Scuba equipment

In short

A scuba unit combines a cylinder of compressed air, a regulator that delivers this air at ambient pressure and a buoyancy control device (BCD) that adjusts your buoyancy. Mask, fins, exposure suit and weights complete the equipment. Knowing what each part does, how to assemble them and how to check them before every dive is the first skill of a diver. This guide describes the equipment in general terms, without recommending any brand: your club or dive centre will show you the model you will use.

Why it matters

Underwater, your equipment gives you air, vision, propulsion and stability. A valve that is not fully open, a disconnected inflator or a loose strap is enough to turn a nice dive into a stressful one. Understanding your equipment lets you set it up correctly, spot a problem before entering the water and react calmly if something does not work as expected.

The cylinder

The cylinder (or tank) holds air, or another mix such as Nitrox, compressed at high pressure.

• Common sizes: 10, 12 or 15 litres for recreational diving; 12 litres is the most common in clubs.
• Working pressure: usually 200 or 230 bar, sometimes 300 bar. A 12-litre cylinder filled to 200 bar therefore holds about 2,400 litres of air at atmospheric pressure.
• Steel or aluminium: steel is heavier and becomes only moderately more buoyant as it empties; aluminium, bulkier for the same volume, becomes clearly more buoyant as it empties, which affects your weighting (see the weighting guide).
• Markings: stamped on the shoulder, they show the manufacturer, the volume, the working and test pressures and the inspection dates. A cylinder whose inspection is out of date must not be filled.

The valve

The valve connects to the regulator in one of two ways:

• Yoke (INT): the regulator's yoke clamps onto the valve's O-ring; simple and very common when travelling.
• DIN: the regulator screws directly into the valve, which protects the O-ring better at high pressures. Adapters let you switch from one system to the other.

A twin-outlet valve lets you fit two independent first stages: if one free-flows, you close its outlet and breathe from the other. It is recommended, or even required, in cold water and for some dives.

The regulator

The regulator reduces the air pressure in two stages:

1. The first stage, attached to the valve, lowers the cylinder's high pressure to an intermediate pressure (about 10 bar above ambient pressure).
2. The second stage, the mouthpiece you hold in your mouth, delivers air at ambient pressure, only when you breathe in.

The first stage also feeds:

• an alternate second stage (often called an octopus), brightly coloured, meant for a buddy who runs out of air;
• the low-pressure inflator hose, which supplies the BCD inflator;
• the pressure gauge (high pressure), which shows the pressure left in the cylinder, or a transmitter that sends it wirelessly to a compatible computer.

The buoyancy control device

The BCD lets you adjust your buoyancy by adding or releasing air.

• Jacket BCD: the air volume surrounds the torso; it is comfortable at the surface and common in clubs.
• Wing: the volume is concentrated on the back, which helps you stay horizontal; it is mounted on a backplate and harness.
• The inflator fills the BCD with air from the cylinder or by mouth; several dump valves (upper, lower, inflator) release air depending on your position.

The rest of the equipment

• Mask: it must fit your face without leaking; it covers the nose so that you can equalise.
• Fins: full-foot in warm water, open-heel with boots elsewhere.
• Snorkel: useful at the surface to save the air in your cylinder.
• Exposure suit: see the guide on thermal protection.
• Weights: see the guide on weighting.
• Computer: see the guide on dive computers.

Assembling and disassembling the unit

1. Check that the cylinder is filled, its inspection up to date and the valve O-ring in good condition.
2. Briefly open the valve to blow out water or dust, pointing it away from yourself and others.
3. Slide the BCD over the cylinder, valve facing the back of the BCD, and tighten the strap at the right height (top of the valve roughly level with the top of the BCD). Wet the strap before tightening: a dry strap loosens in the water.
4. Fit the first stage, primary second stage on the right, without forcing.
5. Connect the inflator hose to the BCD.
6. Open the valve slowly and fully while watching the gauge.
7. To disassemble: close the valve, purge the hoses, disconnect the inflator hose, then remove the regulator and put the dust cap back on the first stage.

Lay the assembled unit down whenever nobody is wearing it: a standing cylinder falls over easily.

Checks before the dive

Before entering the water, do these checks yourself, then cross-check with your buddy:

• Air: valve open, enough pressure, three or four breaths on each second stage while watching the gauge needle, which must not move.
• BCD: the inflator inflates, the dump valves work.
• Weights: in place and releasable with one hand.
• Straps and fasteners: tight, nothing dangling.
• Mask, fins, computer: within reach and switched on.

Key takeaways

• A scuba unit combines a cylinder, a two-stage regulator and a BCD.
• The alternate second stage is for a buddy who runs out of air.
• Wet the BCD strap before tightening it and open the valve fully.
• Check air, BCD and weights, then cross-check with your buddy.
• Doubtful equipment stays out of the water.

Further reading

The equipment care guide explains how to keep this equipment in good condition; the guides on weighting and thermal protection detail the settings that depend on your suit. An illustrated equipment chart is planned: it will be added from the guide image library.