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This is a test of the openness of uncoated papers. We have both a Gurley and Bendtsen porosity tester. The instruments measure 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 porosity test is sometime 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.
Gurley Porosity Test from Applied Paper Technology