Typical recreational scuba divers breathe either compressed air (78 percent nitrogen, 21 percent oxygen) or an oxygen-enriched, nitrogen-oxygen combination called Nitrox (64 to 68 percent nitrogen, 32 to 36 percent oxygen). The gas is contained in a cylinder that you carry on your back. The typical cylinder is made of aluminum, weighs about 31 pounds (14 kg) empty and holds 80 cubic feet (2,265 L) of air at 3000 pounds per square-inch (psi), or 204 atmospheres (ATM). This volume of gas would approximately fill a phone booth and weighs about 7 pounds (3.2 kg).
Scuba gas cylinders
Scuba regulator, showing second stage (left) and first stage (right)
You cannot breathe directly out of the tank because the high pressure would damage your lungs. Therefore, the cylinder is fitted with a regulator. The regulator does two things: It reduces the pressure from the tank to a safe level for you to inhale, and it supplies air on-demand. To accomplish these tasks, regulators have two stages:
- First stage - The first stage attaches to the cylinder. It reduces the pressure from the tank (3000 psi or 204 ATM) to an intermediate pressure (140 psi or 9.5 ATM).
- Second stage - The second stage is connected to the first stage by a hose. It reduces the pressure from the intermediate pressure to ambient water pressure (such as 1 to 5 ATM depending upon depth). The second stage also supplies air, either only when you inhale (typical operation) or continuously (emergency operation).
The first stage consists of high-pressure and intermediate-pressure chambers, separated from each other by either a valve-diaphragm combination or a piston, which is in contact with the ambient water pressure. The high-pressure chamber receives air directly from the cylinder, while the intermediate-pressure chamber is in contact with the ambient water pressure through the diaphragm or piston. The system operates like this:
- You inhale, thereby lowering the pressure in the intermediate-pressure chamber to below the ambient water pressure.
- The water pressure pushes inward, opening the valve or piston.
- The open valve connects the high-pressure chamber with the intermediate pressure chamber.
- Air flows from the high-pressure chamber into the intermediate-pressure chamber, thereby increasing the pressure in the intermediate-pressure chamber.
- When the pressure in the intermediate-pressure chamber equals the ambient water pressure, the valve or piston closes.
- The process repeats when you inhale again.
Operation of a regulator's first stage
The first stage usually has several ports with hoses that lead to the second stage as well as to other devices, such as an additional second stage, tank-pressure gauge and/or buoyancy control device (BCD -- more on this later).
Inside a regulator's second stage
The regulator's second stage consists of:
- Plastic chamber with an outer rubber diaphragm that is in contact with ambient water pressure
- Purge button
- Inner valve that is connected to a movable lever
- Exhaust valve
The second stage is connected by a hose to the intermediate-pressure chamber of the first stage. This is how the second stage operates:
- You inhale, thereby lowering the pressure within the second stage to below the ambient water pressure.
- The water pressure presses in on the diaphragm membrane and moves the lever.
- The lever's movement opens the inlet valve. This allows air to flow into the second stage from the first stage, and into your lungs through the mouthpiece.
- When you exhale, the pressure in the second stage exceeds the ambient water pressure and pushes out on the membrane.
- The membrane moves away, allowing the lever to return to its normal position and thereby closing the inlet valve.
- The increased second-stage pressure opens the exhaust valve and allows the exhaled air to leave the second stage.
- When you inhale again, the exhaust valve closes and the process repeats.
Operation of the regulator's second stage
The regulator must be cleaned with freshwater after each dive to eliminate salt water, silt and debris that would prevent the movements of the various valves and membranes and corrode the parts. Regulators should also be serviced at least once per year. Because the regulator is one of the most important pieces of equipment, many divers choose to purchase their own regulators (instead of rent) so that they can be confident that the regulator is in good working order and has been properly maintained.
The final parts of the breathing apparatus are accessories that contain emergency or alternate air supplies. They include the following:
- Pony tanks - These are smaller cylinders that strap onto the main cylinder. Pony tanks contain air and have their own regulators. They provide enough air for many emergency situations, such as an ascent from a fairly deep depth.
- Spare air unit - The spare air unit has the regulator built directly into the on/off valve. It is lightweight and can be carried in the pocket of a BCD. It is designed to provide only enough air to allow you to ascend from a shallow depth.
- Snorkel - This is a small, J-shaped, lightweight breathing tube with a mouthpiece on one end. It attaches to your mask. When at the surface, the snorkel allows you to breathe outside air when you are swimming face-down, thereby conserving tank air.