The Cascade Impactor : An Instrument for Sampling Coarse Aerosols

Journal/Book: Reprinted from the Journal of Scientific Instruments Vol. 22 No. 10 October 1945. 1945;

Abstract: K. R. MAY Ministry of Supply [MS. first received 29th May 1945 and in final form 10th July 1945] ABSTRACT. A new instrument is described which will sample windborne and stationary aerosols such as natural fogs and clouds fine sprays. insecticidal mists coarse dusts pollen and spores etc. By means of four progressively finer jets impinging on glass slides in series the sample is split up into size-graded fractions in a forte suitable for microscopic analysis. The greatest efficiency of sampling is achieved for particles in the range 50 µ-105 µ diameter. The size-grading greatly facilitates the detailed microscopic examination of heterogeneous samples and in some cases enables approximate size-distributions to be obtained by bulk estimations of the samples without the need for microscopic sizing. Experimental results for the efficiency of jets in impacting particles are correlated by dimensional analysis. and a parameter of general applicability for estimating impaction efficiencies of jets is derived. Descriptions are given of new methods of dealing with volatile droplets and of analysing the samples. ___MH

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