Scuba Regulators Explained: First Stage, Second Stage and Octopus

A scuba tank holds air at around 200 bar, far too high to breathe directly. The regulator reduces that pressure in two steps and delivers air to you on demand, only when you inhale, at the same pressure as the water around you. That's why breathing underwater feels natural at any depth.

The parts of a regulator set

First stage

The first stage attaches to the tank valve (by yoke or DIN) and reduces tank pressure to an "intermediate pressure" of about 9–10 bar above the surrounding water. It has several ports for hoses:

• High-pressure (HP) ports: for the pressure gauge (SPG) or an air-integration transmitter.
• Low-pressure (LP) ports: for the second stages, the BCD inflator hose and (if used) a drysuit hose.

Second stage (primary)

This is the part in your mouth. When you inhale, a diaphragm moves and opens a valve that releases air. When you exhale, air leaves through the exhaust valve and makes bubbles. It has a purge button to blast out water after the regulator has been out of your mouth.

Alternate air source (octopus)

A spare second stage on a longer hose, usually bright yellow, worn in the "triangle" between your chin and lower ribs. It's there for your buddy if they run out of air (and they carry one for you). See out-of-air procedures.

Some divers use an integrated inflator/alternate (such as an Air2-style unit) built into the BCD inflator instead of a separate octopus.

Submersible pressure gauge (SPG)

Shows how much air is left in your tank. It's often combined with a depth gauge and compass in a console. Many divers now use a wireless air-integrated dive computer instead of, or as well as, an SPG.

Regulator design choices

Feature	Options	What it means for you
Balanced vs unbalanced	Balanced first/second stages	Breathing stays easy at depth and as the tank empties. Most good modern regulators are balanced.
Piston vs diaphragm	Piston: simple, high airflow. Diaphragm: sealed from water	Both are excellent; diaphragm "environmentally sealed" models suit cold or silty water.
Cold-water rating	EN 250A tested for <10 °C	Needed for cold lakes, quarries and ice diving to reduce free-flow risk.
Adjustments	Breathing resistance knob, venturi (dive/pre-dive) switch	Helps prevent free-flow at the surface and fine-tunes breathing.
Weight	Travel regulators with lighter materials	Great if you fly to dive.

Regulators are designed to fail safe

A common fear is "What if my regulator stops working?" Modern regulators are designed so the most likely failure is a free-flow, a continuous stream of air, not a sudden stop. You can breathe from a free-flowing regulator by letting the excess air bubble out the side of your mouth while you ascend or switch to your octopus. You'll practise this in your Open Water course.

Choosing a regulator

1. Where will you dive? Warm water only, or cold water too? Choose cold-water rated if in doubt.
2. DIN or yoke? Pick the connection used in your main destinations; adapters exist.
3. Comfort: try the mouthpiece. Many divers swap to a comfort-bite mouthpiece.
4. Service network: choose a brand that can be serviced where you live and dive.
5. Weight if you travel a lot.

A complete set (first stage, second stage, octopus, gauge) typically costs US$500–1,500. Mid-range balanced regulators from major brands are excellent value.

Buy or rent?

Rent while you're learning. Buying your own regulator is usually a later purchase, once you dive regularly. Owning one means you know its history and it's always set up the way you like. See first gear to buy.

Care and servicing

• Keep the dust cap on when the regulator is off the tank, and dry it before replacing it.
• Rinse in fresh water after diving, ideally while still attached to a pressurised tank. Don't press the purge button while rinsing unpressurised, or water can get inside.
• Dry in the shade; don't hang it by the hoses for long periods.
• Service according to the manufacturer, typically every 1–2 years or after a set number of dives.
• Check hoses for bulges, cracks and leaks before every trip.

Frequently asked questions

What is the difference between the first and second stage?

The first stage attaches to the tank and reduces high pressure to an intermediate pressure. The second stage, in your mouth, reduces it again to the surrounding water pressure and delivers air only when you inhale.

What happens if my regulator fails underwater?

The most common failure is a free-flow (constant air). You can still breathe from it carefully or switch to your octopus or your buddy's, then end the dive with a controlled ascent.

How often should a regulator be serviced?

Follow the manufacturer's schedule, usually every one to two years, or sooner if it breathes hard, leaks or free-flows.