EVAC aluminum seals locate on the centering ring groove (inner centering) or outside edge of the flange (outer centering) depending on the size. (Inner and outer centering versions are available for all NW flange sizes.) Aluminum knife-edges are compressed by the sealing surfaces of the mating flanges when the chain clamp or ISO claw clamp is tightened. The resultant seal has a helium leak rate of less than 1x10-11 standard cc/sec. and can be baked repeatedly to 150°C. Proper sealing occurs even if the flanges are slightly scratched or misaligned. The aluminum alloy is soft enough to make a good seal with reasonable force but will not flow at moderate temperatures. EVAC seals will even work with aluminum flanges. Normally the seals are for one-time use, however, by sealing the first time with a low torque and increasing the torque each time thereafter, they can be reused up to five times. If the seal is going to be baked or subject to vibration or pressure, it should be tightened to the recommended torque initially.