These Pfeiffer KF40 centering rings have a PTFE (teflon) ring and Viton (FKM) O ring seal. Maximum bakeout temperature is 200 °C (limited by the the o-ring).

KF-40 Centering Ring vacuum fittings follow ISO-KF standards and have a flange size of NW-40. They are commonly used to build foreline vacuum plumbing and process systems. They are often called quick flange fittings because they are quick to assemble. A circumferential clamp and centering ring form the vacuum seal between the connecting flanges (please see top right figure - click to enlarge). The centering ring contains a rubber elastomer o-ring. The standard sizes for quick flange fittings are KF-10, KF-16, KF-25, KF-40, and KF-50 with flange sizes NW-10, NW-16, NW-25, NW-40, and NW-50, respectively. They are made of corrosive resistant Stainless Steel with VitonŽ O-Ring. VitonŽ is well known for its excellent resistance to heat (400°F/200°C) and to aggressive fuels and chemicals. VitonŽ is a registered trademark of DuPont Performance Elastomers.