The most popular way to measure the air permeability of a filter medium is to fix the pressure drop and measure the airflow. The Frazier Air Permeameter, manufactured by the Frazier Precision Instrument Company, Inc., Hagarstown, Maryland USA, is diagrammed in Figure 6.2. It has been used for years for this type of measurement. The air is sucked through the clamped medium specimen into an enclosed chamber and out through an orifice that measures the flow. A manometer across the medium gives the pressure drop and a manometer across the orifice gives the pressure drop across the orifice for calculation of airflow. The manufacturer provides data tables for converting orifice pressure drop to airflows. Figure 6.3 is a photograph of the Frazier 2000 ®- Air Permeability Instrument. This modern version implements electronic pressure sensing as well as computerized logging and manipulation of the electronic measurements.inclinedverticaloil manometerpartTextest AG, Schwerzenbach, Switzerland markets an air permeability tester that has some convenient automated features including direct digital readout of the permeability. It also has an automated clamping system for the specimen. Orifices are easily changed by use of a turn dial instead of having to replace each one by hand. The airflow measured is based on Darcy's law discussed in Chapter 2:clampreservoirs