Io exp BC

where B is a constant and C is the concentration of powder in the beam. The x-ray density, Xd, is defined as:

The sedigraph is a well-known instrument based on the gravitational settling and x-ray absorption analysis. This instrument automatically presents the cumulative percentage frequency, and the sedimentation cell is driven in such a way that the concentration is recorded directly as a function of the Stokes' diameter. The x-rays from a 22.6 keV source are collimated into a narrow beam and pass through a fixed thickness of suspension. The sedimentation cell (Fig. 2) is filled and emptied with the suspension by a built-in pump. The transmitted radiation is detected as pulses by a xenon-filled scintillation detector. The counting electronics give a voltage that is proportional to the powder concentration. Particle size distribution is calculated by the application of Stokes' law to these data.

Fig. 2 Sedigraph schematic

Centrifugal Techniques. Because the centrifuging process speeds up the rate of settling of particles, it overcomes one of the serious disadvantages of the gravitational sedimentation techniques, which take an unduly long time for particles <5 /''m (200 /''in.). In addition, most sedimentation devices suffer from the effects of convection, diffusion, and Brownian motion. Centrifugal sedimentation can be carried out by either a two-layer or homogeneous suspension technique, either cumulatively or incrementally. The two-layer techniques give rise to streaming, a phenomenon most likely to affect cumulative analyses, in which the fraction sedimented against time is determined (Ref 2).

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