These adapters are KF-40 to quick coupling compression port 1 3/8 inch internal diameter (I.D.). The flange size follows ISO-KF standards and is NW-40 . They are commonly used to make connections between ISO-KF fittings and items with a vacuum tubing connection, e.g., vacuum gauges or instruments. The vacuum seal occurs by compression of a Viton rubber O-ring and is easily removable. The ISO-KF flanges 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 304 Stainless Steel.