This standard describes methods for determining the average particle size of a sample of fine metal powders using air permeability of a packed powder bed. This method is not generally used for mixed powders. The values obtained may not be absolute, but are reproducible, and are highly useful on a relative basis for control purposes. This is an excerpt from Standard Test Methods for Metal Powders and Powder Metallurgy Products, 2016 Edition, with ISBN(s): 9781943694037.