By dial-gauge readings of the deflection of metal foil attached to the bottom of the storage silo, bottom pressures exerted by fine powders of the average particle size smaller than 115 μm were measured for wide ranges of relative humidity and compared with the theoretical values calculated by the Janssen's equation.
Bottom pressures increase with the increasing particle sizes and bulk density of powders. The results are in good agreement with those predicted by the equation. The influence of the relative humidity on the bottom pressures is remarkable, which reach maximum values at 70 % of relative humidity.
As the observed pressures are significantly less than those given by the Janssen's equation, it is considered useful to modify the equation by inserting a dimensionless stability number, c/hρ_B, in order to obtain more accurate estimates of storage pressures.