CALIBRATION AND PERFORMANCE OF AN API AEROSIZER

Two API Aerosizers have been tested with uniform-sized spherical polystyrene latex particles and glass beads ranging in size from 1 to 150 mum aerodynamic diameter as well as with non-spherical natrojarosite particles in the size range from 4 to 18 mum aerodynamic diameter. The effects of reduced ambient pressure and particle shape on the Aerosizer instrument responses have been investigted. The Aerosizer operates by accelerating aerosol particles under non-Stokesian conditions (particle Reynolds number > 1), and measuring their times-of-flight across two laser beams. Such an instrument would be expected to oversize particles under a reduced ambient pressure because the drag force experienced by a particle within the acceleration nozzle is lower than would be experienced by the same particle at ambient pressure. Our results showed that the Aerosizer overestimated the sizes of particles at an ambient pressure of 625 mm Hg (83.3 kPa) by as much as 25%. On the other hand, the Aerosizer underestimated the aerodynamic diameter of natrojarosite particles by as much as 50%. An empirical relationship between the time-of-flight data and a dimensionless Stokes number was established, thus facilitating correction of the size measurement made under different operational conditions.