Patent GB-1522665-A (GB)
Title: Methods of and apparatus for particle examination
IPC: G01N15/10
Filed: 1975-08-05
Abstract: 1522665 Particle analyzing systems COULTER ELECTRONICS Ltd 5 Aug 1975 [6 Aug 1974] 32679/75 Heading GIN In a particle analyzing system for determining particle sizes at predetermined percentiles of the size distribution of a particulate system a correction is made for particles which are too small to be detected. If the mass percentile sizes of a normal or log-normal system are plotted on a probability or log probability graph any two numbers on each side of the 50% percentile size and equally numerically spaced therefrom (e.g. 25% and 75% percentiles) will also be equally physically spaced from the 50% point and a straight line can be drawn through the three points. However if the system is not normal or log normal or if the system contains 10% of particles which are too small to be detected or undistinguishable from noise signals i.e. one of the specific relationships of linearity, symmetry and equal spacing of the particle size signals does not exist with respect to the particle size signals then the probability graph is not a straight line but a curve. Thus if it is known that a system is normal or log-normal any variation in the probability curve can be used to correct the readings to account for undetected particles. Particle pulses from a detector are applied to three percentile discriminating circuits 22, 24, 26 which may be of the type disclosed in U.K. Patent No. 1249470 and provide output voltages e 75 , e 50 , e 25  proportional to the 75th, 50th (mass median) and 25th size percentiles and which are fed to cube root circuits 23, 25, 27 which provide voltages e' 75 , e' 50 , e' 25  proportional to the equivalent spherical diameter of the corresponding particle sizes. If the system is a log normal system log calculators 28, 30, 32 are included otherwise the voltages are applied directly to resistors 36, 40 and input 48 of differential amplifier 46 as shown. The algebraic sum of the 25th and 75th percentile voltages are developed at junction 38 and fed to input 44 of amplifier 46. If the measurements are correct then the output of the amplifier should be zero or a reference level. However if the measurements are incorrect i.e. the probability curve is not a straight line an error signal is obtained from amplifier 46 which is fed back via integrator 54 to the inputs 56, 58, 60 of the percentile circuits where they are combined with the particle size voltages to modify these voltages until the output is zero. At this time the voltage representative of the particle sizes for the 25th, 50th and 75th percentiles will have been corrected for inaccuracies due to non-detection of small particles in the system. Each of the channels may be of the type shown in Fig. 6 of U.K. Patent No. 1249470 with the percentile circuits being part of a summing network and voltage to charge converter. The circuits are modified by including an extra input for the error voltage, Fig. 5 (not shown). The 75th and 50th percentile voltages could be combined and then reduced by a known percentage and the point compared within the measured 25th percentile to obtain the error signal.