For most people a cylinder is the last piece of equipment that they add to their set. Some never see a reason to even add a cylinder to their equipment, because it is to heavy to travel with and is normally very easy to rent at most dive centres and resorts. This leads to people simply diving with any cylinder they are given, and even when they do buy a cylinder it is normally an uneducated purchase.
There are several considerations to be made when selecting a cylinder to dive with, and even more when selecting one to buy. For a start there is a variety of different sizes(volume and pressure), material and even where the cylinder was made. In South Africa the most common cylinder is the 12 litre steel dumpy. You might recognise them as the short fat ones. for most people there is more than enough air in them to finish your average dive easily (45 minutes). Unfortunately not all of us are blessed with naturally good air consumption and might have to opt for a cylinder that can contain more air, for example a 15 litre steel cylinder. These look like the short fat ones from earlier, however they are a little taller and quite a bit heavier. As divers we all have that one friend or buddy that we swear simply doesn’t breath underwater, if you are lucky enough to be one of them you might want to look at a 10 litre steel cylinder. These are a lot lighter than the rest of the cylinders we have looked at so far, and makes your life a lot easier on the boat especially in South African conditions.
The important thing to note about the different sizes is how it is going to effect your diving. All three of the fore mentioned cylinders vary in weight. This means that if you normally dive with a 12 litre cylinder and for some reason switch to a 10 or 15 litre you will in all likelihood either be heavy or to light to complete your dive comfortably. In addition to this the different lengths will also affect your trim, meaning you might need to adjust the positioning of your weight belt. This can especially happen if you switch from a dumpy 12(short fat ones) to a long 12(tall skinny ones).
Although not hugely popular in South Africa, oversees (Europe and USA) Aluminium(Ali’s) cylinders are extremely popular. These can easily be identified by simply looking at the bottom of the cylinder. Ali’s have a flat bottom where steel cylinders have a round bottom. This is also why steel cylinders are commonly seen with boots on them. Ali’s normally come in sizes between 11.5 and 12 litres. Most people assume that ali’s has more air in because they are bigger than a steel cylinders. The opposite is true though, it is true that a 12 litre Ali is bigger than a 12 litre steel if you just look at them. However in terms of volume they are the same size. The reason for this is that aluminium cylinders walls are thicker than a steel cylinder. This also makes them heavier on land, but don’t let that fool you, in water they are lighter(due to the weight to size ratio). Ali’s also becomes noticeably lighter as they get empty sometimes even becoming positively buoyant. This means that if you do the neutral buoyancy test(As thought in the Padi Open Water course) with a full Ali and you adjust your weight accordingly that your weighting might be wrong as the dive goes on. So it is important to keep in mind to add a weight or two when diving with an Ali just to make sure you can do your safety stop with ease instead of struggling to stay down.
The amount of pressure a cylinder can take is also not a constant. Steel cylinders normally take 232 bar or 300bar, where Ali’s only takes 200 Bar. To place a better number on it, a 12 litre steel cylinder will have 2784 litres of air and a 12 litre Ali will have 2400 litres of air. 300 Bar cylinders is quite rare to see in diving, however they are out there. these cylinder are normally a lot smaller(5-9 litres)and used as bailouts or side-mount cylinders.
When buying a cylinder there is 2 other considerations that have to be made. Luckily like the rest it is quite simple if you know what to look for. The 1st thing is the valve, although this is more aimed at buying valves on their own it is nice to know. Not all cylinders have the same thread on the necks(where the valve screws in). This is mostly a feature on Ali’s and depends where they are from. Although this is not always the case the thread from an American made cylinder and a European made cylinder is different. Besides this some valves are preferred because it is easier to get service kits for them, or they might be able to switch between left and right hand valves. Having a left or right hand valve does not really make a difference unless you are planning on using it for side-mount or twin sets. Added to this some valves is made only for A-clamps, meaning it doesn’t have an insert that can be removed in order to use a DIN fitting on it. In my opinion I would try to avoid these valves, even if you are currently using an A-clamp regulator you never know when you might switch to a DIN setup. Lastly how old is the cylinder that you are buying, and has it regularly been Visualled and Hydrod. In order for you to be able to let a cylinder be filled it has to be in date. This means that it has been visualled in the last year and hydrod in the last 4 years. If a cylinder is very old but has been tested(can spot the stamp and the neck of the cylinder) it means that it is still good to go. It is important to note that even if a cylinder looks bad from the outside its the inside that matters, so even if a cylinder is in looks to be in good condition make sure it is in date.