Method of operation of a device for determining the position of a gas/liquid interface of a liquefied gas - helium (including superfluid helium), neon, hydrogen, nitrogen or oxygen - in a cryogenic tank, the device having at least one self-heated resistance temperature detector being mounted inside the cryogenic tank by a support made from a material with low thermal conductivity in comparison to the thermal conductivity of the surrounding media, the detector being connected to a current or voltage pulse generator , the device further having means to read out the temperature of the detector , the readout depending on the electric resistance of said detector , the method of operation comprising the application of at least one current or voltage pulse from the pulse generator to the detector , the power and duration of this heating pulse being sufficient for overheating the detector at the end of this heating pulse to a temperature T heated above at least the temperature of its environment T env plus the temperature resolution Δ( T env ) of the detector at the temperature of its environment: T heated > T env + Δ T env , characterized by performing a temperature measurement with the detector at the end of a time interval t off after the end of at least one heating pulse; the power and the duration of the heating pulse being determined in a preceding test experiment such that the following condition is fulfilled for at least one time interval t off : the difference between the temperature of the detector measured at the end of the time interval t off in gas and the temperature of the detector measured at the end of the time interval t off in liquid is bigger than 2Δ( T env ), where Δ( T env ) is the temperature resolution of the detector at the given temperature of its environment; whereto the power and/or the duration of the heating pulse is/are increased by steps until it is possible to find at least one value of the time interval t off to satisfy the previous condition, choosing a threshold temperature T threshold , satisfying at least the following conditions: • if the detector is immersed in liquid then the threshold temperature T threshold is above the temperature of the detector measured at the end of the time interval t off plus the temperature resolution Δ( T env ) of the detector at the temperature of its environment T env ; • if the detector is immersed in gas, then the threshold temperature T threshold is below the temperature of the detector measured at the end of the time interval t off minus the temperature resolution Δ( T env ) of the detector at the temperature of its environment T env ; deducing a position of the gas/liquid interface of the liquefied gas in the cryogenic tank below the position of the detector if the temperature of the detector measured at the end of the time interval t off after the end of the heating pulse is above the threshold temperature T threshold , and 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 t off after the end of the heating pulse is below the threshold temperature T threshold . • if the detector is immersed in liquid then the threshold temperature T threshold is above the temperature of the detector measured at the end of the time interval t off plus the temperature resolution Δ( T env ) of the detector at the temperature of its environment T env ; • if the detector is immersed in gas, then the threshold temperature T threshold is below the temperature of the detector measured at the end of the time interval t off minus the temperature resolution Δ( T env ) of the detector at the temperature of its environment T env ; <EOS>