An inlet sampler was developed which featured a hemispherical screen in front of the filter to reduce the effects of wind velocity and direction as well as exclude particles greater than 30 micrometers (microm). A metal sheet with a 19% porosity and orifice diameter of 254microm was used to construct a spherical inlet with a subtended angle of 140 degrees. The average air velocity through the screen was 25 centimeters per second for a 2 liter per minute flow rate through a 25 millimeter (mm) diameter filter. Flow pattern visualization showed negligible turbulence effects of wind tunnel smoke streamlines due to the inlet geometry. Overall sampling efficiency of the new sampler was compared to a commercially available closed face 25mm personal sampling filter cassette. Uranine monodisperse particles of 13.5, 20, and 30microm were generated by a vibrating orifice aerosol generator. Both isoaxial and 90 degree sampling were conducted at wind velocities of 100 to 300 centimeters/second. No significant change in sampling efficiency as a function of wind velocity was seen. The isoaxial average overall efficiency of the new sampler was 52%, 47%, and 34%, for particle diameters of 17, 26, and 38microm, respectively. The corresponding efficiencies for 90 degree sampling were 31%, 20%, and 7%, respectively. The filter cassette showed up to two to three fold differences as a function of wind velocity. Microscopic analysis of filter deposit demonstrated standard deviations of 19.2, 19.7 and 33.9 for isoaxial, 45 degree, and 90 degree sampling, respectively. The corresponding standard deviations for the filter cassette were 44.6, 28.2, and 33.6. The authors conclude that the new sampler is less affected by wind velocity and direction than currently used designs.
Klaus Willeke, Aerosol Research Laboratory, Department of Environment Health, University of Cincinnati, Cincinnati, OH 45267-0056, U.S.A.