The Specification and Testing of Fine Particle Filters

Abstract

A method of defining the filtration efficiency of filter media has been devised, in which the transmission factor at a specified size is measured instead of the effective cut‐off size. The new measure is more sensitive and provides a more useful fiure than the old measure of effective cut‐off size. An additional test is proposed to measure the efficiency of removal of particles of smaller sizes; this test requires the derivation of the transmission curve, which is defined. No change is proposed in the method of measurement of flow efficiency in terms of flow constant. It is suggested that the specification of a fine particle filter medium should include the following clauses: (a) A statement of the maximum acceptable transmission factor at a given size (cither 5 microns or 10 microns). (b) A statement of the maximum acceptable transmission factors at various smaller sizes. (c) A statement of the minimum acceptable flow constant. Experimental techniques, including the method of preparing the artificial contaminants, are described. Results are presented on nine materials and a comparison is made for five of these materials between transmission factors at 5 microns and 10 microns and effective cut‐off sizes. The dependence of transmission factor and flow constant on thickness of filter medium is discussed.