Dual mounting head scanners measure the thickness of flexible moving porous webs and employ an air clamp on the operative surface of the lower head to maintain the web in physical contact with a measurement surface. As the web is held firmly by the clamp, the vacuum level that is established is indicative of the porosity of the membrane. As compressed air is supplied to a vacuum generator at a given operational pressure, the rate of airflow through the web can be inferred from the vacuum pressure measurements. The rate of airflow through the membrane and therefore the porosity of the membrane are related to the vacuum level. It is not necessary to measure the airflow through the membrane. From the vacuum pressure measurements, the membrane's permeability can also be determined by correlation to empirical data. Thickness measurements are effected by optical triangulation and inductive proximity measurements.