Patent GB-1316752-A (GB)
Title: Apparatus for analysing suspended particles
IPC: G01N15/02
Filed: 1970-02-12
Abstract: 1316752 Photo-electric analysis of suspended particles UNITED KINGDOM ATOMIC ENERGY AUTHORITY 19 April 1971 [12 Feb 1970] 6926/70 Heading G1A Particles suspended in a fluid are detected photo-electrically by scattering of radiation and elongated particles e.g. fibres, are disguished from symmetrical particles by monitoring the scattered radiation when an electrostatic field is applied. The invention is particularly applicable in the monitoring of airborne asbestos fibres in a factory environment. Apparatus. Air to be sampled is drawn through inner duct 11 to test zone 12 and extracted via duct 21 by a pump. A sheath of clean air at 16 surrounding the sample 17 is provided via duct 14 and flow straighteners 15. Zone 12 is illuminated by lamp 18 via lens 19, an optical element 22 being provided on opposite side of the enclosure 23 to absorb disturbing reflections. Scattered light from zone 12 is directed by lens 25 to a photo-multiplier 24 viewing the zone against the dark background of the other end of the enclosure. The required electrostatic field is provided via electrodes 26-29 or by induction or coupling from a transmission line or waveguide. Identification of fibres. These are distinguished from symmetrical particles by the provision, via the electrodes of a rotating (or oscillating) electrostatic field. Since only the elongated particles follow the field, the modulation in the output of the photo-multiplier is representative of the number of fibres in the air stream. Determination of fibre dimensions. Since the maximum length of a particle which will follow the rotating (or oscillating) field is related to the frequency of rotation (oscillation), a sweep of this frequency enables classification of fibre as to their lengths. In order to avoid inaccuracies in the measurements resulting from the prescence of electrostatic charges on some fibres, the rotating steady field is replaced by a rotating high frequency field of frequency about e. g. 700 KHz. The majority of fibres tend to oscillate about the applied field direction, at a frequency inversely proportional to the fibre length, before aligning with the field direction, resulting in a further detectable modulation in the photo-multiplier output. This is utilized in fibre length determination by square wave modulating the H.F. field so as to effect oscillation or rotation in sharp steps with a dwell at each step position. The time for alignment to occur (in the case of "lead beat" fibres) gives information as to the aspect ratio of such fibres. Limitations. In order to prevent double counting of fibres, the modulation cycle is kept greater than the transit time of fibres in the zone 12. Flow velocity is monitored by detecting, using the main, or a subsidiary, photo-multiplier the transit of fibres across a coded grey filter band. Boundary effects producing inaccuracies are compensated for by ramp function variation of illumination or detection effeciency for the entry and exit region of zone 12. The fibre length must be greater than the wavelength of light used, but the diameter need not be. Modifications. Polarized illumination may be used and fibre, instead of conventional optics. Fibres may also be distinguished from platelets by excluding light scattered at angles close to the plane of rotation of the electric field.