Abstract: An apparatus for measuring the rate of flow of a volume of particulate material comprising a transparent tube of known cross-sectional area, means for supplying a flow of particulate material through the transparent tube, and an optical signal sensor for receiving an optical signal through the wall of the transparent tube from the flow of particulate material. The received optical signal is delivered to a controller for analyzing and determining the relative movement of the flow of particulate material at the wall of the tube and calculating the rate of volume flow through the tube. These observations may be used to control upstream and downstream system processes and equipment.