Comparison of methods for particulate testing of HEPA filters

In 1976 a suggestion was made at a European Economic Community Seminar in Aix-en-Provence that there should be an attempt to correlate results of the various methods of testing HEPA filters. From previous results with the Q127 DOP equipment there seemed to be a reasonable correlation with the sodium chloride test employed in the United Kingdom when sheets of 100 cm2 were tested, while a few tests with HEPA filters also showed fair agreement with figures from the British Standard 3928 method. Little, however, was known of comparisons with other methods when intact HEPA filters were tested. An investigation, begun under an EEC contract in 1979, included as its object both rig and on-site assessments at various European Establishments. Transport of large filters leads to damage of unknown magnitude, whilst seating into test ducts may not always be perfect; on the other hand, testing the same filter many times may lead to erroneous comparisons due to clogging. In consequence in the work, which has continued in 1980, a large number of sheets of 100 cm2 area, held in a leak-proof jig, has also been employed. Some 140 sheets of each of five grades of filter paper were obtained. Tests on randomly chosen sheets indicated consistent penetration figures (variation within any grade less than 15%) so that there were good grounds for assuming that the filters were suitable for comparison purposes. Ten different methods of rig and on-site tests have been investigated, with results reported in this paper. As expected, there are wide differences in absolute penetrations, in velocity/pnentration curves and in sensitivity. Figures for penetration of the paper sheets are still awaited from the ten other test houses and a detailed analysis will not be attempted until these have been received