Calculation of the theoretical response of an optical particle counter and its practical usefulness

Abstract

The response of an optical particle counter (OPC) was modelled from the Mie theory of light scattering and the built-in parameters of the counter. The model allowed the calculation of the output signal for any spherical particle, of a given complex refractive index, on a microcomputer. It was applied to a Royco 225 instrument. The simulation required a calibration with a reference aerosol. This calibration was carried out with monodisperse spherical particles of oleic acid obtained by a vibrating orifice aerosol generator (VOAG). The experimental values of the counter response obtained with monodisperse aerosols of oleic acid and methylene blue were then compared with the results obtained from the model. It was shown that the choice of the material used for generating monodisperse particles in order to calibrate the counter was not important. Particle size distributions were measured by the OPC for two polydisperse aerosols (coal and aluminium oxide particles), by using the response curve predicted from the model and the refractive index of the material, and then compared with those measured using a Coulter counter, after aerosol sampling onto a Nuclepore membrane filter. The comparison shows that the combination of optical measurements with the calculation of the response curve of the OPC yields accurate results when particle shape is close to the sphere, and for nonspherical particles when light absorption is high.