An absolute charge sensor for fluid electrification measurement

An instrument has been developed that provides an absolute measurement, independent of such properties of the fluid as electrical conductivity and trace materials that contribute to zeta potentials. Through a shielded sampling probe, it periodically transfers a small volume of fluid to an electrically isolated Faraday cage. Fluid is pumped in and out by making a part of the Faraday cage a metal bellows driven by a digitally controlled linear actuation. If the fluid is to be returned to the system at a downstream location or to a relaxation region, a pair of check valves are used so that the existing fluid is not in the proximity of the sampling probe. The charge density is calculated from the known filling flow rate and either the current or voltage, measured by low- or high-impedance electrometers, respectively, connected to Faraday cage. The instrument has been designed to measure charge densities as low as 1 mu C/m/sup 3/ using a flow rate of 0.18 cm/sup 3//s with a programmable sampling period on the order of 1 min. A Couette charging apparatus, which simulates the electrification charging properties found in high voltage power transformers, has been used to test this instrument.<>