CAPACITANCE MEASUREMENT APPLIED TO A PNEUMATIC CONVEYOR WITH VERY LOW SOLIDS LOADING

Mass flow measurement in pneumatic conveyors with very low solids loading is difficult, even when using capacitance techniques, due to the baseline drift and limiting sensitivity of the transducer. The authors describe a measurement system specifically designed for low solids loadings. The relationship between number of particles in the sensing volume of the capacitance electrode and the solids loading in the conveyor is derived and shown to be linear and independent of the solids velocity. When the number of particles in the sensing volume is low, a novel capacitance transducer measuring the average value of capacitance fluctuations can be used to determine solids loading. This measurement can be combined with a solids velocity measurement to give mass flow. Good agreement is obtained between calculated and measured results using 4 mm PVC cubes for solid concentrations from 0.05 to 0.30 V/V and velocities from 5.2 to 18.9 m s -1.