Measuring the flow of a packed bed of particles

The flow of a packed bed of particles moving in a primary stream in a standpipe (10) is measured by feeding a secondary stream of particles through a feedpipe (16) having an open end (18) facing downstream in the primary stream to fill a void that would otherwise be formed downstream of the feedpipe due to the passage of the primary stream of particles. The flow of the primary stream is proportional to the flow of the secondary stream. The flow of the secondary stream can be measured either by measuring the amount of particles being metered (24) into the feedpipe (16) or by connecting a hopper to the feedpipe and measuring the rate at which a level of particles in the hopper falls.