air permeability diameter testing

Air Permeability Diameter

This classical analytical technique determines the average particle size by measuring the pressure drop across a packed powder bed using a water manometer. The pressure drop is a function of the permeability of the packed bed. This is related to the surface area of the particles which is then transformed to an “average size” based upon the assumption that the particles are spherical.

The Fisher Sub-Sieve Sizer tests powders quickly and reproducibly for their Average Particle Diameter – also known as the Air Permeability Diameter.


  • Conforms to ASTM STANDARD B330-20.
  • Conforms to USP Monograph for Griseofulvin.


  • This technique does not report on the statistics of the size distribution
  • Density of the sample must be provided or determined prior to calculation of average particle diameter.

Sample Requirements

For griseofulvin, no less than 5 g of sample should be submitted. For other samples, a sample equal in weight (grams) to twice (2x) the true density of the material is required, though higher quantities are recommended.

Detection Range

Covers a nominal particle size span of about 0.2 µm to 50 µm.

Data Reported

Tabulated data summary including the porosity, average particle diameter (aka Fisher Number), and calculated specific surface area is provided.