Steel vs. aluminium
One choice shapes your buoyancy, your weighting and how the cylinder behaves from full to empty. Here’s how the two materials differ — and which role each one is built for.
Buoyancy is the whole story
Both materials get lighter in the water as you breathe the gas down. The difference is where they end up.
Steel stays negatively buoyant for the entire dive. From full to empty it becomes less negative, but it never flips positive — so the cylinder keeps helping to hold you down, and you carry less lead.
Aluminium starts negative when full and swings positive as it empties. For an S80, that’s roughly a +2 kg shift in salt water over the dive. You plan for it: a little extra lead, and the awareness that your trim changes as the bottle empties. That predictable swing is exactly what makes aluminium good as a stage or deco bottle you’ll hand off or drop.
What each is good at
Steel
- Negative buoyancy the whole dive
- Higher capacity, working pressures to 300 bar
- Less lead needed on the belt
- The standard for backmount & back gas
- Can rust — needs rinsing and boot checks
Aluminium (6061)
- Lighter, easy to travel with
- Corrosion-resistant and non-magnetic
- Swings slightly positive when empty
- Ideal as stage / deco / bailout
- Limited to 200 bar working pressure
The differences that matter
| Property | Steel (CrMo) | Aluminium (6061) |
|---|---|---|
| Buoyancy full → empty | Negative → less negative | Negative → positive |
| Working pressure | 200 / 232 / 300 bar | 200 bar |
| Capacity for size | Higher | Lower (thicker walls) |
| Corrosion | Rusts — needs care | Corrosion-resistant |
| Magnetic | Magnetic | Non-magnetic |
| Lead needed | Less | More |
| Best role | Back gas, backmount doubles | Stage, deco, bailout, travel |
Which one for your diving?
RECRecreational single
A 12 L steel cylinder is the standard — negative, high capacity, less lead.
TECHBackmount doubles
Steel as back gas: capacity, higher pressure and stable negative buoyancy.
STAGEStage & deco
Aluminium S80 / S63 — light to sling, positive swing managed as a known.
TRAVELWarm-water travel
Aluminium: no external rust, the global hire standard at warm-water centres.