System for non-intrusive and continuous level measurement of a liquid, where the liquid is enclosed by a solid wall of a horizontally lying, cylindrical vessel (20, 30, 60), the system comprising • an ultrasonic transducer mounted at the outside of the vessel wall for generating an ultrasound wave and for emitting it into the vessel wall, and • at least one electronic control and data processing unit for controlling operation of the transducer and for determining the liquid level from a time of flight of the ultrasound wave, characterized in that • the transducer is a frequency-tunable transducer which is arranged to emit the ultrasound wave as a primary Lamb wave (s1, s2) into the vessel wall so that a part of the primary Lamb wave leaks from the vessel wall into the liquid (22, 32, 62) in form of a pressure wave (s3, s4), • the at least one electronic control and data processing unit is adapted to o change the ultrasonic frequency of the transducer until the transducer receives a reflection of the emitted ultrasound wave in form of a reflection of the pressure wave (s3, s4), o determine the corresponding time of flight, and o determine the liquid level from the time of flight and from a corresponding and predetermined travel length of the emitted and reflected ultrasound wave, taking into account a predetermined speed of sound in the wall, a predetermined speed of sound in the liquid, a geometric dimension of the cross section of the vessel (20, 30, 60) and the fact that the travel length must include a passage where the pressure wave (s3, s4) is reflected by the vessel wall in a perpendicular upward direction to the surface of the liquid (21, 31, 61). • an ultrasonic transducer mounted at the outside of the vessel wall for generating an ultrasound wave and for emitting it into the vessel wall, and • at least one electronic control and data processing unit for controlling operation of the transducer and for determining the liquid level from a time of flight of the ultrasound wave, characterized in that • the transducer is a frequency-tunable transducer which is arranged to emit the ultrasound wave as a primary Lamb wave (s1, s2) into the vessel wall so that a part of the primary Lamb wave leaks from the vessel wall into the liquid (22, 32, 62) in form of a pressure wave (s3, s4), • the at least one electronic control and data processing unit is adapted to o change the ultrasonic frequency of the transducer until the transducer receives a reflection of the emitted ultrasound wave in form of a reflection of the pressure wave (s3, s4), o determine the corresponding time of flight, and o determine the liquid level from the time of flight and from a corresponding and predetermined travel length of the emitted and reflected ultrasound wave, taking into account a predetermined speed of sound in the wall, a predetermined speed of sound in the liquid, a geometric dimension of the cross section of the vessel (20, 30, 60) and the fact that the travel length must include a passage where the pressure wave (s3, s4) is reflected by the vessel wall in a perpendicular upward direction to the surface of the liquid (21, 31, 61). o change the ultrasonic frequency of the transducer until the transducer receives a reflection of the emitted ultrasound wave in form of a reflection of the pressure wave (s3, s4), o determine the corresponding time of flight, and o determine the liquid level from the time of flight and from a corresponding and predetermined travel length of the emitted and reflected ultrasound wave, taking into account a predetermined speed of sound in the wall, a predetermined speed of sound in the liquid, a geometric dimension of the cross section of the vessel (20, 30, 60) and the fact that the travel length must include a passage where the pressure wave (s3, s4) is reflected by the vessel wall in a perpendicular upward direction to the surface of the liquid (21, 31, 61).