NIOSH Manual of Analytical Methods, Fourth edition. Third supplement. Cincinnati, OH: National Institute for Occupational Safety and Health, 2003 Mar; :179-183
The plastic two- and three-piece cassettes commonly used for collection of personal samples of airborne dust in the Americas are prone to bypass leakage if the cassettes are not properly assembled. A cross sectional view of these cassettes is given in Figure 1 . If the pressure used to put the cassettes together is insufficient, or not properly applied, the base and mating piece may not make a proper seal with the filter, permitting sampled aerosol to leak around the filter as illustrated in Figure 2 . Leakage around the filter will result in a loss of dust that should have been collected on the filter, resulting in a measurement that underestimates the concentration of the aerosol sampled. Anecdotal indications of this bypass leakage have cropped up from time to time, indicating that bypass leakage continues to be a problem when these cassettes are improperly assembled. Van den Heever, in an investigation of the leakage problem, presented data on cassettes using a pressure drop measurement to indicate leakage . The average pressure drop for a specific cassette/filter combination was measured for a number of "good" cassettes and then any decrease in pressure drop observed for assembled cassettes was attributed to bypass leakage. The decrease in pressure drop correlated well with reduced sample collection. Establishment of this baseline pressure drop when only assembling a few cassettes was cumbersome and a more direct measure of leakage was needed. A nylon cassette designed to replace the 37-mm cassette for the South African market was shown to have reduced likelihood of leakage; the nylon cassette had a rubber seal for the filter edge and a base that screwed to the cap. Recently, Baron and coworkers developed and evaluated a leak test using ambient aerosol to assess bypass leakage [1, 2, 4]. The test, performed with an optical particle counter or a condensation particle counter, enabled the aerosol concentration upstream and downstream of a cassette to be measured.