Acceptable. Any device, equipment, or appliance that is either approved by MSHA and maintained in permissible condition, or is listed or labeled for the class and location under Part I of this chapter.

Bulkhead. An airtight structure separating the working chamber from free air or from another chamber under a lesser pressure than the working pressure.

Caisson. A wood, steel, concrete or reinforced concrete, air- and water-tight chamber in which it is possible for persons to work under air pressure greater than atmospheric pressure to excavate material below water level.

Cofferdam. A watertight barricade or enclosure erected, sunk, driven or otherwise fabricated to permit the performance of work where hydrostatic pressure exists.

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.

Emergency locks. A lock designed to hold and permit the quick passage of an entire shift of employees.

High air. Air pressure used to supply power to pneumatic tools and devices.

Low air. Air supplied to pressurize working chambers and locks.

Man lock. A chamber through which persons pass from one air pressure environment into another.

Materials lock. A chamber through which materials and equipment pass from one air pressure environment into another.

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.

Rapid excavation machine. Tunnel boring machines, shields, roadheaders, or any other similar excavation machine.

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.

Pressure. A force acting on a unit area. Usually shown as pounds per square inch. (p.s.i.)

Absolute pressure (p.s.i.a.). The sum of the atmospheric pressure and gauge pressure. (p.s.i.g.)

Atmospheric pressure. The pressure of air at sea level, usually 14.7 p.s.i.a. (1 atmosphere), or 0 p.s.i.g.

Gauge pressure (p.s.i.g.). Pressure measured by a gauge and indicating the pressure exceeding atmospheric.

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.

Special decompression chamber. A chamber to provide greater comfort for employees when the total decompression time exceeds 75 minutes.

Working chamber. The space or compartment under air pressure in which the work is being done.

C.F.R. Code of Federal Regulations.

MSHA. Mine Safety and Health Administration.

NIOSH. National Institute for Occupational Safety and Health.