The regulator second stage is the part of the scuba diving regulator a diver breathes from. The function of the second stage is to reduce intermediate-pressure air traveling through a regulator hose to an ambient pressure (the pressure of the surrounding water) that a diver can breathe safely. A primary second stage is one of two second stages on a standard open-water-style regulator. Unless there is an emergency, a diver breathes from the primary second stage during a dive.
1. Purge Button
The purge button is located on the face of the regulator second stage. The purpose of the purge button is to flood the second stage with air, forcing water out of the second stage. Divers use the purge button when the second stage has been allowed to fill with water, for example when a diver removes the regulator from his mouth during the regulator recovery skill.
2. Ease of Breathing Adjustment
Most regulators have a lever or knob that allows divers to adjust breathing resistance. This feature helps to prevent regulator free flow (a state when air flows rapidly out of the regulator second stage without the diver breathing from it) which typically occurs when the breathing resistance has been lowered too much. A free flow can quickly empty a tank. Many second stage adjustments have a setting labeled "pre-dive" to help prevent free flow at the surface, and one labeled "dive" for easy breathing once underwater. As a diver descends, he can adjust the ease of breathing to compensate for the increased difficulty of breathing as he descends.
3. Exhaust Valve
The second stage exhaust valve is the plastic unit that channels exhaled air bubbles away from a diver's face. The exhaust valve is usually located below the regulator's mouthpiece to channel air down and to the sides. This helps to keep a diver's field of vision clear of bubbles.
The mouthpiece is the part of the regulator that a diver bites down on. High quality mouthpieces are made of silicon or soft rubber (not plastic), and come in a variety of shapes and sizes to fit divers' mouths. Mouthpieces are removable and replaceable. A diver should check that his mouthpiece is secured to the regulator second stage with a zip tie or cable tie to ensure that it does not slide off during a dive.