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ASTM C721-20

ASTM C721-20

Standard Test Methods for Estimating Average Particle Size of Alumina and Silica Powders by Air Permeability

2020-10-12 /Active
Significance and Use:

4.1 The estimation of average particle size has two chief functions: (1) 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 (1) 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.


1.1 These test methods cover the estimation of the average particle size in micrometres of alumina and silica powders using an air permeability method. The test methods are intended to apply to the testing of alumina and silica powders in the particle size range from 0.2 to 75 μm.

1.2 The values stated in SI units are to be regarded as standard, with the exception of the values for density and the mass used to determine density, for which the use of the gram per cubic centimetre (g/cm3) and gram (g) units is the long-standing industry practice; and the units for pressure, cm H2O—also long-standing practice.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

air permeability; alumina; average particle size; envelope-specific surface area; Fischer Number; particle size; permeability; porosity; powder; quartz; silica; specific surface;
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