Scuba Tank Sizes: Aluminium 80 vs Steel, Weight and Buoyancy

How scuba cylinders are rated: litres and bar, or cubic feet

Two naming systems describe the same object. A metric cylinder is named by the water it holds when empty and the pressure stamped on its shoulder: 12 L, 232 bar. A US cylinder is named by the air it delivers at the surface, in cubic feet, so the two names never line up.

One sum links them. Gas at the surface (L) = water capacity (L) × working pressure (bar) ÷ 1.01325, and one cubic foot is 28.316846592 L. A 12 L at 232 bar comes out at 2,748 L, or 97.0 cu ft, which is more air than an aluminium 80 and less than a steel HP100. Our dive unit converter runs it both ways, with the common sizes as presets.

The formula treats air as an ideal gas, and real air resists compression more as the pressure climbs. Luxfer's own sheet rates the AL80 at 77.4 cu ft, about 3% under the 80.0 the formula gives for 11.1 L at 3,000 psi. Catalina prints the same figure with a footnote saying the industry-standard S80 has an actual capacity of 77.4 cu ft. At Faber's 3,442 psi the gap widens to 5%. For comparing sizes the formula is close enough; for planning gas, use the maker's rating.

Common cylinder sizes side by side

Every number below comes off a manufacturer specification sheet. Dry weight is the cylinder alone, without a valve. Buoyancy is measured in sea water with the valve on, and a minus sign means it sinks. Luxfer publishes full and 35 bar (500 psi) figures; Faber publishes full and empty.

Two sizes that look alike are worth spotting. An aluminium 80 and a European 10 L steel at 232 bar hold almost the same air, 80.0 against 80.9 cu ft by the formula, and the steel one is 1.3 kg lighter to carry. The aluminium is the one that floats at the end.

The European versions of the steel sizes are stamped 232 bar rather than 237, and Faber's own table shows how much one nominal size varies. A 10 L at 232 bar is 11.5 kg built to EN ISO 9809-1 and 12.2 kg to EN 1964-1; a 12 L runs 12.8 kg to 14.5 kg, a 15 L 16.5 kg to 18.2 kg. That table publishes no buoyancy at all, which is why the steel rows below are the US 237 bar models.

Makers also disagree about the same cylinder. Catalina's S80 shares the 11.1 L capacity and 77.4 cu ft rating, but reads -0.73 kg (-1.6 lb) full and +1.86 kg empty, where Luxfer stops at 35 bar with +1.54 kg. On the 40, Luxfer lists 6.9 kg dry against Catalina's 7.3 kg. The gaps are under half a kilo, so they move the last weight on your belt, not the plan.

Why an aluminium cylinder floats when it is nearly empty

Because the hole it makes in the water never changes, while its weight falls by the mass of the gas you breathe. Whether the cylinder ends the dive positive or negative depends on where it started, and aluminium starts much closer to neutral.

The reason is the metal. Aluminium is weaker than steel, so the wall has to be thicker to hold the same pressure, and the cylinder ends up bulkier and heavier for the air it carries. DAN puts the practical result plainly: steel cylinders tend to be negatively buoyant full or empty, while aluminium ones often start negative and become positive as the diver uses the gas.

The size of the swing is the weight of the air, and the sheets let you check it. Luxfer's AL80 moves from -0.80 kg full to +1.54 kg at 35 bar, a change of 2.34 kg (5.2 lb), and over that pressure drop the converter says it gives up 1,884 L of air. That implies 1.24 g per litre. Faber's HP100 changes 3.66 kg from full to empty across 3,017 L, or 1.21 g per litre. Two makers, two metals, the same gas.

Steel staying heavy is a tendency, not a rule. Faber's own figures put the HP120 at +0.29 kg empty and the HP133 at +0.66 kg, so the largest steels float at the end of a dive too, and Catalina's small aluminium S30 is still -0.09 kg empty. Look up the cylinder you are actually diving.

How the cylinder changes your lead and your trim

Switching from a rental aluminium 80 to a steel 12 L takes roughly 2.5 kg off your belt. Take an 80 kg diver in a 5-7 mm full wetsuit in the sea, with 20 kg of gear. Our scuba weight calculator gives the wetsuit 8 kg, then adds or subtracts the cylinder's buoyancy when empty:

For the Faber 232 bar sizes the calculator shows 7.3-7.8 kg rather than one number, because the two published buoyancy tables it works from differ by about half a kilo. Every figure here is a starting point that the in-water check settles. The full method is in our guide to how much weight you need.

Trim changes too, and in the same direction. Steel carries its ballast high on your back instead of low on your hips, which makes horizontal trim easier to hold. An aluminium cylinder does the opposite near the end: the tail goes light exactly at the safety stop, where you least want your feet drifting up. Divers who fin up at 5 m on the last 30 bar are often feeling the cylinder, not their own technique.

• Aluminium AL80: +2.0 kg buoyant when empty, so 10 kg of lead.
• Steel 12 L at 232 bar: about 0.5 kg negative when empty, so 7.5 kg.
• Faber HP100 steel: 0.15 kg negative when empty, so 8 kg.
• Steel 15 L at 232 bar: about 0.7 kg negative, so 7.5 kg again, because half a kilo rounds away.

How much bottom time does each size give?

At 18 m (60 ft) with a 20 L/min SAC and a 50 bar (725 psi) reserve, the spread runs from 15 minutes on a 40 to 51 on a Faber HP120. The table is our tank duration calculator with the maker's water capacity and working pressure entered for each cylinder.

The pairs are the interesting part. An aluminium 80 and a 10 L steel at 232 bar come out one minute apart, because they hold almost the same air. Moving from that aluminium 80 to a 12 L steel buys 8 minutes at 18 m, and to a 15 L, 17 minutes. These are minutes at depth only: descent, ascent and the stop come out of the reserve, and a shared-gas ascent from 30 m needs more than 50 bar. Our article on how long a scuba tank lasts works that through by depth and SAC rate.

A bigger cylinder does not fix a high breathing rate, it just postpones the turn. It also adds the weight you carry up the ladder: a full 15 L steel at 232 bar is 18.2 kg of metal plus about 4.2 kg of air, against 14.3 kg plus 2.8 kg for an aluminium 80.

DIN or yoke, 232 bar or 300 bar

The valve decides whether your regulator fits. A yoke fitting clamps over the face of the valve and seals against an O-ring set in the valve. A DIN fitting screws into the valve, and the O-ring sits in the regulator instead. Apeks calls its DIN connections by their standard, ISO 12209:2013, and sells its Auto Closure Device in two versions, DIN 300 bar and yoke 232 bar. Those are the pressures each fitting is built for.

That is why a 300 bar cylinder is a DIN cylinder. Apeks warns that adaptors from a yoke first stage to a DIN cylinder valve must never be used, because they would let the yoke connector see a higher working pressure than it is designed to take. The other direction is fine and is how travelling divers cope: a DIN first stage plus the maker's converter dives a yoke valve.

DAN calls the aluminium 80 the standard cylinder, and at its 3,000 psi rating a yoke valve is enough, which is what most US and Caribbean shops fit. European and technical centres run DIN. Ask before you pack, and check the O-ring: Apeks says to inspect the valve O-ring on a yoke cylinder, the fitting's sealing O-ring on a DIN one, and to replace a worn one before mounting the regulator.

Inspection and test rules: the US and Europe do not match

In the US the interval is written into law. Title 49 of the Code of Federal Regulations requires DOT 3AL and 3AA cylinders to be requalified every five years, tested to 5/3 of their service pressure. Scuba cylinders made from 6351-T6 aluminium alloy get an eddy current examination combined with a visual inspection, at the same five-year interval. Luxfer used the 6351 alloy up to and including 1988 and 6061 since 1989.

The annual visual inspection that every US dive shop asks for is industry practice on top of that. Luxfer's inspection guide says a visual inspection should take place at least once every twelve months, and every four months in heavy service, which it defines to include rental cylinders in use during the season and off-season, and cylinders topped off five or more times a week.

Europe works from a standard rather than a fixed number. BS EN ISO 18119 covers the periodic inspection and testing of steel and aluminium alloy cylinders, and HSE says any cylinder filled by a person at work must be inspected and tested by a competent person, with the periodicity taken from the Diving Industry Committee's risk assessments. That document quotes the standard: without local regulation, an internal examination every year and a test every five years, and the internal examination can stretch to a maximum of 2.5 years where a risk assessment shows a low risk of internal degradation. UK recreational diving has used 2.5 and five years for the last decade.

Two rules catch owners out. Cylinders over ten years old carried by road for work need inspection by a body appointed by the Vehicle Certification Agency, every ten years after manufacture. And water is the enemy: HSE says a cylinder suspected of taking water in should leave service until a competent person has inspected it, and Luxfer tells owners never to discharge a cylinder completely, because that is how water seeps in.

Why you fly without a cylinder, and what to check in a rental

A filled cylinder is cargo nobody wants. The FAA's PackSafe guidance treats a scuba cylinder as hazardous material once it reaches 200 kPa (29 psig). An empty one, or one below that pressure, is not restricted, although airlines and screening officers may require the valve to be opened all the way for inspection. Emptying a cylinder to fly it also breaks Luxfer's rule about never discharging a cylinder completely, so this is the one item almost everybody rents.

So spend a minute on the rental cylinder. These are the ones Luxfer classes as heavy service, and the stamps and stickers say whether the schedule has been kept.

Note the metal before you pick your lead, because the difference is 2 kg or more. Then read the gauge instead of trusting the promise of a full fill: an aluminium 80 handed over at 180 bar is not the 31 minutes in the table above.

• Read the neck stamps: specification, service pressure and the last test date.
• Find the visual inspection sticker and the test label, and check the next date has not passed.
• Check the valve type against your first stage, DIN or yoke, and the O-ring on whichever one seals.
• Open the valve and listen and smell: leaks and any odd odour are reasons to swap the cylinder.
• Look at the boot and the base for rust streaks on steel, and dents or deep gouges on either metal.
• Read the gauge with the regulator on, and plan on the pressure you actually have.