Patent GB-966922-A (GB)
Title: Improvements in or relating to a method of determining the relative proportional relationship of the size of particles present in a particulate material, and apparatus therefor
Inventor: WELLER BARTON LEE
IPC: G01N15/02
Filed: 1962-01-11
Abstract: 966,922. Photo-electric particle size measurement. VITRAMON Inc. Jan. 11, 1962, No. 1147/62. Headings G1A and G1S. The invention relates to a method of measuring the size of particles of a material, comprising the steps of forming from a Newtonian fluid suspension of the material a sample of longitudinally varying thickness having particles of different sizes segregated in proportion to the thickness of the sample so that only particles not greater than the thickness of the sample at a point along the sample are present at that point and relating the thickness at the said point to the size of the particles. In the embodiment shown in Figs 6 and 7 to produce a sample of longitudinally varying thickness, a Newtonian fluid (i.e. a fluid in which stress is directly proportional to rate of shear) containing the material is placed in a slot 21 in a metal block 20, the bottom surface of the slot being at a predetermined angle to the top surface of the block so that the depth of the slot varies along its length from the open end 22 of the slot to a zone 23 where it merges with the top surface of the block. A "doctor" knife or blade 26A is then drawn along the top surface of the block 20 from the open end 22 of the slot to the other end thus producing a sample as described above. Thus the discrete particles of the material in the Newtonian fluid are moved in an aperture formed by the knife or blade 26A and the slot 21 so that particles of greater size than the depth of the slot at the position of the blade 26A will be drawn along and eventually out of the slot thus exposing the bottom of the slot at all points where the slot is shallower than the diameter of the particles. Due to the nature of the fluid, at any particular point along the slot the amount of material in the particular slot area will be dependent on the proportion of the particles in the material smaller than the depth of the slot, and at the point where removal of particles first takes place the bottom surface of the slot will appear and the extent thereof will increase until the bottom of the slot is completely exposed and free of particles at which point the depth of the slot is less than the smallest particle present. To determine the area in which material is scraped from the slot, the prepared sample is placed on a conveyer system (Figs 1, 2 and 3 not shown) and moved below a light source 25 and two photo-cells 29, 30 so that it is scanned thereby, along its longitudinal axis from the open end 22 to the other end. Light from the source 25 is directed perpendicularly down on to the sample 24 by a mirror 27 and when depth of the slot at the point 31 being scanned is greater than the largest particles present in the sample light will be diffused by the sample 24 to reach both photo-cells equally. The bottom of the slot 21 has a finish with minute grooves as shown exaggerated in Fig. 5, so that when the bottom of the slot is exposed to the perpendicular beam 28 from the source 25 it will only reflect light in a plane perpendicular to the longitudinal axis of the slot and on to the photocell 30 and no direct light will be received by the photo-cell 29 which is placed along the said longitudinal axis as shown. The signals from the photo-cells are fed to an amplifier (Fig. 9 not shown) where they are combined to form a composite signal which is fed to a recording voltmeter through a valve voltmeter combination. By closing a switch in the valve voltmeter combination the combined signal may be differentiated and shown as such on the chart of the recording voltmeter. An example of a prepared sample in the slot is shown in Fig. 12 and the resulting chart when the sample is scanned is shown in Fig. 11 in which in the area of the slot from B to D no particles are larger than the depth of the slot and both photo-cells receive diffused light from the sample and at D the largest particles present have been scraped away leaving part of the bottom of the slot exposed and thus causing an increase in the light received by the photocell 30. At F the material has been completely scraped from the bottom of the slot showing that 100% of the particles are larger than the depth at this point, and point E represents the mean particle size of the largest proportion of particles in the sample. By differentiating the segments a chart may be produced (Fig. 10 not shown) which shows the proportion of particles of each particle size.