This is a test of the openness of uncoated papers. We conduct this test according to TAPPI Test Method T460, using a Gurley porosity tester. The instrument measures porosity by forcing air through the sheet and measuring the rate of flow. More open sheets allow air to pass through the sheet more rapidly. The units for Gurley Porosity is seconds/100 cc3
The porosity test is sometimes used on coated paper or coated paperboard. These products are not generally porous, so the apparent porosity measurement is in reality a measurement of air leakage from the clamp.