Regulator

The device that reduces high-pressure gas from a scuba tank to breathable pressure, delivering air on demand each time the diver inhales.

The regulator is what turns the compressed gas in your scuba tank into something you can actually breathe. Tank pressure can exceed 200 bar (about 3000 psi), far too high to inhale directly, so the regulator steps it down in two stages and delivers gas only when you inhale, which is why it is also called a demand valve.

A complete regulator set includes the first stage that attaches to the tank valve, the primary second stage you breathe from, an octopus as a backup, a submersible pressure gauge or transmitter, and a low-pressure inflator hose for the BCD. The first stage drops tank pressure to an intermediate pressure roughly 8 to 10 bar above ambient, and the second stage reduces that to exactly the surrounding water pressure so breathing feels natural at any depth.

Modern regulators are extremely reliable when serviced on schedule, typically every year or two depending on the manufacturer. Rinse yours in fresh water after diving, keep the dust cap on, and note any free flow, hard breathing, or wet breathing so a technician can address it before your next trip.