This method is used to measure the air permeance of a circular area of paper using a pressure differential of approximately 10 kPa (1.5 psig). In order to accommodate a wide range of paper products, rubber clamping plates are available for five commonly used orifice diameters: 9.5 mm (0.375 in.), 19.1 mm (0.75 in.), 38.1 mm (1.50 in.), 57.2 mm (2.25 in.), and 76.2 mm (3.00 in.). The air flow range for this method is 0 to 3348 mL/min (0 to 400 Sheffield units). Instruments are available with either variable area flowmeters (glass tubes with internal tapers and floats) or electronic mass flowmeters. This method measures the air that passes through the test specimen, along with any possible leakage of air across the surface; therefore it is unsuitable for papers with rough surfaces which cannot be securely clamped so as to avoid significant surface leakage. For other methods of measuring the air resistance of paper using a 28.6 mm (1.125 in.) orifice diameter, refer to TAPPI T 460 “Air Resistance of Paper (Gurley Method)” for a test that operates at a pressure differential of 1.22 kPa, or TAPPI T 536 “Resistance of Paper to Passage of Air (High Pressure Method)” for 3 kPa.