What is the difference between steel and aluminium scuba tanks? Divers never give much thought to what their air cylinder is made of, but it can affect your dive.
Cylinders are made out of two types of material: steel and aluminium
Aluminium is softer than steel. Aluminium tanks must have thicker walls than steel tanks to hold air at a comparable pressure. Aluminium is softer than steel, it scratches and dents more easily. An aluminium cylinder is larger and heavier than a similar capacity steel cylinder. They also don’t handle overfilling nearly as well. Aluminium tanks are far more tolerant of corrosion from salt water.
Steel tanks may rust in the presence of moisture. They are more likely to be damaged by improper fills containing moisture than aluminium tanks, and may require periodic tumbling (a process which removes oxidation from the inside of the tank).
Buoyancy Characteristics of Steel and Aluminium Tanks
As a diver empties his tank by breathing from it, the tank becomes lighter. Aluminium has a particularly annoying characteristic; it becomes positively buoyant as they are emptied while steel tanks only become less negatively buoyant as they are emptied. Whether a diver dives with a steel or an aluminium tank, he must compensate for the increased buoyancy of his tanks near the end of a dive. A full aluminium cylinder will sink while an empty will float. This makes it harder to pin down a perfect weighting for the entire length of a dive.
This is why during a proper buoyancy check it is recommended to use a near-empty cylinder, or add weight to compensate. The usual recommendation is to add about 2 kg to your base weighting to compensate for an aluminium cylinder.
Durability of Steel vs Aluminium Tanks:
When properly cared for, steel tanks generally last longer. Steel is a harder metal than aluminium, and is less likely to pit or dent. Steel may rust, but with proper care most rust can be avoided. Any rust discovered during a visual inspection can be removed by tumbling the tank.
Aluminium tanks may develop cracks or fractures in the tank neck threads where the valve screws into the tank. These cracks can cause gas loss, and a tank with a cracked thread is unusable. The tank neck threads of aluminium tanks are inspected during the standard visual inspection, and this problem is usually caught before it becomes dangerous
Steel tanks weigh less, are smaller and more durable, and require that a diver use less weight than standard aluminium tanks. However, aluminium tanks are so much cheaper than steel tanks that they have rapidly become the industry standard.
- Sidemount diving (learntodivetoday.co.za)