The Insufficiency of Single Point Diffusive Air Flow Integrity Testing

Abstract

Filter manufacturers characterize a given filter type by a single acceptable maximum diffusive air flow level. A too high value is prudently rejected as signalling the possible presence of flaws. Values lower than the stipulated maximum are accepted. However, the presence of single pores, whatever their sizes, is not being revealed. Only their influence within the total porosity is measured by the diffusive airflow. Single point diffusive airflow integrity testing may invite misleading conclusions because it offers possibilities for lower airflows to mask the presence of flaws or of outsized pores. This is particularly so where initial (prefiltration) and final integrity are being compared. The filtrative accretion of retained material, viable or not, must inevitably come to block or diminish the size of the pores. This results in decreased total porosity, directly expressed in lower diffusive airflow values. Sufficient drop in such final air flow values may mask the presence of a flaw developed in the filter subsequent to its initial testing. For this reason a filter’s entire airflow permeation curve, particularly including its bubble point, must be characterized. It is this measurement, and this measurement alone, that is capable of unambiguously revealing the existence of insufficiently retentive pores. The inquiry into such presences is the very purpose of filter integrity testing.