Automatic Grain-size Analyser for Sedimentological Investigation

The methods for grain-size analysis are discussed and new instruments for the purpose are described. Analysis should be made automatically and within a short time on all of the grain-sizes of the sediments by a sedimentational technique in order to avoid possible technical errors. The result of the analysis should be recorded directly as a continuous frequency curve of the grain-sizes and the fractions of the sediments should be collected for further analysis. The newly designed instruments include a separator, and two automatic grain-size analysers for both coarse and fine grains (Figs. 1 and 2). The instruments include electronic recorders to obtain automatically the continuous frequency curves of the grain-sizes of the sediments during the analysis. To save time for grain-size analysis, the apparatus separates the sediment samples into fractions of grains coarser and finer than 5φ. The apparatus consists of a plastic tube, 35 mm in inner diameter and 37 cm in length with a funnel in the top and glass tubes in the middle part and at the bottom of the cylinder. The grains of the sediments accumulate in the tubes separately according to their settling rates in the cylinder filled with water. The main part of the analyser for grains coarser than 5φconsists of a glass tube of 19 mm in inner diameter and 160 cm in length with a funnel at the top, a collector at the bottom, and the measuring equipment located around the lower part (Fig. 2). The stopper of the funnel is removed automatically by an electromagnet. The measuring equipment consists of a projector charged with stabilized electricity and a CdS-cell receiver which transmits the light value projected through the glass tube to the electronic recorder. The collector comprises seven rotative cylinders which are stopped at the bottom of the the tube in turn by a microswitch (Fig. 3). The glass tube is filled with water pouring from the lower part. The prepared sample of sediments in the funnel is thrown into the tube. The sediment grains settle at different rates according to their shapes and weights. The shading effects of the falling grains are recorded continuously by the CdS-cell receiver. The grains are deposited separately according to their settling rates in the successive cylinders of the collector intermittently rotating at the bottom of the tube. Each part of the analyser works under control of signs recorded on tape. The analyser for the grains finer than 4φis set in the centrifuger. It consists of a test tube with measuring equipments at two levels which are connected with the electronic recorder. The electronic recorder automatically draws the curve of the differences in the shading effects of the settling grains at the two levels of the tube filled with distilled water in which the grains are suspended. The curve drawn by the recorder represents the frequency curve of grain-sizes (Fig. 4). The changes in the rotation of the centrifuger are known by the repeated measurements (Fig. 5). The settling velocities of glass balls of 2.51 in density ranging from -0.1 to 3.5φin size were examined by the analyser as a reference for the interpretation of the curves drawn by the automatic grain-size analyser. The balls were sieved at intervals of 0.5φby a Ro-Tap shaker and the maximum and minimum diameters in each fraction of the sieved grains were measured under a binocular microscope. The fractions of the glass balls were analyzed at water temperature of 10.0, 15.0, 18.0, 19.5 and 22.0℃. The settling velocities of the grain-sizes of every fractions are shown on the curves of Fig. 6. The result of the measurement shows that the settling velocity of the glass balls of the size ranging from -1.0 to 3.5φcoincides well with that calculated by the equations of Karman and Stokes (Fig. 7). The table 1 shows the times for settling of quartz balls of 2.65 in density, which are used usually as standard grains in sedimentology, through the water column 130 cm high at 2℃ intervals of water temperature, which was calculated from the measured velocities of the glass balls after density correction by the equations of Karman and Stokes. This table gives the sizes of the quartz balls equivalent to various grains settled in water at the same velocity. Some technical problems on the automatic grain-size analyser are mentioned in the text