Effect of impactor inlet efficiency on the measurement of wood dust size distribution

Two designs of cascade impactor inlets were evaluated experimentally to determine their particle sampling differences. One was the standard shrouded inlet provided with the low flow rate (2 L/min) Marple Personal Cascade impactor (Marple). The other was a simple vertical tube used with medium flow rate (15-30 L/min) impactors used as area samplers (e.g., Microorifice Uniform Deposit Impactor, Andersen, Sierra, Berner, or University of Washington [UW] impactor). When two impactors (Marple and UW) were used side-by-side to measure particle size distributions in the wood products industry, they often produced very different size distributions. In the laboratory the particle aspiration efficiencies of both impactor inlets were determined using monodisperse solid particles with aerodynamic diameters (Da) ranging from 5 to 68 microm at wind speeds of 0.55 and 1.1 m/sec. For particles with Da greater than 10 microm, the sample obtained using a simple vertical inlet tube was more representative of the environmental concentration than that obtained using the shielded inlet provided with the Marple impactor. Tests conducted using the Marple impactor inlet without a visor produced aspiration efficiencies that varied with inlet orientation and wind speed. The vertical sampling tube was found to be a better sampler inlet for indoor aerosol sampling.