Patent GB-968491-A (GB)
IPC: H03K21/02
Abstract: 968,491. Counting particles. W.H. COULTER. Aug. 4, 1960 [Aug. 19, 1959], No. 27115/60. Heading G1N. [Also in Divisions A5, G3, G4, H3 and H4] In apparatus for counting those particles of a liquid suspension which lie within a predetermined and adjustable size range, in which a measured quantity of the suspension is drawn through a submerged orifice in a container and the resistance change arising from the passage of each particle through the orifice produces a pulse in a circuit connected to two electrodes located one on each side of the orifice, the circuit includes: a constant current D.C. generator supplying the orifice current; a low input impedance amplifier, in series with a capacitor, connected across the electrodes; and pulse analysing circuits for counting, and displaying on an oscilloscope screen, only pulses of an amplitude lying between predetermined lower end upper threshold values. The maintenance of a constant orifice current, and the use of a common voltage source for the constant current generator and the threshold determining circuits, obviates the need, otherwise necessary in order to maintain proportionality between pulse amplitude and particle size, for recalibration whenever the standing orifice current changes. The test cell orifice 26, Fig. 1, is formed by an annular sapphire wafer fused over an orifice in the side of an insulating tube. Mercury is pushed up into reservoir 40 when valve 30 is opened, and drops back when the valve is shut, drawing the suspension 24 through orifice 26, and energizing and de-energizing the circuitry as it contacts electrodes 54 and 52 respectively, such that a known volume of suspension is examined. The apparatus is applicable to the examination of blood, bacteria, ink, and mud, and to particles of metal, alumina, and petroleum catalyst when suspended in a suitable liquid; it is stated that the suspending fluid may be different on the two sides of the orifice. Polarisation effects are reduced by reversing the current applied to the electrodes between successive measurement runs. Circuitry. The pulse-e.g. 154 in A of Fig. 3- produced by the passage of each particle through the orifice passes via blocking capacitor C2, Fig. 7, to the low input impedance pre-amplifier 86, Figs. 1 and 7, and main amplifier 88 (see Divisions H3 and H4) to an oscilloscope 116 and to expander amplifiers 108 and 110, small segments of the output pulses of which are selected, at the chosen lower and upper threshold levels, by circuits 128 and 130 comprising pairs of semi-conductor diodes, to yield pulses D and E of unequal duration. The latter pulses are employed respectively to brighten and dim the oscilloscope display, whereby display of the original pulse is largely suppressed if it is of an amplitude such that it rises above the upper threshold 158. For counting by counter 67, both pulses D and E are subjected to further amplification and shaping, F, G to J. K, Fig. 3 and a pulse L derived from the trailing edge of lower threshold pulse J is transmitted to a monostable circuit 66 to produce a square pulse M for transmission through gate 176 to counter 67. If the original pulse 154 rises above the upper threshold 158, thereby giving rise to a pulse E, a pulse N derived from the leading edge of pulse E is transmitted to the bi-stable circuit 170 to switch the latter to a state in which its output signal P closes the gate 176 to prevent the subsequent transmission therethrough of the associated pulse M to the counter. The pulse O, derived from the trailing edge of pulse M is transmitted via line 194 to circuit 170 and, if the latter is in the state corresponding to closure of gate 176, switches it back to terminate signal P and thereby open the gate ready for the next particle pulse 154. A pulse derived from the trailing edge of signal P may be passed via line 196 to monostable circuit 66, to disable the latter for a set period subsequent to the termination of each signal P, to prevent the improper counting of the second pulse of a pair of rapidly succeeding pulses both of which have an amplitude above the upper threshold level (see Division G4). Specifications 722,418, 968,492, 968,493 and 968,494 are referred to.