1.1 ,These test methods use air permeability to determine an envelope-specific surface area and its associated average equivalent spherical diameter (from 0.2 to 75 µ,m) of powders. Two test methods are described: One test method for inorganic materials (Test Method 1), and another test method for organic materials (Test Method 2). The values obtained are not intended to be absolute but are generally useful on a relative basis for control purposes.