The 100936525 Overpressure Centering Ring Assembly is designed for NW25 size ISO-KF vacuum flanges and features a Viton® O-ring, stainless steel construction, and an aluminum overpressure ring. Series 31 Centering rings are inserted between two same size ISO-KF 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.