First there is the volume of a tank which is not always apparent by its size. It is a combination of size and something called working pressure. The working pressure of a cylinder, which is always stamped in the neck of the tank, is the maximum allowable pressure a cylinder may be pumped to. Once it reaches this pressure it will contain the rated volume. For example most standard 'aluminum 80s' have a working pressure of 3000psi. PSI (pounds per square inch) is a unit of pressure. For sport diving 80 cubic feet of gas is almost always enough. Those who enjoy longer dives or wish to eventually dive deeper may want to look at purchasing larger tanks or twin tanks which I will explain later. If you find that you need a larger tank because your air consumption seems to drain an '80' fast then you have to ask yourself an important question. Is my air consumption high because I am not as fit as I should be and maintain poor breathing skills or is it because I have an exceptional high lung tidal volume? If it is the first reason a bigger tank is not your solution.

There are all sorts of tanks on the market today with a variety of different working pressures. Low-pressure cylinders can be as low as 2015psi or less. Many of the low-pressure tanks are around 2400psi and on some steel cylinders there may be a (+) stamp or rating that qualifies a tank to be over-pressurized by 10% of its working pressure.

It is common to go to some parts of the world and find technical divers over-pumping their cylinders to maximize their dive. Quite often it is justified by comparing the cylinder's European counterpart, which sometimes has a much higher working pressure. It is true that the steel vessel itself may by built to withstand the higher pressures but North American valves and valve fittings are not. Do not risk your lifefor an extra couple of feet of gas over-pressurizing is not worth it.

Next there are medium or standard working pressures that most aluminum and some steel tanks are rated to (3000psi or 204bar). Nearly every compressor in the country can pump to this pressure easily. Finally we have high-pressure tanks which can be as high as 3500psi. The problem with high-pressure tanks (usually steel) is that they heat up very fast during filling and stay hot. This makes filling difficult and sometimes riskyin fact many shops and charters boats can't fill past 3000psi (204bar). High-pressure tanks are not recommended for technical diving.

The size of the tank should not be confused with its volume. The size includes length and diameter. For many divers long tanks like standard 80 or 90 cubic feet (10 or 12L) are just too large and can cause imbalance and/or serious strain on the back. There are a few solutionsshorter high-pressure tanks or doubling up smaller tanks like 45 or 50 cubic foot tanks. I like the second option better personally because it provides balance across the back, a better center of gravity and the opportunity to employ redundancy. For stage bottles short, fat tanks can interfere with configuration styles. A popular choice for most stage bottles are 30 cubic foot aluminums, 45 steels or for longer deeper technical dives 66 or 72 cubic foot steel tanks.

Aluminum is very inexpensive and does not corrode easily. Nearly all aluminum cylinders start off negatively buoyant when full and become positive as they are breathed. For warm or fresh water divers where over-weighting is an issue aluminum tanks have good buoyancy characteristics. For cold water divers who wear lots of extra lead to counter the buoyant neoprene, a few extra pounds for the empty aluminum tanks makes swimming that much more difficult. It especially becomes a factor when they dive deep with multiple cylinders; at depth when the neoprene crushes and their tanks are full and negative they rely heavily on their BCDs and suits to maintain neutral buoyancy. As they ascend their suit expands, there is less gas in their breathing cylinders and they become much lighter. They have to carry a lot of extra weight to keep them down for their stops during ascent. Steel tanks maintain a negative weight throughout the dive and therefore are the choice among many cold water technical divers. Steel tanks are subject to higher corrosion rates, but some of the newer galvanized cylinders resist oxidization very well.

More and more we are moving into the European DIN style valve. The trapped O-ring provides a greater margin of safety reducing the risk of gas loss. Many manufacturers now offer a plug that can be screwed into the valve converting it for a regular yoke regulator. When choosing a tank, also look into getting a valve that has modular capabilities. In other words you can convert it to an H-valve or incorporate it into a dual-outlet manifold system. This allows your system to grow with you.

Finally be sure your tank has the proper markings, in Canada every tank must have a current hydro stamp and a TC or CTC stamp. Without it they cannot be filled. As a last piece of advice I recommend that, should you consider purchasing a tank that you have not used before, get it in the water and try it out first. This includes determining its buoyancy characteristics when it is nearly empty (do this in a shallow area for safety reasons).