The choice of cylinders should be made to accommodate body size, breathing rate, dive profiles and diving environments. Improper cylinder choice contributes to buoyancy control problems, and thus increases environmental damage and diver risk. Failure to match the appropriate cylinders with your style of exposure suit and buoyancy control system can also prove fatal-- steel back cylinders and stages, when used with a thin wetsuit, can result in uncontrolled descents.  

1. AluminiumIn general terms aluminium cylinders change from being negatively buoyant to positively buoyant as the volume of gas they contain reduces. This characteristic, when combined with correct weighting can be capitalized upon to provide redundant buoyancy. This makes the use of aluminium cylinders when wet suit diving an absolute necessity.

2. SteelThe use of steel cylinders when wearing a wet suit should be avoided at all costs. Steel cylinders represent total negative buoyancy. Whilst they too reduce in overall weight as the gas volume reduces they never achieve positive buoyancy and remain negative through out the dive no matter what the circumstances. 

3. So what?Imagine diving in a wet suit and experiencing a BC failure, the only option would be to fin yourself back to the surface. If you're using a steel cylinder no matter what equipment you ditch you'll still have to swim up carrying on average a 5kg lump of steel, twice as much if you're using twins. 
However if you use an aluminium cylinder and have balanced your equipment correctly, by removing your ditchable weight you will be able to take advantage of the the cylinders integral buoyancy during your swim back to safety. Now, in an emergency what would you prefer a 5kg millstone or a gentle helping hand?
And before you ask, no, a double bladder isn't an acceptable solution. Please refer to our BC section for further information. This subject cannot be adequately covered in this limited space but is expanded on in other areas. And of course you can always call us or email us for a fuller explanation.

4. High Pressure CylindersHigh pressure cylinders such as 300 bar should be avoided as they place additional stress on manifolds, first stages, HP hoses and O rings etc. High pressure cylinders by their nature weigh more because they require a thicker cylinder wall to contain the elevated pressure. As a consequence they are considerably heavier than their standard counterparts. Coupled with the issue of additional stress these cylinders become undesirable. 
Contrary to popular belief high pressure cylinders do not offer a proportional increase in available gas volume. Many divers don't realize that as pressures exceed 200 bar the relationship between pressure and volume changes. Consequently there is not the significant gain in available gas that some divers expect but there is a considerable weight penalty. Remember the millstone effect referred to earlier?  

5. Correct weightingSome tanks are fairly heavy when full yet will become neutral or even positive when empty. It's important to ensure that a diver wears sufficient weight to remain submerged at 6m with near empty cylinders. There are several methods of offsetting this problem which include the conventional weight belt, weight placed behind the Backplate and heavier stainless steel plates.

6. Hot-dip galvanized tanks Hot-dip galvanized tanks appear far more rust resistant but environmental considerations and cost seem to be reducing their availability. There's also a maintenance consideration. Depending on where in the world you live, once the galvanized coating is sufficiently damaged it may have to be replaced before the cylinder will pass its periodic inspection.
7. CharacteristicsCylinders come in many capacities and are made of various compounds. Each has a specific characteristic, the desirability of which varies according to the kit configuration and circumstances in which you plan to dive. Choosing the correct cylinder for the circumstances is an essential safety consideration.

8. Cylinder sizesIts important to choose the correct cylinder for the job as addressed earlier the cylinder needs to be appropriate for the type of dive being undertaken. merely grabbing the largest possible cylinder is not the answer. As described earlier each cylinder has a different personality and its important to ensure the correct one is being used. For wet suit diving Luxfer aluminium S80 and S40 cylinders are best suited.