Equivalent diameter sedimentation technique

The particle size deterrnined by sedimentation techniques is an equivalent spherical diameter, also known as the equivalent settling diameter, defined as the diameter of a sphere of the same density as the irregularly shaped particle that exhibits an identical free-fall velocity. Thus it is an appropriate diameter upon which to base particle behavior in other fluid-flow situations. Variations in the particle size distribution can occur for nonspherical particles (43,44). The upper size limit for sedimentation methods is estabHshed by the value of the particle Reynolds number, given by equation 11 ... 

Other techniques of deducing shape (or size) include using sieve analysis or sedimentation. The equivalent diameter for a sieve analysis is the mesh size of the upper sieve through which particles pass. (A more precise definition of the sieve diameter is the mean between the mesh size through which the particles pass and on which the particles are retained.) The characteristic diameter of a sedimentation technique would be the diameter of a sphere that has the same settling velocity. 

The calculation of average, volume-equivalent, diameters was based upon total suspended particulate mass and the total number of particles, the latter determined by a condensation particle counter. Aerodynamic size distributions were determined via sedimentation. The operational assumption in this technique is that an insignificant... 

If we use a sieve to measure the particle size we come up with an equivalent sphere diameter, which is the diameter of a sphere passing through the same sieve aperture. If we use a sedimentation technique to measure particle size then it is expressed as the diameter of a sphere having the same sedimentation velocity under the same conditions. Other examples of the properties of particles measured and the resulting equivalent sphere diameters are given in Figure 1.2. 

Various techniques and equipment are available for the measurement of particle size, shape, and volume. These include for microscopy, sieve analysis, sedimentation methods, photon correlation spectroscopy, and the Coulter counter or other electrical sensing devices. The specific surface area of original drug powders can also be assessed using gas adsorption or gas permeability techniques. It should be noted that most particle size measurements are not truly direct. Because the type of equipment used yields different equivalent spherical diameter, which are based on totally different principles, the particle size obtained from one method may or may not be compared with those obtained from other methods. 

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