text: The particle size, or fineness, of powder materials is very often critical to their performance.
  Measurement of air permeability can be performed very rapidly, and does not require the powder to be exposed to vacuum or to gases or vapours, as is necessary for the BET method for determination of specific surface area. This makes it both very cost-effective, and also allows it to be used for materials which may be unstable under vacuum.
  When a powder reacts chemically with a liquid or gas at the surface of its particles, the specific surface is directly related to its rate of reaction.  The measurement is therefore important in the manufacture of many processed materials.
  In particular, air permeability is almost universally used in the cement industry as a gauge of product fineness which is directly related to such properties as speed of setting and rate of strength development.
  Other fields where air permeability has been used to determine specific surface area include:
  * Paint and pigments
  * Pharmaceuticals
  * Metallurgical powders, including sintered metal filters.
  In some fields, particularly powder metallurgy, the related Fisher number is the parameter of interest. This is the equivalent average particle diameter, assuming that the particles are spherical and have uniform size. Historically, the Fisher number was obtained by measurement using the Fisher Sub-sieve Sizer, a commercial instrument containing an air pump and pressure regulator to establish a constant air flow, which is measured using a flowmeter. A number of manufacturers make equivalent instruments, and the Fisher number can be calculated from air permeability specific surface area values.