Background: Electrical mobility techniques such as the Differential Mobility Analyser (DMA) have been used for many years by aerosol scientists to classify particle size, and their operation for filtration measurements is covered in ISO 29463 (high efficiency air filters) for determining the most penetrating particle size (MPPS) of a filter sample. Since 2016, classification by a particle’s aerodynamic diameter is possible using the Aerodynamic Aerosol Classifier (AAC), which uses rotating concentric cylinders with a controlled sheath flow to induce known centrifugal and drag forces on each particle sampled. Crucially, the AAC holds the advantage of operating independently of particle charge. This means there is no need to condition the aerosol charge state for classification as with the DMA, which is designed to transmit only singly charged particles (generally a fraction of the particle population) yet larger, multiply charged particles with the same electrical mobility may also pass. The AAC consequently offers significantly higher transmission efficiency, and its output is truly monodisperse. These advantages may be important for precise size-resolved measurements for filtration efficiency required in ISO 29463 and future standards.

Aim: Evidence of limited discrepancies in particle penetration measurements between the DMA and AAC was previously presented at FILTECH (Payne et al., “A New Methodology for Measuring Filtration Efficiency as a Function of Aerodynamic Diameter Using a Monodisperse Aerosol Source”, FILTECH 2018). The present research explores scenarios in which these discrepancies significantly worsen due to multiple charging artifacts, depending on the position of the DMA setpoint relative to both the peak of the input aerosol size distribution and the MPPS of the filter. This can result in incorrect reporting of the MPPS and local and integral filtration efficiencies.

A neutraliser device (usually containing a radioactive source or X-ray emitter) is mandatory in many filter testing standards to ensure the media are challenged with particles possessing an equilibrium charge distribution; examples were considered in this study where the input aerosol contains few charges, and the neutraliser was omitted from the filter testing set-up. This may be of significant benefit considering resources available at testing locations, and health and safety regulations.

Method: An AAC and a DMA were used back-to-back to measure particle penetration through flat sheet media samples of high efficiency air filters certified according to EN 1822 and ISO 29463. In tests with the AAC, results were compared with and without a Kr-85 neutraliser device between the classifier and filter sample, while with the DMA, results were compared with and without an impactor at the classifier inlet. The particle charge state of the nebulised Di-Ethyl-Hexyl-Sebacate (DEHS) test aerosol was also characterised.

Results: It was found that multiple charging artifacts in the DMA output can have a substantial effect on ...