System for non-intrusive and continuous level measurement of a liquid, where the liquid is enclosed by a solid wall of a vessel (41, 51), the system comprising • an ultrasonic transmitter (38, 48) for generating an ultrasound wave and for emitting it into the vessel wall, • an ultrasonic receiver for receiving the ultrasound wave through the vessel wall, • at least one electronic control and data processing unit for controlling operation of the transmitter (38, 48) and of the receiver and for determining the liquid level from a time of flight of the ultrasound wave, characterized in that • the transmitter (38, 48) is a frequency-tunable transmitter which is placed at a first position ([h1, d1]) at the outside of the vessel wall and below the level of the liquid surface (44, 54) in such a way that the transmitter (38, 48) is able to emit the ultrasound wave as a primary Lamb wave into the vessel wall so that a part of the primary Lamb wave leaks from the vessel wall into the liquid in form of a pressure wave (33, 43) in an inclined and upward direction towards the liquid surface (44, 54), • the receiver is placed at a second position ([h2, d2]) at the outside of the vessel wall and below the level of the liquid surface (44, 54) in such a way that the receiver is able to receive a secondary Lamb wave which is generated by the pressure wave (33, 43, 45) hitting the vessel wall after having been reflected by the liquid surface (44, 54), • the at least one electronic control and data processing unit is adapted ∘ to control the transmitter (38, 48) to emit the primary Lamb wave at different frequencies, ∘ for each of the different frequencies, to determine the time of flight (t) of the pressure wave (43, 45) and/or the amplitude of the received secondary Lamb wave, ∘ to change the frequency (f) of the transmitter (38, 48) until the determined time of flight reaches a minimum (t min) or until the determined amplitude reaches a maximum, and ∘ to determine the liquid level based on the relationship that the minimum time of flight (t min) or the time of flight corresponding to the maximum amplitude of the secondary Lamb wave, respectively, equals the length of the travel path (a1+a2) of the pressure wave divided by the speed of the pressure wave in the liquid (cL). • an ultrasonic transmitter (38, 48) for generating an ultrasound wave and for emitting it into the vessel wall, • an ultrasonic receiver for receiving the ultrasound wave through the vessel wall, • at least one electronic control and data processing unit for controlling operation of the transmitter (38, 48) and of the receiver and for determining the liquid level from a time of flight of the ultrasound wave, • the transmitter (38, 48) is a frequency-tunable transmitter which is placed at a first position ([h1, d1]) at the outside of the vessel wall and below the level of the liquid surface (44, 54) in such a way that the transmitter (38, 48) is able to emit the ultrasound wave as a primary Lamb wave into the vessel wall so that a part of the primary Lamb wave leaks from the vessel wall into the liquid in form of a pressure wave (33, 43) in an inclined and upward direction towards the liquid surface (44, 54), • the receiver is placed at a second position ([h2, d2]) at the outside of the vessel wall and below the level of the liquid surface (44, 54) in such a way that the receiver is able to receive a secondary Lamb wave which is generated by the pressure wave (33, 43, 45) hitting the vessel wall after having been reflected by the liquid surface (44, 54), • the at least one electronic control and data processing unit is adapted ∘ to control the transmitter (38, 48) to emit the primary Lamb wave at different frequencies, ∘ for each of the different frequencies, to determine the time of flight (t) of the pressure wave (43, 45) and/or the amplitude of the received secondary Lamb wave, ∘ to change the frequency (f) of the transmitter (38, 48) until the determined time of flight reaches a minimum (t min) or until the determined amplitude reaches a maximum, and ∘ to determine the liquid level based on the relationship that the minimum time of flight (t min) or the time of flight corresponding to the maximum amplitude of the secondary Lamb wave, respectively, equals the length of the travel path (a1+a2) of the pressure wave divided by the speed of the pressure wave in the liquid (cL). ∘ to control the transmitter (38, 48) to emit the primary Lamb wave at different frequencies, ∘ for each of the different frequencies, to determine the time of flight (t) of the pressure wave (43, 45) and/or the amplitude of the received secondary Lamb wave, ∘ to change the frequency (f) of the transmitter (38, 48) until the determined time of flight reaches a minimum (t min) or until the determined amplitude reaches a maximum, and ∘ to determine the liquid level based on the relationship that the minimum time of flight (t min) or the time of flight corresponding to the maximum amplitude of the secondary Lamb wave, respectively, equals the length of the travel path (a1+a2) of the pressure wave divided by the speed of the pressure wave in the liquid (cL).