The resistance to air flow (defined for this paper as the pressure drop at a specified velocity) of fine fiber filter media is an important property since the flow resistance of any fabricated filter device will depend heavily on the resistance of the media used. Equally important is the fact that filtration efficiency and capacity depend in large part upon the same properties of the media that control the flow resistance. For fibrous media, these properties include the fiber diameter, media density, thickness, binder effectiveness (which to some extent controls the effective thickness and density under the forces of an air stream) as well as the fiber orientation and pack uniformity. Thus when used in conjunction with suitable control of other variables in the manufacturing process, the measurement and control of flow resistance can significantly aid in insuring that consistent and reproducible filtration properties will result. For these reasons specifications for filter media usually state the· nominal flow resistance along with allowable tolerances.

Since such specifications serve as a basis for the manufacture and sale of the media and the design and production of tne finished filter, it is imperative that test methods and equipment be standardized and accurately calibrated. Standardization is necessary since several variables of the test must be chosen arbitrarily; These include the type of backing screen or support, the method of edge sealing, and the related effects of sample shape (round or square) and the size (as it affects the edge treatment). The need for accurate calibration is obvious, though it must be recognized that calibration to an absolute standard may not be necessary as long as all testers are calibrated the same by the use of transfer standards. Calibration traceable to absolute standards is desirable however in order to gain universal acceptance of the method and its results.