This International Standard specifies a method of measuring the air permeability and the porosity of a packed bed of metal powder, and of deriving therefrom the value of the envelope-specific surface area. The permeability is determined under steadystate flow conditions, using a laminar flow of air at a pressure near atmospheric. This International Standard does not include the measurement of permeability by a constant volume method.Several different methods have been proposed for this determination, and several instruments are available commercially. They give similar reproducible results, provided the general instructions given in this International Standard are respected and the test parameters are identical.This International Standard does not specify a particular commercial apparatus and corresponding test procedure. However, for the convenience of the user, an informative annex has been included (annex A) which is intended to give some practical information on three specific methods:- the Lea and Nurse method, involving an apparatus which can be built in a laboratory (See A.1);- the Zhang Ruifu method similar equipment (see A.2);- the Gooden and Smith metod, involving an apparatus which can be built in a laboratory but for which a commercial apparatus also exists (see A.3).These metods are given as exemples only. Other equipment available in various countries is acceptable within the scope of this International Standard.