Parts of a Scuba Regulator – First and Second Stage

All 15 parts on the diagram

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The whole set 8

First stage It takes the cylinder's high pressure down to a steady middle pressure. It sends the air along the hoses.
Second stage (primary) It gives air only when the diver breathes in. It takes the air down again, to the pressure of the water around the diver.
Octopus (alternate second stage) It lets a second diver breathe from the same cylinder. It is also called the secondary demand valve.
Submersible pressure gauge It shows how much pressure is left in the cylinder. That tells how much air is left. Divers call it the SPG or contents gauge.
Yoke It fits round the valve and its screw is tightened by hand. The other standard fitting, called DIN, screws into the valve instead.
Low-pressure hose It carries middle-pressure air to the part the diver breathes from. It is also called the intermediate pressure hose.
High-pressure hose It is joined to the high-pressure port of the first stage. The gauge then reads the full pressure in the cylinder.
Inflator hose It plugs into the inflator of the buoyancy compensator. That is the vest that lets a diver float or sink.

Second stage, cut through 7

Mouthpiece The diver breathes in and out through it.
Purge button Pressing it pushes the diaphragm in by hand. That opens the valve and lets air flow without breathing in.
Diaphragm When the diver breathes in, the pressure in the case drops. The diaphragm then bends inward, pressing on the lever.
Demand lever The diaphragm pushes it down. It lifts the valve off its seat so air comes in.
Valve seat A spring holds the valve shut on it. That lasts until the lever lifts the valve off.
Exhaust valve Breath going out lifts it. It shuts again so that no water comes in.
Exhaust tee It protects the valve and sends the bubbles out to the sides. That is away from the front of the diver's face.

How this list was compiled: Part names checked on 2026-09-30, a name kept only when two of these use it for the part. (1) NOAA Diving Manual, 1991 edition, section 5.1.1 Demand Regulators and the equipment lists of later sections, a US government work, full text at https://archive.org/details/noaadivingmanual00unit (first and second stage, intermediate pressure hose, mouthpiece, diaphragm, valve seat, purge button, exhaust valve and ports, octopus second stage, submersible pressure gauge on a high-pressure hose, inflator hose). (2) Wikipedia, text only: Diving regulator (first stage, second stage, yoke clamp and DIN fitting, low pressure hose, high pressure hose, submersible pressure gauge, low pressure inflator hose, octopus, mouthpiece, purge button, diaphragm, lever, valve seat, exhaust valve, exhaust tee). (3) A regulator maker's assembly and maintenance guide, full text at https://archive.org/details/manualsbase-id-657679 (first and second stage, yoke, dust cap, mouthpiece, purge, diaphragm, lever, seat, exhaust valve, exhaust tee). Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry, a composite of no one make; nothing traced. Licence: own work

How it works

A scuba regulator takes the high-pressure air in the cylinder down to the pressure of the water around the diver, and gives it only when the diver breathes in. These drawings name 15 parts. They are not dive training.

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The set, laid out

The first stage fastens to the cylinder valve with a yoke. Hoses run from it to the second stage the diver breathes from, to a spare second stage called the octopus, to the inflator of the buoyancy compensator, and to the submersible pressure gauge.

Inside a second stage

The second drawing is a second stage cut through. Breathing in lowers the pressure in the case, so the diaphragm bends inward and presses the lever, which lifts the valve off its seat and lets air in. Breathing out lifts the exhaust valve, and the exhaust tee sends the bubbles out to the sides.

The pressure gauge is on the one high-pressure hose; the other three hoses carry air at a lower, middle pressure.
This page only names the parts. Learning to use a regulator belongs in a dive course.

Questions and answers

What are the parts of a scuba regulator called?

The first stage on the cylinder, the second stage in the mouth, and the hoses between them. Most sets add a spare second stage called the octopus, an inflator hose and a submersible pressure gauge on a high-pressure hose.

What is the difference between the first stage and the second stage?

The first stage is mounted on the cylinder valve and lowers the cylinder pressure to a steady middle pressure. The second stage has the mouthpiece; it lowers the pressure again to that of the surrounding water, on demand.

What is the octopus on a scuba regulator?

A second second stage on its own hose, joined to the same first stage. It is there so that another diver can breathe from the cylinder as well. It is also called a secondary demand valve.

What does the purge button do?

Pressing it pushes the diaphragm in by hand. That opens the valve, so air flows through the second stage without the diver breathing in.