Scuba Diving Cylinder FAQ

This will depend very much on what type of diving you are doing and your own personal preference. 

For recreational Scuba there are basically two type of gas mixes available, standard Air and Nitrox (Also known as EAN32 or EAN36 which stands for Enriched Air Nitrox, followed by the percentage of Oxygen in the mix). Standard scuba tanks come with compressed air, for ease of calculations we say air contains 79% nitrogen and 21% oxygen. The actual breakdown of air is closer to 78.09% nitrogen, 20.95% oxygen and 0.96% covering all the other gases in our atmosphere, such as Argon, Neon Helium, Methane and Krypton. Nitrox, or EAN (Enriched Air Nitrox) is also used recreationally and is simply a mixture of air that has had the percentage of Nitrogen reduced and the percentage of Oxygen increased. Nitrox offers the diver extended dive time, because of the lowering of the Nitrogen content, but has the slight drawback that the diver had a reduced maximum depth limit as the percentage of oxygen increases. Nitrox is only available to divers who have done additional training after their initial dive course and are certified as Nitrox Divers.

Some divers continue their training far beyond what is considered “Recreational Scuba” and enter the realms of Technical Diving. Tech divers operate at the extremes of the sport with deeper depths, longer bottom times and dives that require several “decompression” stops on the way back to the surface. With this also comes an array of different gas mixes, that are designed for use at different depths. These mixes have varying percentages of oxygen and nitrogen, along with the addition of extra gases like helium.

If you have just started diving, then you may not know if this sport will suit you yet.  It can be a good idea to rent equipment in the beginning (usually available directly from the diving school or your local Adreno Store).  Once you have decided you like diving and want to do more of this sport then you can invest in your own equipment. Owning your own tanks, frees you from the requirement of going to the local dive store to pick up your hire tanks, which is especially useful if you want to dive with just your buddy from  a private own vessel, rather than being restricted to the dive shop day trips. 

If you are going on holiday diving, then you will not be able to take your tanks with you. Airlines will not transport pressurised Scuba tanks, you can however rent the tanks you need from the diver resort, once you arrive. 

Generally, yes. Provided that the maximum fill pressure of the tanks is the same, then a physically larger air tank will hold more air than a smaller one. The most popular type of tanks are steel tanks, which are measured and named after their internal capacity, also called their “water volume”. For instance, a 12 litre steel tank has an internal capacity of 12 litres. The pressure of cylinders in Australia is measured in the metric system, of Bar (1 bar is equal to air pressure at sea level). Generally, the steel tanks that are the most popular in Australia are rated to a maximum fill pressure of 230 bar, though many fill stations will not be able to go much over 220. So, a 12 ltr. Tank, filled to 220 bar, would hold 2,640 litres of air, If released and measured at sea level. 

Older, less popular, aluminium tanks are named after the total amount of air they carry at a given maximum pressure and use the imperial system of Cubic Feet. For instance, an 80cu ft cylinder rated to 3000 psi (the equivalent of 207 bar) would have an internal volume of 11.1 litres. So, an 80 cu ft Aluminium cylinder is smaller than a 12 litre steel cylinder, regardless of the fact that it also has a lower rated maximum pressure.
 

How long a tank lasts will depend on two main factors.  

• Firstly, how much air the diver consumes.
• Secondly, what depth the diver is descending too.  

It is almost impossible to come up with a universal formula for how much air someone will use when diving. Respiration rates, lung volume, physical fitness depth of the dive and so many other factors come into play.  A good guide to working out what size of tank to buy would be to keep note of the size of cylinders you have used on your dive courses or day trip dives. Note down how long the dive lasted, the maximum depth and what your ending tank pressure was. 

For instance, you go on a day trip, and the guide on the boat said that the maximum dive time was an hour, and the minimum pressure to return to the boat with was 50 bar. Your dive though, only lasted 35 minutes, because you reached the minimum tank pressure quicker than the rest of the divers, then you would consider buying a larger tank than the one used on the day trip. On the other hand, if another diver did the same day trip, and came back to the boat after the maximum one hour dive time, and still had 100 bar left in their tank, then they could consider a smaller cylinder as they are better on their air consumption.

If you keep your logbook up to date, you will soon get very accurate measure of how long a specific size of tank lasts you, whilst diving to a variety of depths and you can then decide what size tanks to buy.

Firstly, you always need to plan a safety margin into your dive planning.  You should plan to end your dive, with a certain amount of air left in your tanks, in case of emergencies. If you are diving on a commercially operated trip, this return pressure will be mandated by the dive operator. You will be told to return to the boat with at least ‘x’ bar left in you tank. This varies slightly between the states and territories of Australia. Queensland, which possibly has the largest dive industry of all the states, has a lower limit of 50 bar remaining at the end of a dive.

Secondly, if you completely empty your air tank then there is no pressure left inside to keep moisture or particles from entering your tank.  Moisture entering your tank makes an increased likelihood of corrosion, which would reduce the life of your air tanks as rust forms internally.  Particles, such as dust or other contaminants, should not be allowed to enter your scuba tanks. Keeping pressure in your tank will ensure that any unwanted particles in the mouth of the valve, are blown clear by the pressurised air if the valve is opened, rather than entering you tank. 

If you buy your own air tanks for diving, then you will need to make sure you get them hydrostatically tested every year.  This service can usually be organised through your local dive store or directly with a certified testing facility. 

When talking to other divers you may hear them referring to tanks being “in test”, which is the common way of saying that a tank has a valid hydrostatic test date, stamped on the cylinder neck. If you attempt to get a tank filled with an expired test date on the neck, then the operator will refuse to fill it. You would not be able to fill the cylinder again, till the Hydrostatic test has been completed and the new test date stamped into the metal on the shoulder of the cylinder.

Tanks are crucial items of equipment. When you are on a dive it is highly recommended that you never attempt to use tanks that do not have a valid date stamp for the hydrostatic test, even if the tank was filled before the date ran out. These are high pressure items, which rarely go wrong, but when they do the outcome is always catastrophic. The national requirement for annual hydrostatic testing, is the way that government ensures that the tanks that are in circulation and in use, are safe to be used. This ensures that injuries, or worse, from Scuba equipment are kept as close to zero as possible. 

In Australia, the most common Scuba air tanks are made from steel. Although there are still some older, less popular, aluminium units still in circulation. Each of these metals has their own advantages and disadvantages:

• Steel – steel is much harder than aluminium and is more resistant to dents and other damage.  Steel is heavier than aluminium, which means you do not need to wear as much weight on your belt/suit. Also, since steel is so much heavier than aluminium, even when the tank is empty, a steel tank will be negatively buoyant, which does not change the diver’s overall buoyancy at the end of a dive.  Steel can rust over time if not cared for properly and excessive corrosion can make the tanks unsafe to use, in extreme cases.  Steel tanks are a much better investment, they last much longer than aluminium, which more than offsets the initial purchase cost. Generally, steel tanks have a higher working pressure than aluminium, 230 bar as opposed to 207 bar (3000 psi) for aluminium, so a steel tank will hold more air than an aluminium one of the same size.
• Aluminium – aluminium is much softer than steel and therefore is more likely to get dents, be damaged and get scrapes.  Although aluminium does oxidize it is much less prone to this damaging reaction than steel and this mean these tanks require less maintenance.  If you can still find a company selling Aluminium tanks, they are likely to be slightly less expensive than steel tanks, but they do not last as long. Making this initial cost saving a false economy. Aluminium tanks have a lower working pressure than steel cylinder, so hold less air than a steel one of the same size. Also, due to the light weight of aluminium as a material to make tanks, they will affect a diver’s buoyancy as the air is consumed. An aluminium tank that is completely full will be negatively buoyant and sink, whereas an aluminium tank that is less than one quarter full will be positively buoyant and float, adding lift to the diver’s buoyancy profile. If diving with aluminium tanks, it is necessary to perform your weighting checks with an empty cylinder, ensuring that you have enough lead on your belt, to keep you correctly weighted, even with a buoyant tank. Failure to do this can lead to an uncontrollable ascent near the end of a dive, where a diver has no means of overriding the buoyant affect of the tank, which can lead to serious injury, decompression sickness or even death.

No. These types of cylinder are made for use on land for situations such as firefighting applications.  They are not used for diving as they have a high positive buoyancy, and a lower working pressure which would give them a much shorter dive time than metal tanks, and require significantly more weight that metal ones.

This is a throwback to when aluminium tanks were the most common ones available, and the majority of which were either manufactured in the U.S. or had the U.S. in mind as their primary use destination. The American method of calculating a tank’s volume, works on how much total air (measured in cubic feet) is pumped in to a tank to bring it up to a certain pressure (measured in PSI {pounds per square inch}). So when an aluminium tank is stamped to say  80 cu ft, at 3000 PSI, then that means 80 cu ft of air (or 2265.34 litres) needs to be pumped into the tank to bring it up to 3000 PSI (approx. 207 BAR). Working the maths backwards, an 80 cu ft aluminium tank would have an internal volume of 10.94 litres (2265.34 ÷ 207 Bar = 10.94ltrs) which then allows us to compare it with a steel cylinder, measured in litres.

In Australia, as a metric country, we are more familiar with the use of litres for volume and bar for pressure, so steel cylinders make more sense to us in terms of how they are rated and designated. When a steel cylinder is advertised as a 12 lt tank, that means that it has an internal volume, or ‘water capacity” of 12 litres. It will then have a maximum working pressure, usually 230 bar, also stamped on the tanks. Generally, when getting your cylinders filled, a fill of 220 bar is considered a “good fill” and fills of 210 to 220 are considered acceptable. So, a 12ltr tank, filled to 220 bar, would mean that tanks contains the equivalent of 2640 litres of air, if released at sea level. 

So, if you compare two tanks, one steel, one aluminium, that look roughly the same size on the outside, then the steel one will hold significantly more air. The two cylinders appear the same on the outside, as the thickness of the cylinder wall needs to be much greater on the aluminium tanks to hold the pressure. So even though they look roughly the same size on the outside, the aluminium has a smaller internal volume

Conversion factors:

• 1 Cu Ft = 28.316 litres
• 1 ltr = 0.035315 cu ft
• 1 Bar = 14.503 PSI
• 1 PSI = 68.947 Millibar (0.068947 Bar)

Note to work out the litre volume of an Aluminium Cylinder in Cu Ft, you need to work out the total volume of gas (i.e. 80 cu ft, or 2265.3 ltrs) and the pressure it is filled to (i.e. 3000 PSI or 207 Bar) the divide the total litres by the fill pressure, to give you the internal volume (water capacity) of the Aluminium tank. 

2265.34 ltr (80 cu ft) ÷ 207 Bar (3000 PSI) = 10.94ltrs

Basically when you fill a Scuba tank with a breathing gas (typically air), the gas will be forced (compressed) into the tank. This air, moving at speed into the tank, under pressure from the compressor, causes friction as the molecules in the air collide with the metal of the tank, and with each other. This friction from the agitated gas molecules, can be felt as heat on the wall of the tank. The faster a tank is filled, then the more the molecules will move (in terms of Physics, molecules moving more quickly, is the textbook definition of heat being added to a collection of molecules). 

Most fill station operators will try to fill (pressurise) a cylinder at the slowed rate possible and many will immerse the tank being filled in a water bath to try to dissipate some of the heat generated. They do this to try and get the most air into the cylinder. Since heat in the cylinder means faster moving molecules, and fast moving (warmer) molecules exert a greater pressure than slow moving (colder) molecules, then it is definitely desirable to keep the cylinder as cool as possible when filling, to get the best fill. That is why, when you pick up your cylinders from the dive shop and they show 220 bar as a warm cylinder, they might drop to 210 bar when you get home and the tank has cooled down. Always try and measure your fills when the cylinder is cold, that way you guarantee what you are getting and know you will have the most air in your tank.