The use of electrets in dust sampling and particle charge analysis.

Investigations have been carried out of the capture of charged dust particles by electrets, with a view to implementing the process in a passive dust sampler that is simple and light-weight and therefore, acceptable to the wearer. The electret collects dust particles at a rate that is proportional to their electrical mobility but independent of the velocity of the ambient air outside its surface provided that a critical velocity is exceeded. In addition a miniature device has been developed which measures the mean electrical mobility of an aerosol using an electric field to precipitate the aerosol onto parallel plates between which it is drawn. A large dynamic range is obtained if the charge on the electret is made spatially variable so that the aerosol encounters first a weak then a strong electric field. The theory of action of the devices is discussed and the results of laboratory and field tests are presented.