An application relates to a system and a method for non-intrusive and continuous level measurement in a cylindrical vessel. The system comprises an ultrasonic transducer for generating an ultrasound wave and for emitting it into the vessel wall, and an electronic control and data processing unit (ECDP) for controlling operation of the transducer and for determining the liquid level from a time of flight of the ultrasound wave. The transducer emits the ultrasound wave as a primary Lamb wave into the vessel wall so that a part of the primary Lamb wave leaks into the liquid. The ECDP changes 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, determine the corresponding time of flight, and determine the liquid level from the time of flight and from parameters of the ultrasound waves.