 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.Measurement consists of packing the powder into a cylindrical "bed" having a known porosity (i.e.