Vacuum anchors use suction to provide a tie-off point for fall protection equipment on clean and smooth non-porous surfaces. They are non-marring so they are often used when performing aircraft maintenance. They are also used for ship and rail maintenance, inspecting facilities, and working when surfaces cannot be penetrated. The system uses a compressed air source from a shop line or portable cylinder to create the seal. A gauge indicates when the vacuum seal is safe for use. Each anchor supports one person. Two vacuum anchors can be used to attach a horizontal lifeline system to support up to two people. Anchors should always be inspected and replaced after being exposed to a fall.