This method is used to measure the air resistance of approximately 6.4 sq. cm. (l sq. in.) circular area of paper using a pressure differential of 1.22 kPa. The recommended range of the liquid column instrument is from 5 to 1800 seconds per 100 mL cylinder displacement. For more impermeable papers the time requirements become so excessive that other techniques are preferable.
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 rough-surface papers which cannot be securely clamped so as to avoid significant surface and edge leakage.
For a similar method of measuring air resistance that tests paper at a higher pressure (approx. 3 kPa), and has higher resolution in measuring smaller air volumes, refer to TAPPI T 536. For a method of measuring air permeance at pressures up to 9.85 kPa, using both smaller and larger test areas, refer to TAPPI T 547.
