These ISO-F DN 250 bored blank flanges follow vacuum technology standards as described in DIN 28404 and ISO 1609. ISO-F fixed flange vacuum fittings have a bolt circle where bolts align the two flanges. A centering ring which contains a rubber elastomeric O-ring is used to form the vacuum seal between the two flange faces. They are commonly used to build process vacuum plumbing and systems for industrial vacuum applications. The ISO standard sizes for large LF flanges are ISO-F DN 63, 80, 100, 160, 200, 250, 320, 400, 500, 630. These vacuum fittings are made of corrosive resistant 304L stainless steel.