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;