Patent GB-1450712-A (GB)
Title: Method and apparatus for particle length measurement
IPC: G01N33/48
Filed: 1973-11-06
Abstract: 1450712 Analyzing particles electrically COULTER ELECTRONICS Ltd 6 Nov 1973 [6 Nov 1972] 51530/73 Heading G1N In the determination of the length of long particles (three times the length of the aperture) by means of a Coulter-type apparatus the length of each particle produced pulse is measured at the same specific percentage of its maximum amplitude. The measurements made in this way are independent of differing particle cross-sections. In one embodiment, Fig. 4 (not shown) the particle pulses generated are passed to a threshold circuit the output of which is fed to a gate circuit to which a reference voltage is also fed whereby a constant amplitude (reference voltage) pulse is produced the duration of which corresponds to the period during which the amplitude of the respective particle pulse exceeds the threshold level set by the threshold circuit. The constant amplitude pulses are then integrated and the resultant ramp signal applied to a pulse height analyzer, the output from which gives the length of the particle. In order to compensate for velocity changes due to different lengths of particles a non-linear function can be built into the analyzer. The setting of the threshold circuit to the required percentage level depends upon the manner by which the maximum amplitude of each pulse is ascertained. If prior to analyzing it is known that all the particles in the sample suspension have the same cross-section, generating pulses having the same maximum amplitude which is of known value then the threshold circuit is merely set to give the required percentage value of this. If the level is other than 50% then the final value of particle length has to be suitably modified. In the case where the particles are known to be of the same cross-section but generating unknown amplitude pulses then the maximum amplitude can be determined using a portion of the sample prior to the analysis or instead of actually making a measurement the output from the threshold circuit, set at a very low level, is fed to a pulse ratemeter. This counts the number of pulses generated in a fixed period of time. The threshold is then adjusted upwards until the counting rate is halved which threshold is then the median maximum amplitude of the pulses and the required percentage level can be then set. If the cross-sections of the particles are not known or are known to differ, the circuit of Fig. 5 may be used. Each particle pulse 71, Fig. 6 is stretched 77 by diode capacitor pulse stretcher 73 and then applied via an attenuator 79, set to attenuate to a set percentage level 81, to one input of a comparator 83. At the same time pulse 71 is delayed by circuit 75 and the delayed pulse 85 fed to the other input of comparator 83. Comparator 83 supplies to integrator 55 a constant output signal of duration t 4 -t 1  so that a ramp signal 87 is produced which may be fed to a pulse height analyzer as before. If velocity compensation is required circuit 89 is employed. Attenuator 79 in addition to attenuating to level 81 provides a further attenuation level 99 and this attenuated signal is applied to comparator 91 which compares the level with the delayed pulse 85 and generates an output for the duration t 2  to t 3  that pulse 85 exceeds level 99. Vetogate 93 receives outputs from both comparators 83, 91 and generates output signals 101, 103 which are applied to an integrator of velocity computer 95. The output 111 of the integrator has a final amplitude B proportional to the reciprocal of the average velocity of the particle. Ramp signal 87 from integrator 55 is then divided by signal 111 at 97 and the resultant waveform 113 has a final amplitude C proportional to particle length which is absent of velocity variables. Waveform 113 is gated 117 by a timer unit 119 so that signal 115 is obtained which is applied to analyzer 57.