This test method provides a procedure 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 this test method will be defined as a Fisher Number. The degree of correlation between the results of this test method and the quality of powders in use will vary with each particular application and has not been fully determined.

This test method is generally applicable to all metal powders and related compounds, including carbides, nitrides, and oxides, for particles having diameters between 0.5 and 50 μm. It 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 method described only by agreement between the parties concerned. This test method 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 such as that referenced in 1.1 may be used if agreed upon between the parties concerned.

When an "average" particle size of powders is determined using the Fisher sub-sieve sizer, 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.