Selecting the right Cylinder & Valve for you
Select your dive cylinder taking into account your size, fitness, where you will be diving, as well as the type of diving you will be doing. These days, you can buy scuba cylinders in a wide variety of colours including some with patterns and pictures.
When purchasing a dive cylinder, it is important to consider your strength capabilities. Think about what you can manage because, although cylinders feel weightless underwater, you will need to be able to transport it and carry it comfortably out of the water too. There is no point in purchasing a 15 litre cylinder to give yourself a lot of underwater time if you aren't able to carry it to and from the dive site.
The common sizes which cylinders come in are 8, 10, 12 and 15 litres. The fitter you are the more muscles your body uses and therefore the more fuel it needs. So, unfortunately one downside to being a fit diver is that you will use up more air than a not so fit diver. If you are going through air much faster than your buddy, it's a good idea to upgrade to a bigger size to even the game, but don't opt for a bigger cylinder in attempt to outlast your buddy as remember, you'll just have to surface when he does!
Type of Diving
It is rare that you will need more than a 12 litre cylinder for off-shore diving, however, you will probably only want to purchase one cylinder so it is important to make sure this will be suitable for all your uses. For instance, on boat dives you will be diving deeper and this increased depth means higher pressure which causes you to use air up faster. Therefore you will probably want a bigger tank than usual when partaking in a boat dive, as there is also no need to carry it to and from your home. Some boats will lend you a cylinder if you go along with organised dive groups or with a certified dive school abroad; it's a good idea to check before you go.
Dumpy cylinders hold the same amount of litres as regular tanks but are a slightly different shape which makes them wider and shorter, usually 2/3rds the height of the usual tall model. This makes them quite desirable to shorter people to prevent them from digging into your head or back in the water. It is also a big advantage to carrying and storage, as you can lift it off the ground with ease and obviously it fits into smaller spaces. Best of all, it is much easier to sit down comfortably when wearing your gear, especially useful on a boat!
The pony bottle is an alternative air source that you can use independently as it has its own regulator. It is a small SCUBA cylinder which normally straps in next to your main cylinder, and fits securely into your BCD. Most divers who carry a pony bottle also have an alternate second stage for sharing air with an out-of-air-buddy.
Most divers prefer alternate second stages and alternate inflator regulators because they cost less and are less bulky than pony bottles. However some divers prefer the added security that a pony bottle offers in some diving situations. Again, it is down to personal preference.
Aluminium vs. Steel
Both Aluminium and Steel are perfectly good for containing the high pressure of air that is demanded from a SCUBA cylinder. Whilst there is not a great difference in price, you need to think about which will be better for you. Aluminium cylinders resist corrosion much better meaning you will have to get it tested less often and it will probably have a longer life span than a steel one. Whereas the advantage of stainless steel ones is that they can hold a similar quantity of air in a smaller cylinder by storing it at a higher pressure. Think about whether you want a compact cylinder or one which will resist corrosion better, read more about cylinder features here.
The other factor to consider when trying to decide between the two, is that steel cylinders are still slightly negatively buoyant when low on air; this means you don't have to wear extra lead to counteract that buoyancy at the end of the dive which if you struggle with weights can be a real plus. Weight up the pros and cons and figure out which one is better suited for you.