Abstract
The filtration efficiencies of 5, 8 and 12 μm pore size Nuclepore filters have been measured by detecting the penetration of monodisperse particles with an optical counter. Within a filter lot, the standard deviation of efficiency near the 50 % cutpoint is less than 1 %, whereas the variation is much larger between lots. For liquid particles the filtration efficiency increases slowly with face velocity showing that the principal filtration mechanism is interception. At low face velocity the data agree with a theoretical calculation of interception in the creeping flow approximation, and at higher face velocity the data agree with a numerical calculation by Parker and Buzzard including particle inertial effects. The filtration efficiencies for solid particles decrease with increasing face velocity, indicating the occurrence of particle bounce. Coating the filters with grease reduces but does not completely eliminate particle bounce. Both the filter efficiency and pressure drop change significantly for loadings of only a few particles per pore. For ambient sampling, optimum results were obtained with an 8 μm grease-coated filter at 1.8 cm s −1 face velocity.
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