The measurement of bipolar charge in polydisperse powders using a vertical array of Faraday pail sensors

A vertical array of Faraday pail sensors was developed to partially separate bipolar charged polydisperse powder and measure the charge-to-mass ratio of the powder deposited in each Faraday pail. In our experiments, the system consisted of seven Faraday pails, six special Faraday pails, which were mounted vertically in cascade, and a normal Faraday pail located at the bottom. The inner and outer pails of the special Faraday pails had open holes on the upper and lower covers. The bipolar charged polydisperse powder was sampled from an earthed metallic fluidized bed with an earthed metallic vertical tube at the axis of the bed. As the powder falls, individual particles experience separation forces due to the gravity segregation and space charge repulsion. The particles thus deposit into different inner Faraday pails according to their charge, size and mass. Each powder sample was collected and the charge-to-mass ratio was measured. The size distribution of the particles sampled from each Faraday pail was analyzed to clarify the effect of this factor on contact charging between particles.