Second Stage

The part of the regulator you hold in your mouth, which reduces intermediate hose pressure to ambient pressure and delivers gas each time you inhale.

The second stage is the piece of the regulator that goes in your mouth. It performs the final pressure reduction, taking the intermediate pressure supplied by the first stage and delivering gas at exactly the ambient pressure of the water around you, but only when you inhale. That on-demand design is why breathing from a well tuned second stage feels almost as easy as breathing on land.

Inside is a simple lever and diaphragm: inhaling flexes the diaphragm, the lever opens a valve, gas flows, and exhaling closes it again while your exhaled bubbles exit through the exhaust tee. A purge button on the front lets you clear water from the housing, which is exactly what you practice in regulator recovery and clearing drills during open water training.

Many second stages have adjustments such as a cracking effort knob or a venturi switch that prevents free flow at the surface. Your backup second stage, the octopus, is usually detuned slightly and colored bright yellow so it is easy to find and share in an out-of-air situation.