector at the temperature of the detector environment, wherein Theated>Tenv+Δ(Tenv);
• b) performing a temperature measurement with the detector at an end of a time interval toff after the end of at least one heating pulse, wherein a power and a duration of the heating pulse is determined in a preceding test experiment such that, for at least one time interval toff, a condition is fulfilled, the condition specifying that a difference between a temperature of the detector measured at an end of the time interval toff in gas and a temperature of the detector measured at the end of the time interval toff in liquid is greater than 2Δ(Tenv), where Δ(Tenv) is the temperature resolution of the detector at the temperature of the detector environment, a power and/or a duration of the heating pulse thereby being increased by steps until at least one value of the time interval toff is found which satisfies the condition;
• c) choosing a threshold temperature Tthreshold, such that, if the detector is immersed in liquid, then the threshold temperature Tthreshold is above a temperature of the detector measured at the end of the time interval toff plus the temperature resolution Δ(Tenv) of the detector at the temperature of the detector environment Tenv and, if the detector is immersed in gas, then the threshold temperature Tthreshold is below the temperature of the detector measured at the end of the time interval toff minus the temperature resolution Δ(Tenv) of the detector at the temperature of the detector environment Tenv;
• d) deducing a position of a gas/liquid interface of the liquefied gas in the cryogenic tank below a position of the detector if the temperature of the detector measured at the end of the time interval toff after the end of the heating pulse is above the threshold temperature Tthreshold;
• and
• e) deducing a position of the gas/liquid interface of the liquefied gas in the cryogenic tank above the position of the detector if the temperature of the detector measured at the end of the time interval toff after the end of the heating pulse is below the threshold temperature Tthreshold.