If you take away one fact, it should be that not all dive cylinders are created equal. Second should be that the way we colloquially refer to cylinder size is misleading. Choosing a cylinder is like choosing a car, there are many to choose from a most will do the job fine. Getting the best one for you money however is a little tougher and requires a good understanding of the characteristics of each.
Size and Capacity
The two common ways to refer to cylinder size are by cubic foot (cf) capacity and liquid volume (litres). For example, a 100cf cylinder refers to the fact that when filled to its working pressure it holds 100cf of gas when expanded. If under or over filled, it’s size will vary so the 100cf rating is misleading. The same cylinder might have a liquid volume of 12 litres (L) – imagine we took out the valve and pour 12 litres of water into it. Again this is misleading as the actual capacity is relatively to the fill pressure, and you can get 12L cylinders with 232 and 300 bar ratings.
In technical diving, expanded litres are used as a reliable measure for gas volume. When applied to the gas in a cylinder it gives you an absolute measure of the amount of gas you have. Let’s say you have a 12L cylinder, filled to 200 bar, this will give you 12 * 200 = 2400 litres. We then determine that our rate of breathing is 15 litres per minutes (on the surface), so 2400 / 15 = 160 minutes. Once you factor in changes for depth you can calculate the exact amount of gas a diver will need.
For many, the aluminium or alloy 80cf is the baseline for cylinder size. Before they hit the market all you could get was a steel 72cf, which look similar to modern steel cylinders (especially when re-sprayed) but only have fill rating of 170 bar. In contrast the 80cf went all the way to 207 bar and held a little more gas, then it’s successor the 88cf got even bigger. Years past and along came the newer steel cylinders, which pushed capacity up to 100cf/12L and now even 150cf/18L! For the purposes of this article we’ll consider steel cylinders to refer to the new style steel cylinders (since you can’t buy the older ones anymore).
Materials & Construction
Scuba cylinders are typically made from either alloy or steel, though other materials such as carbon fibre are also used. Although both appear heavy on land, in the water they take on quite distinct buoyancy characteristics. Steel cylinders are quite negative, whilst alloy ones vary from barely negative to quite negative depending on their construction. Each type of cylinder has its pros and cons, but generally speaking steel are desirable (in cool water like Sydney) because they enable lead to come off your weight belt.
Alloy cylinders today come in either standard or shot ballasted varieties. The latter literally have lead shot in the bottom of them to ensure they’re always negatively buoyant. One of the problems with cylinder buoyancy is that the gas they contain has a substantial impact on the overall cylinder buoyancy, meaning that all cylinders become less buoyancy when empty. Steel cylinders are notably negative when full and negative when empty, but there is a change. Standard alloy cylinders are negative when full but positive when empty, with the change being more much more noticeable than with steel cylinders.
The way you look after a cylinder appears to have more of an impact on longevity than construction material. We’ve just as many rusted steel tanks as we have seen corroded alloy ones. New alloy cylinders are different to old ones which have had issues with cracked necks and even explosions.
What does all this mean?
It can be confusing, but below is a summary of cylinder characteristics:
- Cost less
- Holds a standard amount of gas (e.g. 80cf/11L)
- Primary cylinder sizes are limited
- Stage and pony cylinder sizes vary nicely
- Are generally only slightly negative to positively buoyant:
- great in the tropics with lightweights suits as the tank doesn’t mess with you trim
- great for stage cylinders as they’re easy to handle in the water
- not so great for cold water as more lead is required
- Cost more
- Come in a variety of shapes and sizes (e.g. 10.5L/85cf, 12L/100cf, etc…)
- Physically much smaller for the same volume of gas (compared to alloy)
- Are always negatively buoyant:
- great for primary cylinder
- impacts diver trim underwater
- not so great for stage and deco cylinders (makes them hard to handle)
- For tropical diving, alloy 80cf/11L cylinders are great
- For Sydney diving, steel cylinders are best
- Shore Dives: 10.5L/85cf are small and light
- Boat Dives: 12L/100cf gives you more gas
- For technical diving:
- Twin 12L cylinders
- Alloy for stage/deco
Note: Faber 300 bar cylinders were becoming popular for a while. Whilst on paper they hold more gas, they’re super heavy and very negatively buoyant making them annoying in and out of the water. It’s also very difficult to get a proper 300 bar fill (due to the wall thickness and internal heat), so for this reason are not recommended.