Continuous Monitoring of Bulk Density and Particle Size in Flowable Powders and Grains

Both bulk density and particle size are powerful indicators of product quality. Bulk density is an elusive parameter, but can be measured directly, and with considerable precision, with an instrument consisting principally of a vertically mounted vessel attached to a load cell. The test material flows in at the top and leaves through an orifice which chokes the flow and keeps the sensing vessel full at all times; bulk density is obtained from the (known) active volume of the sensing vessel and the weight of material in it as measured by the load cell. As well as defining and maintaining a constant volume of product for continuous density measurement, the sensing vessel effectively maintains uniform flow conditions within the flowing powder mass, and so is an ideal location for taking measurements of powder properties. Accordingly, our apparatus was designed to include a capacitance probe, responsive to changes in moisture content and sensors for mean size determination from sound velocity measurements (the velocity of an audio-frequency acoustic wave traversing a particulate medium correlates well with mean particle size). In this paper we present results on simultaneous monitoring of the bulk density and particle size of an industrial material, and demonstrate the use of density compensation to improve the precision of moisture measurements made with a simple capacitance probe.