The Fig. 6 presents the comparison of Ih (Vmix) for three mixtures of beads of same size but different specific mass: Glass beads of 5mm (w), Alumina beads of 6 mm ( ) and mixture 3 (50% Glass/50% Alumina, /). The pressure drop curve for the mixture lies be-tween the two single-type cases and seems to be well modelised by the mean of the two curves: the solid red line in Fig. 6 is a fit for the Alumina, the solid blue line a fit for Glass and the black line is the mean of these two curves.