Five basic parts are usually included in a standard open water scuba diving regulator.
1. First Stage
The regulator first stage attaches the regulator to the scuba tank. Remember, a diving regulator reduces the air from the scuba tank in stages as it travels from the tank to the diver. The first stage of the regulator is named for its function: it accomplishes the first stage of pressure reduction by reducing the high pressure air in the tank to an intermediate pressure. The air travels through the low pressure (LP) regulator hoses at this intermediate pressure (perhaps they should actually be called "intermediate pressure hoses"!) Air at this intermediate pressure is still at too high a pressure to be breathed directly, and requires further reduction.
2. Primary Second Stage
The part of the regulator that a diver puts in his mouth is called the second stage. The regulator second stage is attached to the first stage by a low pressure hose. The name “second stage” comes from this part's function as the second stage of pressure reduction. It takes the intermediate pressure air from the regulator hose and reduces it to ambient pressure, a pressure equivalent to the air or water pressure surrounding a diver, allowing a diver to breathe from the second stage safely. The primary second stage is one of two second stages attached to a standard open water regulator, and is the second stage a diver normally breathes from during a dive.
3. Alternate Second Stage
The alternate second stage (also know as an alternate air source, buddy regulator, or octopus) does the exact same thing as the primary second stage: it reduces intermediate air pressure supplied by a low pressure hose to an ambient air pressure that a diver can breathe. The difference between the primary second stage and the alternate second stage is that the alternate second stage is a back-up. It enables a diver to share air from his tank with a second diver in case of an out-of-air emergency. Alternate second stages are usually bright colors, such as neon yellow, which allows them to be quickly located. As diver education and safety procedures have evolved, alternate second stages have become standard scuba diving safety gear, allowing any diver to breathe from any other diver's tank.
4. Submersible Pressure Gauge and Gauge Console
The submersible pressure gauge (also called a pressure gauge or SPG) allow a diver to monitor the amount of air in his scuba tank so that he doesn't run out of air underwater. The pressure gauge is connected to the regulator first stage by a high pressure hose (HP hose) that feeds high pressure air from the tank directly to the pressure gauge. Frequently, the console containing the pressure gauge also holds a variety of other gauges, such as a depth gauge, compass, or dive computer.
5. Low Pressure Inflator Hose
This low pressure hose carries intermediate-pressure air from the regulator first stage to the Buoyancy Compensator's (BC) inflator. This allows divers to add air to the BC from the tank at the touch of the button.