This EVAC Aluminum Metal OUTER Seal NW40 KF40 flange is for converting Elastomer (Viton/Buna) rubber seals to UHV (Ultra High Vacuum) aluminum metal seals. This allows for a lower vacuum compared to elastomer (Viton/Buna) rubber seals. Other benefits include: reduced outgassing, no permeation , no hydrocarbons, and resistance to radiation and higher heat. The resultant UHV seal has a helium leak rate of less than 1 X 10 -11 standard cc/sec. They can also be baked repeatedly to 150°C (302°F). The EVAC chain clamp and an aluminum knife edge seal form the UHV vacuum seal between the connecting KF flanges. (please see far right graphic above - click to enlarge). This EVAC metal seal is made of aluminum. This is a KF40 NW40 aluminum metal knife edge seal only, the stainless steel chain clamp IS NOT included but available on this website. For a complete explaination of this UHV Seal and dimensions of the aluminum metal knife edge seal, download the EVAC Flange Brochure .pdf below.