To ensure adequate upstream aerosol challenge mixing, it is strongly recommended to introduce the test aerosol at least ten duct diameters upstream of the filter(s) under test. If 10 duct diameters is not feasible, or adequate mixing is not obtained, sparge pipes, upstream baffles, turning vanes or a Stairmand disc, located four to six duct diameters upstream, will achieve adequate mixing.
A sparge pipe improves the testing of clean air cabinets, safety cabinets, isolators, and any installed HEPA filter where the distance between the point of aerosol injection and the filter is short, and upstream challenge measurement shows poor mixing. ATI provides a sparge pipe kit with bag (P/N 630-703) that includes pipes of varying lengths, single and double-drilled, providing end users with maximum flexibility for use. For more information, see Sparge Pipe Kit.
A Stairmand disk is a plate with the same geometric shape as the duct section blocking the center of the duct area. Air flowing past the disk creates vortices on the leeward side that compel turbulent and thorough mixing of the introduced aerosol and the dilution air stream. The required diameter of the Stairmand disk may be calculated using the following information.
A Stairmand disk diameter equals the pipe (or duct) diameter divided by the square root of two (1.414213). E.g., a 12-inch duct would require a Stairmand disk 8.5 inches in diameter.