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 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). Eg. a 12 inch duct would require a Stairmand disk 8.5 inches in diameter.