The ISO 5636-4:2013 standard specifies a method for determining the air permeability of paper and Cardboard in the medium range using the Sheffield method. This method is suitable for paper and Cardboard with a Porosity between 0.02 μm/(Pa · s) and 25 μm/(Pa · s). It is not suitable for materials with rough surfaces. Flow rate of air through the sample under pressure difference is tested by the Sheffield unit Measurement unit to evaluate the breathability performance of paper. This standard is widely used in quality control of paper and Cardboard, especially in the fields of Encasement, filtrated materials, etc., to help evaluate the vapor transmission rate and breathability uniformity of paper.