In this paper, theoretical and experimental results are presented which indicate that the influence of material surcharge level can be significant for relatively low permeability materials. Three regions of particle permeability are proposed to generalize the effect of surcharge level. For very fine powders an unsteady condition of particle flow (periodic flow) was observed and it was noted that the lowest flowrate occurring in the unsteady flow can be regarded as the maximum attainable steady flowrate when a feeder is used to diminish the flowrate fluctuation. It it found that the extent of this flowrate oscillation is material level dependent. © 1993.