Pre-filter SR 221 is a disc that is meant to catch coarse particles and by that extend the lifetime of the particle filters SR 510 P3 R and SR 710 P3 R, combined filter SR 299-2 ABEK1-Hg-P3 R and SR 599 A1BE2K1-Hg-P3 R. The pre-filter is placed in the prefilter holder and then snapped on to the particle filter or combined filter.