Today, with a few improvements and refinements, we still use the aqualung of 50 years ago. The aqualung, or scuba (Self Contained Underwater Breathing Apparatus) gear, is made up of an air cylinder, regulators, and a harness.
High-pressure cylinders of steel or aluminum alloy contain the air supply. The cylinders are pressurized to about 3000 psi, equivalent to 100 times the pressure of a car tire!
Two vital parts of the scuba gear are the regulators, which reduce the air pressure to a breathable level in stages. Attached to the valve of the cylinder is the first stage of the regulators; this drops the pressure down to about 140-psi. It then transfers the air over to the second stage via a high pressure hose. Connected at the mouthpiece, the second stage reduces the pressure to that of the surroundings.
There are two kinds of first stage regulators, the diaphragm and the piston. Both types utilize water pressure and a spring to push the valve open. When the airflow equals intermediate pressure, the pressure from the tank will close the valve.
The second stage regulator is shaped like a cup with a flexible diaphragm on the top and a mouthpiece connected to the bottom. When the diver inhales, it creates a vacuum inside the cup. Lowered pressure pulls in the diaphragm, moving a lever that causes the valve to open. This lets air flow from the high-pressure hose into the regulator. When inhalation ceases, a buildup of pressure returns the diaphragm to normal position, closing the valve and stopping the flow of air. Exhaling increases pressure inside the mouthpiece and opens the purge valve, allowing used air to escape.