Experimental studies have shown that outlet cavitation of a Francis turbine is influenced by the active nuclei content of the test loop water, up to a limit curve called the saturation characteristic. To determine the importance of this influence in operating conditions, tests are performed for a nu= 0.38 Francis mode!, for many operating points. Moreover, based on the resolution of the Rayleigh-Plesset equation, it can be shown that, even with a low nuclei content of the plant water, the cavitation characteristic of the full-scale machine should correspond to the saturation curve determined on the model. To verify the condition on this minimum amount of nuclei, in-situ measurements are performed.