This paragraph is devoted to the comparison of the pressure drop curves with various concentrations, specific masses and sizes for identical spherical beads. The reference case is the Alumina beads of diameter 6 mm, of specific mass s = 3.65, and at a deliv-ered concentration C = 5%. In this case, the order of magnitude of the minimal pressure drop at critical velocity Vcrit ,h 2.4 ms-1 is Icrit ,h 0.11 m/m