10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
A Novel Technique for Testing Filter Media Using Monodisperse Aerosol as a Function of Aerodynamic Diameter
Simon Payne, Martin Irwin, Tyler J. Johnson, JONATHAN SYMONDS, Cambustion
Abstract Number: 204 Working Group: Control and Mitigation
Abstract The standard ISO29463:2011 method of testing filter media efficiency as a function of particle size uses a Differential Mobility Analyzer (DMA) to select particles by electrical mobility. Depending on the size range of the source aerosol, it is possible for larger particles than selected to pass through the DMA, due to multiple-charging induced by the particle charging process used prior to classification. This can affect filter efficiency measurements if not taken into account. In addition, most DMAs are size limited to less than 1 micron.
The Aerodynamic Aerosol Classifier (AAC, Tavakoli and Olfert, 2013) is an instrument which classifies particles by aerodynamic diameter instead of electrical mobility diameter. Because it does not require particles to be electrically charged, this mitigates the DMA multiple-charging artefacts previously described. Furthermore, the currently available version of the AAC can classify particles from 25 nm to over 6 μm. This paper explores substituting an AAC for the DMA usually used in ISO29463:2011 filter efficiency tests to mitigate the issues described above.
Samples of filter media from two different types of half-mask respirators were mounted in an in-line filter holder. In turn, both sodium chloride and dioctyl sebacate aerosol sources from a nebuliser were classified using an AAC, to provide a monodisperse aerodynamic size selected aerosol. The penetration efficiency of the filter media in response to the monodisperse aerosol was evaluated by comparing particle number concentrations measurements upstream and downstream of the filter media using a condensation particle counter (CPC). This process was repeated over an aerodynamic size range from 50 nm to 3 μm (the upper limit was limited by the maximum detectable particle size of the CPC). As a comparison, the AAC was replaced with a DMA, to conform with the method prescribed by ISO29463:2011 (although only up to 750 nm due to the size limitation of the DMA).
On equivalent diameter size axes, the penetration efficiency curves between the AAC and DMA methods are similar, except for classification sizes smaller than the peak size of the nebulised source aerosol. This difference is attributed to a significant number of the particles selected by the DMA having 2 or more charges, and thus being much larger than intended. Due to the fact that a large proportion of particles remain uncharged in a neutraliser, the transmission efficiency of the AAC was found to be 1.5 to 5.5 times higher than the neutraliser-DMA system. This high transmission efficiency could be useful for more exotic test aerosols available only in limited concentrations.
References:
ISO 29463:2011 High-efficiency filters and filter media for removing particles in air.
Tavakoli, F., & Olfert, J. S. (2013). An Instrument for the Classification of Aerosols by Particle Relaxation Time: Theoretical Models of the Aerodynamic Aerosol Classifier. Aerosol Science and Technology, 916-926.