Air lock is a chamber with two airtight doors. It is used on submarines and spacecraft and in underwater construction. One door opens to the outside environment. The other door opens to a structure called a pneumatic caisson. The caisson contains compressed air. Caissons cannot be opened directly to the outer environment because air loss will occur. All movements to and from the caisson must take place through an air lock. The pressure in the air lock is regulated, depending on whether people or materials are entering or exiting the caisson. Separate air locks are usually provided for people and for materials.
To enter the caisson, a person first steps into the air lock. Compressed air holds the inner door of the air lock tightly shut. The outer door is then closed, and air with a pressure equal to that of the caisson is slowly introduced. When the pressures within the air lock and the caisson are equal, the inner door is opened and the person can enter the caisson. When exiting the caisson, a person moves into the air lock and the pressure in the lock is made to match the pressure outside. When they are equal, the outer door is opened and the person can leave. Lowering of the air pressure, or decompression, must be done slowly. If the pressure drops too rapidly, the person may develop a painful and dangerous condition called bends.
