Investigation of Filter Bypass Leakage and a Test for Aerosol Sampling Cassettes

Plastic filter cassettes (37 and 25 mm), which are press fitted together to seal and hold a filter in place, are commonly used for sampling aerosols. Aerosol bypass leakage around the filter has been reported by several researchers and attempts have been made to test for leakage and to reduce the likelihood of leakage by improving cassette design. Under typical sampling conditions, there is often no indication to the user that leakage may have occurred. In the present study, a particle count leak test was developed that used a particle counter that measured the particle number concentration of ambient aerosol (primarily submicrometer particles) upstream and downstream of the filter cassette. The relationship between leak test results and particle loss from the filter depended on particle size and type in a complex fashion. The mechanisms of particle loss were investigated and the losses increased for particles above 2 w m and were much greater for solid and fume aerosols than for oil droplets. Although the test could not be used to predict particle mass loss during sampling, the test was a sensitive indicator of cassette bypass leakage and was used to establish compression pressures needed for proper assembly of these cassettes.