(1) Each bulkhead in tunnels of twelve feet or more in diameter or equivalent area, shall have at least two locks in perfect working condition, one of which shall be used as an air lock. An additional lock for use in case of emergency shall be held in reserve.
(2) The air lock shall be large enough so that those using it are not compelled to be in a cramped position, and shall not be less than five feet in height. Emergency locks shall be large enough to hold an entire heading shift.
(3) All locks used for decompression shall be lighted by electricity and shall contain a pressure gauge, a time piece, a glass "bull's eye" in each door or in each end, and shall also have facilities for heating.
(4) Valves shall be so arranged that the locks can be operated both from within and from without.