The 100996303 Centering Ring Assembly is designed for NW100 size ISO-MF, ISO-BF, or ISO-KF vacuum flanges and features a Viton® O-ring, 304 stainless steel construction for chemical resistance, and includes a screen. These centering rings are inserted between two same size ISO flange faces to provide the vacuum seal when joining them. The included Viton O-ring is recommended when high temperature, chemical resistance or long installed life make them beneficial.