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.
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What is Nitrox?
Enriched Air Nitrox or EANx or simply Nitrox is a breathing gas that has the level oxygen increased to displace the nitrogen in the mix.
Why is it used?
The nitrogen in the air that we breathe when we are diving is absorbed into our tissues at depth, however unlike oxygen which our body easily metabolizes, nitrogen that is absorbed must work its way back out the same way it came in. Under pressure it works its way into our tissues and if we take in to much nitrogen under pressure, we can no longer ascend directly to the surface and must 'decompress' by doing planned, manditory stops to try and let the nitrogen bubbles work its way out. As recreational divers, we follow the no-deco limits and thus nitrogen that we breath is the limiting factor of the length of a dive at any given depth.
By limiting the nitrogen in our breathing gas we are able to extend our bottom times beyond the air no-deco limits, or we dive the air tables on Nitrox with a far greater safety margin because we have less nitrogen in our system.
EANx does have its own risks, but when managed breathing Nitrox is a safe way to enjoy the sport of Scuba.
Does this mean I can dive deeper using Nitrox rather than air?
No, actually it is the opposite. The richer your Nitrox blend (the higher the percentage of oxygen) the shallower you will need to limit your dive. Nitrox is all about limiting your nitrogen intake and making your dive safer and nothing to do with diving deeper.
Who should learn to dive with Enriched Air Nitrox?
Anyone that plans to dive with Nitrox must learn the risks associated with the gas. Taking the Enriched Air Nitrox class also gives you a greater understanding of the air you breathe while diving. Even if you decide you won't be diving Nitrox on a regular basis, you will have a greater appreciation of how you body works under the stresses of diving.
Although limiting the nitrogen has definite benefits, there are also certain risks associated breathing oxygen under pressure such as oxygen toxicity. In any Enriched Air Nitrox class you will learn about all the risks and what precautions you must take to ensure you are using Nitrox safely.
If you think you are ready, sign up for an Enriched Air Nitrox course.
Do I need any special equipment to dive with Nitrox?
Yes. All of the equipment should be Nitrox compatible, from your tank through to your regulator.
Most regulators these days are Nitrox compatible for recreational Nitrox blends(up to 40%) from the manufacturer and you probably don't need to do anything with them.
Your scuba cylinder and valve on the other hand will likely need to be properly cleaned and overhauled. Many dive shops use the 'partial-pressure' blending method when filling scuba cylinders with Nitrox which means that at some point, there will be 100% oxygen being delivered into your cylinder. This means you tank and valve must be thoroughly cleaned to be devoid of any hydrocarbon residue which could lead to fire or explosion when exposed to 100% O2. All the o-rings in the valve and cylinder combination will have to be replaced with oxygen compatible materials such as Viton. This is an annual expense at a minimum, and must be done after every air fill from an air station that does not fill using 'hyperclean' or oxygen compatible air.
Enriched Air Nitrox when used correctly can make your scuba adventures safer or longer, and even if you aren't going to dive it regularly, just taking the class will improve your understanding of the physiological changes your body undergoes each time you slip beneath the surface.
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