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01 / 07 — Life-support stack

Scuba Cylinder

Your entire atmosphere, compressed into a backpack.

The cylinder stores breathing gas at 200–300 bar — two to three hundred times the pressure at the surface. Everything else in the kit exists to make that stored gas breathable, measurable and controllable.

The pressure you plan to surface with

50bar

Working pressure
200–300 bar
Common size
11.1 L (AL80)
Gas inside
~2,300 L at 207 bar
Full weight (AL80)
~18 kg

The pressure vessel, part by part

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Scuba Cylinder. This 3D plate needs WebGL; the parts list below carries the same content.

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FIG 1.1 — Aluminum, Assembled

Scale: the bar is 10 centimetres on the model.

Try the variations

Material

Toggle the material to see how size, weight and buoyancy shift — it changes how much lead you wear.

01Cylinder valve

The gatekeeper. A handwheel lifts a seat off its seal, letting 207-bar gas out through the outlet to the regulator. A burst disc in the valve body — rated near the cylinder’s hydrostatic test pressure — vents a grossly overfilled or fire-heated tank instead of letting it rupture.

Parts list5 parts

  1. Cylinder valve

    The gatekeeper. A handwheel lifts a seat off its seal, letting 207-bar gas out through the outlet to the regulator. A burst disc in the valve body — rated near the cylinder’s hydrostatic test pressure — vents a grossly overfilled or fire-heated tank instead of letting it rupture.

  2. Neck & threads

    Precision-machined threads mate the valve to the cylinder. O-rings make the seal — the single most common source of a slow leak, and the first thing a divemaster checks.

  3. Shoulder

    The dome that distributes stress from the cylindrical body into the neck. Stamped nearby: alloy, working pressure, serial number and hydrostatic test dates — the tank’s passport.

  4. Pressure vessel

    A seamless aluminum or steel shell engineered so the hoop stress from 207 bar stays well inside the metal’s elastic limit. Aluminum walls are thicker because the alloy is softer; steel is thinner, denser, and stays negatively buoyant even when empty.

  5. Tank boot

    A rubber base that lets round-bottomed aluminum cylinders stand upright and protects the base from dings. Many steel tanks have a flat base instead and skip it.

Two hundred atmospheres, one handwheel

  1. Air is squeezed 200×

    A compressor pushes filtered air in until the gauge reads 207 bar (3,000 psi). An 11.1-litre AL80 now holds about 2,300 litres of surface air. Keep 50 bar in reserve and breathe a relaxed 18 L/min, and that is 97 minutes at the surface — but 48 at 10 m and 32 at 20 m.

    Boyle's law

    At 10 m the ambient pressure is 2 bar — double the surface — so each lungful pulls twice as many molecules from the tank. Your gas lasts half as long at 10 m and a third as long at 20 m. Pressure, not fitness, is why gas planning exists.

  2. The valve meters release

    Opening the handwheel lifts a seat off its seal. Gas flows to the regulator’s first stage, but the full 207 bar never reaches you — taming it is the regulator’s job. Open the valve all the way: a valve cracked a quarter-turn breathes fine at the surface and starves you at depth, when every breath needs three or four times the gas.

    Dalton's law & nitrox

    The pressure of a gas mix is the sum of the partial pressures of its parts. Air at 30 m (4 bar) delivers 0.84 bar of oxygen and 3.16 bar of nitrogen. Nitrox swaps some nitrogen for oxygen — EAN32 is 32% O2 — so you load less nitrogen and get a longer no-stop time. The trade: oxygen turns toxic above about 1.4 bar, so every mix has a maximum operating depth, MOD = (1.4 ÷ FO2 − 1) × 10 m. For EAN32 that is 33 m; for EAN36, 28 m; for air, 56 m.

  3. Pressure falls as you breathe

    Every breath drains the reserve. Your SPG or computer tracks the drop, and the rule is to be back on the surface with 50 bar still in the tank. That is not spare gas — it is the margin for a buddy who runs out, a current, or a slow ascent.

    Buoyancy swing

    A tank is the one piece of gear that changes weight during the dive. An AL80 swings ~2.7 kg towards positive as you breathe it down — so your weighting has to hold you at the safety stop with a near-empty cylinder, not just at the start.

  4. The tank changes buoyancy

    Compressed air has real mass — about 2.7 kg in a full AL80. Breathe it down and an aluminum tank goes from about 0.9 kg negative to 1.8 kg positive. A steel HP100 swings the same way, but starts so negative (−5.4 kg) that it still sinks when empty (−2.0 kg).

Breathe a tank down

Dive profile — one timeline, seven instruments

T+12 minAL80 · SAC 18 L/min · 5 mm suit

Cylinder

136bar

Wing

1.6L

Suit

2.5mm

Mask air

113mL

NDL left

10min

Ceiling

0m

Ascent

0.0m/min

Depth 30.0 m · ambient 4.00 bar. Drag the needle, or use the arrow keys.

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A scuba cylinder emptying, with its buoyancy arrow. This view needs WebGL.

The cylinder gets lighter as you breathe it

Buoyancy & gas

Air weighs something

Drain the cylinder and the buoyancy arrow swings: an aluminium tank ends the dive wanting to float, steel stays sinky. Move the diver deeper and the same gas buys fewer minutes, because every breath at 2 bar takes twice the molecules.

Challenge

Breathe the aluminium AL80 down to its 50-bar reserve and read which way the arrow points.

Cylinder

207bar
18m
18 L/min

Buoyancy

−0.9kg

Time at depth

35min

Gas aboard

2.7kg of air

Working pressure

207bar

At 18 m you breathe 50 litres of free gas a minute, so the 1743 usable litres aboard last 35 minutes — and the empty cylinder is +1.8 kg buoyant when you get there.

Takeaway — locked

Drain an aluminium AL80 to 50 bar to unlock what this proves.

11.1 L · 207 bar · 2.7 kg of gas. Buoyancy figures are typical; makers differ by about a kilogram.

Aluminum, steel, or a second bottle

You rarely choose your first cylinder — the rental fleet chooses for you, and it is almost always an AL80. The choice arrives when you buy: steel trades a bigger price tag and rust-vigilance for more gas and less lead; a pony is not a bigger tank but a second, independent one.

Aluminum (AL80)

What it is
The world’s rental fleet standard. Thick walls, soft alloy, very corrosion tolerant.
For
  • Cheap and widely available
  • No rust: the aluminum oxide skin is its own protective layer
  • Every AL80 fills, weighs and floats the same, so your weighting carries over between rentals
Against
  • Goes positively buoyant when empty (+1.8 kg)
  • Bulkier for the same gas volume
  • Lower pressure rating (207 bar)
  • Softer alloy — dents easily, and a deep dent fails the visual inspection
Best for
Warm-water recreational diving, travel, rental fleets.

Steel (HP100)

What it is
High-pressure steel holds more gas in a similar-sized, heavier shell that stays negative all dive.
For
  • About a third more gas than an AL80 in a similar-sized shell
  • Stays negative all dive (−5.4 kg full, −2.0 kg empty) — carry less lead
  • Higher pressure ratings (232–300 bar)
Against
  • Rusts inside if moisture gets in
  • More expensive
  • Heavier on land (~19 kg full)
Best for
Cold water, wreck and technical diving where gas volume and trim matter.

Pony / stage bottle

What it is
Small 2–6 L cylinder (13–40 cu ft) with its own regulator — a fully independent bailout gas supply.
For
  • True redundancy, independent of the main set
  • Compact, clips to your harness
Against
  • Extra weight and clutter
  • Another reg to maintain
  • Tempts people into dives they should not be doing alone
Best for
Solo-leaning divers, deep or overhead-adjacent recreational dives.

What fails, and what you do

What the cylinder can do to you

A positively buoyant aluminum tank can float you off your safety stop at the end of a dive if you are under-weighted — do your weight check with a near-empty tank, not a full one.

Steel’s constant negative buoyancy lets you drop lead from your belt, improving trim and comfort.

A burst disc in the valve is rated near the cylinder’s hydrostatic test pressure (about 1.5–1.7× working). It protects against gross overfilling and fire — not a warm car park, where a full tank gains only ~30 bar. When it goes it empties the cylinder in a minute or so: safe compared with a rupture, not gentle.

Hydro stamps matter: a cylinder out of test cannot legally be filled, and internal corrosion is invisible from outside — the annual visual inspection exists to find it.

Where this chapter connects