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The setting level of a hydraulic machine, specially for low head machines, is decided with respect to the possible alteration of the efficiency due to the cavity development. This alteration can easily be noticed by following the evolution of the efficiency 'eta' as a function of the Thoma number 'sigma' leading to the so-called 'eta'-'sigma' cavitation curves. Observation of the cavity extent in the flow passage of the runner allows to associate the drop of efficiency with a particular type of cavity development. However, depending on the type of cavities this drop cannot be very easily explained. Obviously, for a leading edge attached cavity corresponding to high head operating points, the presence of the vapour phase on the blade suction sicle limits the pressure at the vapour tensile strength value which causes the flow alteration. In the case of travelling bubble cavitation, corresponding to the outlet cavitation at the nominal head, previous experiments with a 2-D NACA profile show that the modification of the mean pressure field is mainly due to the bubble dynamics. The aim of this paper is to present the results and the analysis of two experiments intending to explain the influence of the nuclei content on the mean pressure field correction due to the bubble dynamics.