An acoustic wave measurement system for measuring the fill level (N R) of a tank, said tank storing a first single-phase fluid (F 1) having first physical properties (P 1) and a second single-phase fluid (F 2) having second physical properties (P 2), said first physical properties (P 1) comprising a first density ρ 1 and said second physical properties (P 2) comprising a second density ρ 2 strictly lower than the first density ρ 1 so that the single-phase fluids (F 1, F 2) are vertically superimposed in the tank, the first single-phase fluid (F 1) being located in the lower part of the tank, the second single-phase fluid (F 2) being located in the upper part of the tank, said first single-phase fluid (F 1) and said second single-phase fluid (F 2) being separated by a substantially horizontal interface, said tank comprising an envelope extending longitudinally along an axis X10, the envelope comprising an inner face in contact with the single-phase fluids (F 1, F 2) and an outer face, the envelope comprising a cylindrical median portion, the measurement system being characterized in that it comprises: - at least three transducers (22a-22g) configured to be positioned at different vertical heights on the outer face of the cylindrical portion of the envelope, - each transducer being configured, on the one hand, to emit, upon receiving a first electric signal (U 1), an incident acoustic wave (O 1) to the outer face of a wall of the envelope and, on the other hand, to emit a second electric signal (U 2), upon receiving a reflected acoustic wave (O 2), corresponding to the reflection of the incident acoustic wave (O 1) on the inner face of said wall, the reflected acoustic wave (O 2) having passed only through the wall of the envelope, the second electric signal (U 2) being a function of the difference in acoustic energy between the incident acoustic wave (O 1) and the reflected acoustic wave (O 2), - at least one calculation member configured to determine, from the electric signals (U 1 -U 2) and physical properties (P 1 -P 2) of the single-phase fluids (F 1, F 2), the presence of the first single-phase fluid (F 1) at each vertical height of the transducers and to deduce the fill level (N R) therefrom, the calculation member being configured to determine, for each transducer (22a-22g), a lower state (ET 1) in the presence of the first single-phase fluid (F 1) or an upper state (ET 2) in the absence of the first single-phase fluid (F 1), the calculation member is configured to determine the height of the interface from the height of the two successive transducers (22e, 22d), one of which is in a lower state (ET 1) and the other in an upper state (ET 2). - at least three transducers (22a-22g) configured to be positioned at different vertical heights on the outer face of the cylindrical portion of the envelope, - each transducer being configured, on the one hand, to emit, upon receiving a first electric signal (U 1), an incident acoustic wave (O 1) to the outer face of a wall of the envelope and, on the other hand, to emit a second electric signal (U 2), upon receiving a reflected acoustic wave (O 2), corresponding to the reflection of the incident acoustic wave (O 1) on the inner face of said wall, the reflected acoustic wave (O 2) having passed only through the wall of the envelope, the second electric signal (U 2) being a function of the difference in acoustic energy between the incident acoustic wave (O 1) and the reflected acoustic wave (O 2), - at least one calculation member configured to determine, from the electric signals (U 1 -U 2) and physical properties (P 1 -P 2) of the single-phase fluids (F 1, F 2), the presence of the first single-phase fluid (F 1) at each vertical height of the transducers and to deduce the fill level (N R) therefrom, the calculation member being configured to determine, for each transducer (22a-22g), a lower state (ET 1) in the presence of the first single-phase fluid (F 1) or an upper state (ET 2) in the absence of the first single-phase fluid (F 1), the calculation member is configured to determine the height of the interface from the height of the two successive transducers (22e, 22d), one of which is in a lower state (ET 1) and the other in an upper state (ET 2).