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HVAC and PPE filter testing

Determination of most penetrating particle size of filter media samples

Standards such as ISO 29463 define how the most penetrating particle size (MPPS) for filters may be measured, using size classified particles to measure the penetration at each particular size.

The Differential Mobility Analyser

Current standards including ISO 29463 specify the use of the differential mobility analyser (DMA) to produce a monodisperse aerosol according to electrical mobility.

Although specified in standards, the DMA has downsides for this application:

The aerosol output from the DMA is monodisperse in electrical mobility, but not necessarily in electrical mobility diameter. This is a consequence of multiple charging in the DMA, which occurs at larger particle sizes and could lead to errors or uncertainty in measuring the true size dependent filtration efficiency.

To minimise multiply charged particles in the DMA output, the input size distribution should be chosen so that particles are classified from the right hand side of the distribution, meaning larger, multiply charged particles with the same mobility are in the far tail of the distribution where the concentration is low. In practice this could mean an array of aerosol generators are required with the DMA to cover the required range in classified particle sizes for testing.

The transmission efficiency of the DMA is inherently dependent on the particle charging (which only ever approaches a fraction of the population), resulting in reduced classified concentrations available for testing.

The Aerodynamic Aerosol Classifier

The Aerodynamic Aerosol Classifier is an instrument which produces a monodisperse aerosol according to aerodynamic diameter.

Available commercially since 2016, the AAC has several features which make it attractive for filter testing:

The AAC's output is monodisperse in aerodynamic diameter, the ideal metric for many filter studies. Aerodynamic diameter is important for particle interception and inertia driven processes, including filtration of larger particles.

Unlike the DMA whose monodisperse mobility output may contain particles of multiple diameters due to charging artifacts, the AAC does not require charging and thus the output is free from charging artifacts.

The AAC has higher transmission efficiency than the DMA across the size range.

The AAC offers a wide size range, 25nanometers - 5microns.

To learn more about the application of the AAC to filter testing, download the application note.

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