A gas diffusion device used in conjunction with a flow of gas from a
conduit utilizing a sheet having a peripheral edge defining a surface
inwardly from the edge. The surface possesses a first zone, and a second
zone between the first zone and the peripheral edge. A first plurality of
apertures through the sheet lie in the first zone. A second plurality of
apertures through the sheet lie in the second zone. The percentage of
open area of the first plurality of apertures relative to the area of the
first zone is less than the percentage of open area due to the second
plurality of apertures through the second zone.