The debate over which tanks are better, steel or aluminum has raged for many years. Some divers prefer aluminum while some divers prefer steel. As a new diver buying equipment, you need to know which tanks are best for you.
Aluminum scuba cylinders most commonly come in an 80 cuft (actually 77 cuft) variety. They tend to be cheaper, lighter and also come in a wide variety of colours. Aluminum 80's or AL80 are the most common rental tank for dive shops around the world. They also come in a less common 100 cuft variety. The AL100 is larger - but also much heavier.
Steel scuba cylinders are a bit more complicated. While the options for aluminum tanks are fairly limited, steel tanks come in a greater number of sizes and two different working pressures. Low Pressure or LP steel tanks have a working pressure of 2640 PSI / 184 BAR while the High Pressure or HP steel tanks have a working pressure of 3442 PSI / 237 BAR. They are less commonly used as rental tanks but have gained popularity because of their higher gas volumes.
A quick breakdown of some common scuba cylinder characteristics ....
|Buoyancy(lbs) @500 PSI:||+2.8||+1.8||+1.16||-0.35||+1.46||-2.38||-2.66||-1.73||-1.19||-0.08|
|CuFt @500 PSI:||12.83||15.15||16.1||17.99||20.45||22.73||11.62||14.53||17||19.32|
*Always check the manufacturer specifications for your own model of tanks for the most accurate information.
The working pressure is the fill pressure to which the tank has been rated and is directly related to it's fill volume. The working pressure is always marked permanently on the shoulder of a scuba cyliner as well as its full volume. Many different sizes of tanks have the same working pressure ratings, so it is the volume that is most important. If someone tells you they are starting their dive with full working pressure of 3000 PSI, you really do not know how much air they have unless you also know the volume of the cylinder. A 3000 PSI fill in a 19 cuft tank is vastly less air than a 3000 PSI fill in an 80 cuft tank. Though the working pressure is the same, the 19 cuft tank is much smaller and has 1/4 of the internal volume. There are three common working pressures found on tanks. The most common is the aluminum cylinders (AL) with a working pressure of 3000 PSI, nearly as common is the low pressure (LP) steel cylinders with working pressures of 2640 PSI and finally the high pressure (HP) steel cylinders with a working pressure of 3442 PSI. There are other, less common working pressures as well.
Hydrostatic Testing and the mysterious PLUS rating.
Both aluminum and steel scuba cylinders must be hydrostatically tested every 5 years in Canada and in the US.
In the US, a typical 2400 PSI low pressure steel tank can be tested for a PLUS (+) rating which allows the filling station to increase the fill pressure to 2640. They come from the factory with this plus rating, however it must be specially tested to be renewed every 5 years otherwise the working pressure rating on the tank drops back down to 2400 PSI.
Transport Canada does not recognize the (+) system and tanks are marked according in the metric pressure rating (BAR) and you will notice on your LP steel tanks (Marked 3AA) that while the rating in PSI is 2400, the rating in BAR will be 184. (184 BAR = 2669 PSI!!!)This is the normal working pressure as determined by the permit issued by Transport Canada. There is no (+) required, ever.
When air is compressed to fill a scuba cylinder, it heats up dramatically which causes the air to expand. Though the initial fill pressure may read 3000 PSI, that pressure will drop as it cools back down leaving your cylinder less than full. For this reason, it is always best to take your time when having your tanks filled. If possible, leave them at the dive shop over night so that they can be filled slowly.
Different scuba cylinders will become more or less buoyant as they are emptied. It is important that you plan carefully for this additional buoyancy and ensure you have enough weight on your weight belt to do a comfortable safety stop at the end of your dive. If you do not have enough weight you will find yourself fighting to keep from popping up at the end of your dive.
In Recreational Scuba diving, it is important to remember that you are carrying the breathing gas for not only yourself, but for your buddy in the event there is an emergency. Ensure that you are aware of what is a safe 'turn pressure' (at what point you must end your dive) so that you always have ample breathing gas to share with your buddy to make a safe ascent should something go wrong. Remember that the turn pressure will vary depending on the size of your scuba cylinder. As shown in the chart above, 500 PSI in an AL80 is quite a bit less than 500 PSI in an LP120. It is always best to plan to end your dive with between 13 and 15 cuft of gas remaining in your tank, which for most cases should be ample reserve. For this reason it is a good idea to understand how many PSI or BAR 15 cuft of gas is in your individual tank(s).
So ... Steel or Aluminun?
For the kind of diving we do in Canada, which tends to be deeper, colder and a bit more challenging, I prefer steel tanks. They have better buoyancy characteristics and they are available in a larger variety of sizes that can give you more bottom time without sacrificing your safety margin. With proper care and maintenance, steel tanks will last the same or longer than aluminum tanks and are quickly becoming the status quo for many divers up north.