scope:

This method is used to measure the air resistance of approximately 6.4 sq. cm. (1 sq. in.) circular area of paper using a pressure differential of approximately 3 kPa. The recommended range of this instrument is for papers that require 10 or more seconds for 10 mL of air to pass through. Refer to the manufacturer's instructions for the upper range limits. For more permeable papers, other techniques are preferable. Instruments are available with automatic timing devices.

This method measures the time it takes for a given volume of air to pass through the test specimen, along with any possible leakage of air across the surface; therefore it is unsuitable for rough-surface papers which cannot be securely clamped so as to avoid significant surface and edge leakage.

NOTE 1: If increasing the compression force on the lower clamp decreases the air leakage rate, this can indicate air leakage through surface roughness or through the edge of the specimen, then the selected clamping force is too low.

For a similar method of measuring air resistance that tests paper at a lower pressure (approx. 1.22 kPa), and has higher volume capabilities, refer to TAPPI T 460 "Air Resistance of Paper." For a similar method of measuring air permeance at pressures up to 9.85 kPa, using both smaller and larger test areas, refer to TAPPI T 547 "Air Permeance of Paper and Paperboard (Sheffield Type)."