Electrostatic particle counting for automated filtration efficiency testing of medical masks

The unprecedented global demand for medical protective equipment has exposed a gap in the quality-control infrastructure available to local manufacturers, whose compliance testing depends on imported optical light-scattering particle counters. This study reports the development, theoretical formulation and validation of an automated filter tester in which the optical detector is replaced by an electrostatic particle counter. The instrument integrates a multi-aerosol generator, a bipolar charge conditioner, a standardized pneumatic filter holder, and a detector comprising a unipolar corona-needle charger, a shielded Faraday cup and an LMC662 electrometer. A general inversion is derived that converts the measured electrometer current into a number concentration for an arbitrary polydisperse challenge, through a distribution-weighted effective elementary charge for which a closed form is obtained for a lognormal aerosol. The same framework yields the bias incurred when size-dependent penetration reshapes the downstream size distribution, and shows that this bias is proportional to penetration and vanishes for a monodisperse challenge. The polydispersity of the sodium chloride challenge is prescribed by NIOSH 42 CFR 84 rather than chosen, and the parallel ASTM F2299-03 path, which uses classified 0.1 µm polystyrene latex, provides an internal control in which the bias is smallest. Aerosol characterization confirmed compliance with NIOSH 42 CFR 84 (CMD = 57.4 nm, MMAD = 0.264 µm, GSD = 1.83), close to the lower bound of the permitted CMD band. Calibration against a reference condensation particle counter gave adjusted R2 between 0.998 and 0.9999 for the response in number concentration at fixed size-distribution shape, with a size-dependent calibration constant consistent with diffusion-charging theory. Filtration efficiencies obtained for surgical masks, N95 respirators and E10, E11 and H13 filter classes agreed with the optical method to within 0.48 to 1.98 percentage points. With a commercial reference filter tester to within 0.03 percentage points at a penetration of 5 × 10−4. The measurement is shown to be metrologically favorable for filtration efficiency specifically, because the dominant systematic terms cancel in the upstream-to-downstream current ratio.