GB845917A - Improvements in or relating to particle sizing apparatus - Google Patents

Abstract

845,917. Photo-electric particle counters. RANK CINTEL Ltd. Jan. 23, 1958 [Jan. 23, 1957], No. 25l0/5 Class 40(3). [Also in Group XL(c)] In particle counting apparatus of the kind described in Specification 835,901, in which quantized signals developed when a scanning- element being traversed in successive adjacent lines over a field containing particles encounters a particle or is suppressed during the flyback interval, are tested as to whether their duration exceeds a predetermined time, thus classifying the particles scanned according to size, and compared so that signals due to a single particle may be associated and a single count only recorded in respect of each particle, the said quantized signals are fed to a spacestretching circuit in the output of which the spaces between pulses have a predetermined minimum value. The object is to prevent miscounting due to the fact that the frequency response of the quantizer is superior to that of subsequent apparatus, so that when a particle is close to another particle or to the margin of the scanned area the quantized "space" signals are too short to evoke adequate equipment response. As shown, Fig. 5, a transparent field member 11, upon which are dispersed relatively opaque particles 12, is placed between a CRT1, upon whose screen 4 a luminescent pattern is scanned by the electron beam and focused on to the field 11 and a photo-cell 13. The output from cell 13 is amplified at 14; alternatively a photomultiplier may be used at 13. The amplified signal is fed to a quantizer 15 comprising a bistable trigger circuit which changes from a first stable condition to a second when the input exceeds a first datum value and reverts to its original condition when the input falls below a second, lower, datum value, the quantized signal being passed, via a space-stretcher 16, to a gating device 17 which is opened by "particle" pulses, and to a clipper device 18 which suppresses a predetermined initial portion of each such pulse, such portion being equal in duration to the pulse produced by a particle of the maximum size not to be recorded. The remaining pulse is injected into a circulatory loop comprising injector 19, gate 17, extender 20, and delay circuit 21, each pulse being recirculated as long as the same particle is encountered by the scanning element in each successive line. Signals from either end of delay circuit 21 are applied to a counter 22 which includes anticoincidence means, e.g. of the kind described in Specification 778,288, so that each particle is counted only at the beginning or the end of a succession of pulses occurring at one-line intervals. Space-stretcher circuit. Fig. 3. Pulses from the quantizer are applied via input terminal 11, to the grid of one triode 12 of a bi-stable circuit comprising also triode 13 which has an output terminal 14 in its anode circuit, "particle" pulses making the grid more positive and "space" pulses making it more negative. On the arrival of a "space" pulse, triode 12 cuts off, and the raising of its anode potential causes a positive spike to be developed by a differentiating circuit comprising a capacitor 15 and the resistance between the H.T. negative and the grid of a triode 16 which, with another triode 25, forms a monostable trigger circuit, triode 25 being normally conductive. The positive spike is passed via a crystal diode 17 to the grid of triode 16 to initiate the unstable condition in which triode 25 is cut off, the duration of this condition being predetermined by the values chosen for a coupling capacitor 24, a resistor 26, in the grid circuit of triode 25, and the grid bias of triode 16, supplies from a potentiometer 20 across the H.T. supply. During this unstable condition, the positive potential at the anode of 25 is transmitted, via a cathode follower 28 and a diode 39, to the grid of triode 13 to prevent the latter becoming non- conductive even if a "particle" or fly-back pulse appears at terminal 11. Thus the "space" signal at terminal 14 persists until the end of the predetermined unstable period of circuit 16, 25, or the end of the "space" pulse at terminal 11, whichever is the longer.