These adapters are KF-40 to 0.250 in. Stainless Steel Tube. The flange size follows ISO-KF standards and is NW-40. These tube fittings are commonly used to adapt KF vacuum connections to common tube connections for gas or fluid delivery. 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 KF flanges and the centering ring contains sealing 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. These are all stainless steel construction.