GB708347A - Improvements in or relating to machines for counting and sizing spray droplets, stains, craters and the like - Google Patents

Abstract

708,347. Photo-electric particle counter. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 21, 1952 [May 31, 1951], No. 12851/51. Class 40 (3) Small particles or stains on a two dimensional record are counted and sized by means of a scanning beam producing a succession of signals of duration corresponding to the lengths of chords of the particle, a number of counters being arranged to count the number of signals the scanning period of which is above predetermined time duration. Thus by subtraction the number of chords in each group may be determined and the Specification shows how, from this, the number of particles in each size group may be inferred. As shown, the specimen is scanned by a slow speed scanner A which may be a picture telegraph transmitter of the type in which the record to be analysed is fastened on to a rotary drum, a mirror drum or prism being used to cause the scanning spot to move with constant velocity along the axis of rotation of the drum. When the specimen must be scanned as a plane surface two mirror drums or prisms rotating on axes at right angles may be used. The ouput of the scanner which consists of a high frequency carrier modulated by the scanner signal is demodulated at B and the resultant pulses applied to a squaring circuit C and are passed through a protective gate D to be described later to a linear voltage sweep generator E. This circuit is initiated by the start of pulses to produce a voltage which increases linearly with time until the end of the pulse resets the circuit. The output of the circuit is applied to a number of voltage level discriminators F ... O of the type in which a biased diode forms part of the feed-back circuit of a blocking oscillator. When the output of the circuit E reaches the bias level of any of the diodes the blocking oscillator generates a pulse but by the time the oscillator has recovered the output of the circuit E is such as to prevent a repetition of the action. These pulses are counted on one of counters P-Y which may be the cold cathode type in which the pulses are stored as the position of a glow within the tube. Gate circuit D is inserted between squaring circuit C and linear voltage generator E and is made non- conducting by a pulse from discriminator O which accepts the largest pulse. Thus pulses too large for the counters are automatically shortened and included in the largest group. A rate voltage proportional to the speed of the scanning motor may control the slope of the output waveform of voltage generator E to correct for speed variations. Contacts on the drum shaft may black out the circuit when spring clips holding down the specimen are traversed by the scanning beam. Variations in the " white level " of the pulses are compensated by applying a bias derived from a circuit of long time constant compared with the pulse duration to the squaring stage.