The 100938658 Overpressure Centering Ring Assembly is designed for NW100 size ISO-MF vacuum flanges and features a Viton® O-ring, stainless steel construction, and an aluminum overpressure ring. Series 76 Centering rings are inserted between two same size ISO-MF 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.