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 steady-state 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, using similar equipment (see A.2); the Gooden and Smith method, involving an apparatus which can be built in a laboratory but for which a commercial apparatus also exists. (Two types of commercial apparatus exist; one of these is no longer available for purchase, but is still being used - see A.3.). These methods are given as examples only. Other equipment available in various countries is acceptable within the scope of this International Standard. This testing method is applicable to all metallic powders, including powders for hardmetals, up to 1 000 ¼m in diameter, but it is generally used for particles having diameters between 0,2 ¼m and 50 75 ¼m. It should not be used for powders composed of particles whose shape is far from equiaxial, i.e. flakes or fibres, unless specifically agreed upon between the parties concerned. This testing method is not applicable to mixtures of different metallic powders or powders containing binders or lubricant. If the powder contains agglomerates, the measured surface area may be affected by the degree of agglomeration. If the powder is subjected to a de-agglomeration treatment (see annex B), the method used shall be agreed upon between the parties concerned.