In powder mixers with internal moving parts, as ploughshare mixer, the rapid rotation of the blades involves the movement of the powder, which is aerated as in a fluidised bed. A capacitance sensor is used to measure the local concentration of a catalyst powder, in a laboratory pilot plant of Lodige type mixer. The variation of local solid hold-up is measured by two couples of electrodes glued at the external surface of the trough, at two levels: in the upper third and lower third. This local solid hold-up is a function of the angular position of the blade. The hold-up in the upper part of the trough increases significantly with the loading of the mixer and with the rotation speed. The electrodes are also introduced into the trough and located here and there of a blade, in order to follow the mass of powder that is lifted by the blade.