A filterless nozzle-to-plate Faraday cup electrometer: design, computed and measured collection efficiency

Electrical aerosol instruments measure aerosol charge by depositing it on a collector inside a Faraday cup, and in almost every published design that collector is a filter, a consumable whose loading raises the flow resistance. Here the filter is replaced by a high-voltage-biased 1 mm nozzle facing an electrically isolated 20 mm impaction plate inside a grounded cup, so that charge is captured on a rigid, cleanable electrode with no consumable. Because the collection efficiency of such a collector is neither unity nor known a priori, it was computed with a coupled model of the impinging jet, the electrode field and the particle trajectories, and measured with a mobility-classified, unipolar-charged aerosol and a charge balance over the plate, the cup and a reference filter. Over 50–150 nm at a nominal 1 L min −1 the two agree to 2.9 percentage points on average across twelve conditions spanning 49–98 % efficiency. Inertia captures nothing in this size range, so the bias is the enabling mechanism: the computed efficiency at 100 nm rises from below 1 % unbiased to 98 % at 4 kV and 1 L min −1 , and falls to 36 % at 5 L min −1 . The computed efficiencies collapse onto a Deutsch-form relation with an exponent of 1.43, allowing this collector to be sized without a trajectory calculation. The insulation requirement and the discharge margin at the plate are derived, the margin verified experimentally. Efficiency at higher flow rates and an absolute calibration of the current scale remain to be measured.