GB1417691A - Methods of measuring sizes of particles - Google Patents

Abstract

1417691 Photo-electric investigation of size distribution of particles CAMBRIDGE CONSULTANTS Ltd and WELWYN HALL RESEARCH ASSOCIATION 21 Nov 1972 [21 Aug 1971] 39298/71 Heading G1A Size and size distribution of particles moving through a liquid are investigated by utilizing the Doppler change in frequency when electromagnetic radiation is scattered by the particle(s). In the arrangement of Fig. 1, a helium-neon laser and beam-splitter, 3, provide two coherent beams 5, 7 to be focused on a volume 11 within a jar. The particles in the contained liquid are investigated when settling under gravity such that Stoke's law is applicable. The light scattered from volume 11 is detected at 13 and analyzed by processing circuit 17 which may be as in Figs. 1 A or 1B (not shown). In the former, used when one particle is being scanned at a time, the output is applied to a bank of ten narrow pass filters 21 coupled to respective counters. The detector 13 produces an output at a frequency f equivalent to the Doppler change in frequency of the laser beam, this being dependent on the particle velocity V, which itself is proportional (under Stoke's law) to D<SP>2</SP>, the diameter of the particle. The counters in circuit 17 as in Fig. 1A (not shown), therefore provide the numbers of particles in a plurality of size ranges. In the second case, Fig.1B (not shown), circuit 17 comprises a time compression spectrum analyzer coupled to a normalizing circuit; this provides a continuous size distribution spectrum to be displayed on a cathode ray tube, pen recorder &c. In the arrangement of Fig. 2 (not shown), a single beam from a laser is focused by a lens on to a small volume within the sample. The emergent light is allowed to pass through two apertures in a mask, and the resultant narrow beams focused on the detector. In a further arrangement, Fig. 3 (not shown), light from the laser is divided into two beams by a beam-splitter, one beam being applied to the sample and the other reflected from a mirror to a further beam-splitter where it is combined with scattered (and Doppler shifted) light from the sample.