DN Half Nipple, Long Weld Stub DN-25 Vacuum Fittings, ISO-DN Flange, Stainless Steel, 55 mm Long, 28 mm OD Tube. They are commonly used to build foreline vacuum plumbing and process systems. The standard sizes for fittings are DN-10, DN-16, DN-25, DN-40, and DN-50. 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 KF O-rings and clamps can be used for DN fittings, only the OD tube size is different than a standard Quick Flange (KF) fitting. They are made of corrosive resistant stainless steel.