Abstract: A monitoring system measures the flow of solids in a pneumatic conveying system having a conduit with an air inlet, a solids inlet, and a mixture outlet. A pressure transducer responds to pressure in the conduit to produce a pressure-representing signal. Prior to the introduction of solids into the conduit and while only air flows through the conduit, the pressure-representing signal is sampled to provide a signal which is stored. After solids are introduced into the conduit, the pressure-representing signal is sampled again, preferably on a repetitious basis, and supplied to apparatus such as a computer which determines the difference between the pressure of air plus solids and the pressure of air, and multiplies this difference by a constant to determine the mass flow of solids.