Subpart S—Underground Construction, Caissons, Cofferdams and Compressed Air
§ 1926.804 Definitions applicable to this subpart.

(a) Bulkhead —An airtight structure separating the working chamber from free air or from another chamber under a lesser pressure than the working pressure.
(b) Caisson —A wood, steel, concrete or reinforced concrete, air- and water-tight chamber in which it is possible for men to work under air pressure greater than atmospheric pressure to excavate material below water level.
(c) Decanting —A method used for decompressing under emergency circumstances. In this procedure, the employees are brought to atmospheric pressure with a very high gas tension in the tissues and then immediately recompressed in a second and separate chamber or lock.
(d) Emergency locks —A lock designed to hold and permit the quick passage of an entire shift of employees.
(e) High air —Air pressure used to supply power to pneumatic tools and devices.
(f) Low air —Air supplied to pressurize working chambers and locks.
(g) Man lock —A chamber through which men pass from one air pressure environment into another.
(h) Materials lock —A chamber through which materials and equipment pass from one air pressure environment into another.
(i) Medical lock —A special chamber in which employees are treated for decompression illness. It may also be used in preemployment physical examinations to determine the adaptability of the prospective employee to changes in pressure.
(j) Normal condition —One during which exposure to compressed air is limited to a single continuous working period followed by a single decompression in any given 24-hour period; the total time of exposure to compressed air during the single continuous working period is not interrupted by exposure to normal atmospheric pressure, and a second exposure to compressed air does not occur until at least 12 consecutive hours of exposure to normal atmospheric pressure has elapsed since the employee has been under pressure.
(k) Pressure —A force acting on a unit area. Usually shown as pounds per square inch. (p.s.i.)
(l) Absolute pressure (p.s.i.a.)—The sum of the atmospheric pressure and gauge pressure (p.s.i.g.).
(m) Atmospheric pressure —The pressure of air at sea level, usually 14.7 p.s.i.a. (1 atmosphere), or 0 p.s.i.g.
(n) Gauge pressure (p.s.i.g.)—Pressure measured by a gauge and indicating the pressure exceeding atmospheric.
(o) Safety screen —An air- and water-tight diaphragm placed across the upper part of a compressed air tunnel between the face and bulkhead, in order to prevent flooding the crown of the tunnel between the safety screen and the bulkhead, thus providing a safe means of refuge and exit from a flooding or flooded tunnel.
(p) Special decompression chamber —A chamber to provide greater comfort of employees when the total decompression time exceeds 75 minutes.
(q) Working chamber —The space or compartment under air pressure in which the work is being done.