What size and type of cylinder do I need?

Cylinders come in different sizes and two type of materials - aluminum and steel.
Sizes range from 3lts to 18lts (although not many 18lts around these days).
Then we have dumpies and talls! So what is right for you?

Lets look at aluminum -
These cylinders are very common on liveabaords in Egypt. They are lighter than steel and more resiliant to sea water. Light cylinders are good for smaller build people / young people but if you need weight to get down you may need more weight on your weight belt and some feel its better to have the weight on your back (easier to carry it there than your waist). They tend to have lower operating pressure (c.207bar) and slightly different capacity (i.e. 11lts). In todays technical diving community they are used for stage cylinders.
Steel cylinders -
In the UK most people dive with steel cylinders as the need weight to off-set the buoyancy from dry suits. Steel cylinders have a round bottom so will need a boot to allow them to stand up. They come in two pressure ratings 232bar and 300 bar (higher the specification - higher the cost). Like most dive equipment cylinders needs tests - visual inspection every 2 years and hydrostatic (pressure) testing every 4 years.
Cylinder sizes -
3lts cylinder were used as 'pony bottles' a small reserve cylinder that would attach to the main cylinder and used in emergencies. 5lts can be used for emergency oxygen systems, both 3lts and 5lts are too small capacity for diving. 7lt cylinders are useful in pool sessions for children / small people who find the larger cylinders to heavy to handle, they are also used in technical diving as stage cylinders. 10lt cylinders are also used in the pool but also used in open water for people who don't use a lot of air. Both 7lt and 10lt cylinder can also be used in 'twin sets' (more later). 12lt cylinders come in 'dumpy' and 'tall' options; the thing to consider is body length, if you are short a tall cylinder can be too long and if you are tall then a dumpy cylinder can put the weight high up on your shoulders so can result in you diving head down. The other consideration is do you think in the future you may like to try technical diving (dive deeper and long with redundant air source)? if the answer is possible then the tall is a better option as you can the buy another 12lt tall cylinder and join them together. The last option for most people is the 15lt cylinder. This has a large capacity of air and is the heaviest cylinder (again weight on the back - less on the waist!)....these can be twined but very heavy maybe too heavy! How do you know how much air you use? To find the capacity of breathable air in a cylinder is simple...cylinder capacity X pressure(so 12lt X 200bar = 2400lts air) the difficult bit is knowing how many Lts you breath per min! Depth and conditions will vary greatly during a dive so you could work out a theoretical average or a much better view is to look back through your log book and look at the dive you have done to work out a trend. There is much discussion about the amount of air used on a dive between divers as to how much air is left is a sort of competition, in my opinion this adds to poor breathing techniques...you use what you use and as long as you stay within the safety limits then it doesn't matter how much air you use...never empty your cylinder! Try to be on the surface with 50 bar the red part of the contents gauge is there for a reason!test-date-sticker Cylinder values are universal for both the A clamp and din fiittings (for din you need to take out the insert).
If you plan on diving on Enriched air / nitrox then cylinders need to be oxygen cleaned for safety reasons. So considering your size, strength, air consumption and possible future diving plans will determine which cylinder is right for you! When buying a cylinder new or second user check for the date of the last test click here to learn more on cylinder markings, if buying second user and its out of test...there is not guarantee that it will pass!

The image above shows the next expiry date -
month on the left, and year on the right.