Feedback on the operation of French sodium fast reactors (SFR) revealed the necessity to improve continuous monitoring, particularly in terms of the primary system instrumentation. Among others, research and development is performed in order to characterise the continuous presence of microbubbles in the primary liquid sodium. A water-air bench has been experimented as the first step of the industrial development of acoustical techniques. The generated bubble cloud characteristics are very similar to the reactor case (bubble radiuses and void fractions). Non-linear acoustic methods have been investigated to determine the histogram of bubble radiuses. Our work involved using the non-linear data resulting from the mixing of two frequencies. Sweeping at low frequency makes it possible to determine the non-linear resonance of the bubbles present. Then the non-linear mixing with a high-imagery frequency is used to build the bubble radius histograms. In addition to this method, measurement of the low-frequency acoustic velocity using a Helmholtz resonator is proposed to determine the void fraction. Future industrial prospects for this work look promising