Electrical charge characteristics of dry aerosols produced by a number of laboratory mechanical dispensers

Apparatus comprising a split-flow electrostatic elutriator and an optical particle counter has been used to measure the charge distributions of aerosols of coal, quartz, and mica, each dispensed by four commonly used laboratory dust generators. The optical particle counter has five selectable particle size ranges allowing the relationship between particle size and charge distribution to be determined in each case. It has been found that the levels of charging produced by Wright, Timbrell, and turntable-Venturi dispensers do not differ markedly for any of the materials and that the numbers of negative and positively charged particles are approximately equal in all cases. Particles generated by the fluidized bed dispenser however, were found to carry a generally higher electrical charge and, in the case of quartz and mica, to be predominantly negatively charged. Comparison of these results with similar measurements carried out earlier in a wide range of workplaces shows that the former three dispenser types generate aerosols with charge characteristics similar to those normally found in the factory whereas the latter does not. Comparison is also made with theoretical predictions of the minimum magnitude of particle charge required for enhancement of lung deposition due to electrical image forces. It is found that the Wright, Timbrell, and turntable-Venturi dispensers produce aerosols wherein the predominant number of particles carry less than this minimum charge referred to, whereas a large proportion of particles generated by the fluidized bed are charged to levels well above this threshold.