Calibration and Use of the Aerodynamic Particle Sizer (APS 3300)

Abstract

The Aerodynamic Particle Sizer (APS) has been in use at the National Institute for Occupational Safety and Health (NIOSH) for over two years, beginning with a prototype model and more recently with a commercial version (Model 3300). The APS has been tested and used in a variety of laboratory and field situations. It has been a very useful instrument for testing aerodynamic sizing devices and provided a much needed means of rapid aerodynamic sizing of particles. Limits to the accuracy of the APS in determining aerodynamic diameter of particles were investigated.

The calibration of the APS was originally carried out by using monodisperse di-octyl phthalate (DOP) oil aerosol in the 3–15 μm range. Using a laser imaging system, the flattening of droplets into oblate spheroids was observed for larger particles (20–100 μm). The APS was recalibrated with solid latex particles and the DOP particles were measured to determine the effect of the droplet flattening. A 15 μm droplet is measured as being 20% smaller by the APS. A semiempirical equation was developed to fit the droplet deformation data. Particle measurement in the APS takes place largely outside the Stokes regime. Therefore, it has been predicted by Wilson and Liu (1980) that the measured diameters will be dependent on density. Monodisperse particles of density 1.15 and 2.15 were generated. In the range of 8–14 μm there was a difference of up to 8% in the measured size for particles of the same aerodynamic diameter. Particle coincidence can modify the measured size distribution in a different way than for other optical particle counters. The APS has circuitry to reduce the effects of coincidence that can also modify the meausred distribution. Calculations were carried out to simulate the effect of coincidence. Several potential problems and improvements for the APS were found.