Equivalent diameter sieve analysis

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. 

Since the reported diameter is an equivalent volume diameter, proper consideration must be taken when comparing the results with other particle size analysis methods. For instance, the equivalent volume diameter reported for any non-spherical shaped particle will generally be higher than the particle size reported by sieve analysis, as the equivalent volume diameter is based on the volume of a perfect sphere. This theory becomes apparent if one compares the equivalent volume diameter with the sieve diameter of a square that has dimensions of 2(X)pm by 200 pm by 200 pm and that passes through a U.S. ASTM sieve 70 (212 pm) and is retained on a U.S, ASTM sieve 80 (180 pm). In this case, the sieve diameter reported will be 212 pm but the... 

Feret s Diameter Feret s diameter is determined from the projected area of the particles by using a slide gauge. In general it is defined as the distance between two parallel tangents of the particle at an arbitrary angle. In practice, the minimum Xjrmm d maximum Feret diameters the mean Feret diameter Xp, and the Feret diameters obtained at 90 to the direction of the minimum and maximum Feret diameters X/rmax9o used. The minimum Feret diameter is often used as the diameter equivalent to a sieve analysis. 

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. 

Most calculation methods are based on one dimension of the particle, usually the equivalent diameter. If this dimension is obtained from a sieve analysis, it will be the sieve aperture diameter, ds.a., but as crystals are never true spheres, this diameter will normally be the second largest dimension of the particle. Figure 2.14 demonstrates some particle shapes that would, in a sieve analysis, all yield the same value for 4.a.- One potential source of error is thus clearly seen. 

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