4.1 The estimation of average particle size has two chief functions: first, as a guide to the degree of fineness or coarseness of a powder as this, in turn, is related to the flow and packing properties; and, second, as a control test on the uniformity of a product.

4.2 These test methods provide procedures for determining the envelope-specific surface area of powders, from which is calculated an "average" particle diameter, assuming the particles are monosize, smooth surface, nonporous, spherical particles. For this reason, values obtained by these test methods will be reported as an average particle size or Fisher Number. The degree of correlation between the results of these test methods and the quality of powders in use will vary with each particular application and has not been fully determined.

4.3 These test methods are generally applicable to alumina and silica powders, for particles having diameters between 0.2 and 75 μm (MIC SAS) or between 0.5 and 50 μm (FSSS). They may be used for other similar ceramic powders, with caution as to their applicability. They should not be used for powders composed of particles whose shape is too far from equiaxed-that is, flakes or fibers. In these cases, it is permissible to use the test methods described only by agreement between the parties concerned. These test methods shall not be used for mixtures of different powders, nor for powders containing binders or lubricants. When the powder contains agglomerates, the measured surface area may be affected by the degree of agglomeration. Methods of de-agglomeration may be used if agreed upon between the parties concerned.

4.4 When an "average" particle size of powders is determined using either the MIC SAS or the FSSS, it should be clearly kept in mind that this average size is derived from the determination of the specific surface area of the powder using a relationship that is true only for powders of uniform size and spherical shape. Thus, the results of these methods are only estimates of average particle size.