Faraday pail with self regulating ion repulsion

Measurements of charge-to-mass ratios of paint powders applied electrostatically with corona guns are often subject to error because ions are collected at the same time as the charged powder. The authors present the results of tests with a Faraday pail with self-regulating ion repulsion, using a Teflon shield with holes mounted between the corona painting gun and the pail. The shield acquires a high surface charge from the ionic bombardment. When the charge reaches a saturation level, it repels the ionic bombardment almost completely and only powder is entrained in the Faraday pail in the flow. The calculated charge-to-mass ratios of silica dust powder with and without the Teflon shield are shown. Without the Teflon shield and without taking into consideration the free ionic current which reaches the Faraday pail, the charge-to-mass ratios are shown to be one order of magnitude higher than the true charge-to-mass ratio values.<>