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Buying a Scuba Cylinder

Buying a Scuba Cylinder

So you're ready to buy your first scuba diving cylinder. You've decided you have enough compelling reasons to own your own cylinder(s) (tank / bottle), instead of continuing to rent. These include:

  • The right size cylinder for you is not readily available for rental.
  • Your dive centre is still some distance away, making that additional trip to return rental cylinders expensive in terms of both time and travel.
  • The satisfaction of simply owning all of your own dive gear.
  • The overall convenience of not having to rent anything.

But, before you go ahead and buy a dive tank, there are some questions you'll want to answer to make sure you are spending your hard-earned money as wisely as possible. The answers to these questions will vary depending on who you are, where you live and dive, and on the type of diving you anticipate doing.

Here's a video that from SDI that answers some of the questions, and below we'll try to answer them all for you...

How Many Cylinders Do You Need?

For most divers, a typical day of local diving involves at least two dives. For example, a common type of boat dive is a two-tank boat dive. Often you might travel to a remote site and it's not convenient to get to a dive shop for air fills in between dives.

This is why most divers will want to own more than one tank. Doing so will save you the inconvenience of renting a second, or even a third, cylinder.

Should I Buy a Steel or Aluminium Dive Cylinder?

In the southern waters of Australia most people dive with steel cylinders as they are tough, more resistant to external damage, and are negatively buoyant helping to offset the buoyancy from your thick exposure suit. Less weight is needed on your weight belt.

In the northern waters of Australia, the use of aluminium cylinders is more common. They are typically much cheaper.

However diving cylinders can be made from either steel or aluminium. Even though both types are metal, the steel and aluminium cylinders are manufactured differently, look slightly different, have different working pressures and behave very differently in the water.

Steel Cylinders

Steel cylinders are made by drawing and spinning the cylinder from heated steel to a thickness of 4-5mm. This manufacturing technique means that the cylinder is finished with a rounded bottom that will require a rubber boot in order to stand upright. However, it also means that steel cylinder have nearly the same wall thickness everywhere and thus excellent fore and aft balance characteristics. The cylinders are typically galvanised before a protective paint finish is applied to the outside of the cylinder to help prevent rust forming and damaging the steel.

Steel scuba cylinders are negatively buoyant (more so when the cylinder is full) and are the typical dive cylinder to be found in southern Australia. Steel cylinders are available in a range of sizes including: 3, 5, 7, 9, 10.5, 12.2, 15 and 18 litres with a maximum operating pressure of 232 bar (sometimes referred to as low pressure). Steel or 300 bar (high pressure) cylinder are also available in 12, 7, 5 and 3 litre sizes.

Aluminium Cylinders

Aluminium cylinders are made using an extrusion and forming process that uses a ram to effectively force the aluminium into a mold and create the internal space in one movement. This manufacturing technique gives the cylinder a flat bottom and negates the need for a separate rubber boot. It also means there is more metal at the bottom, which makes an aluminium cylinder more tail heavy, affecting diver trim.

Aluminium is a softer metal and requires a thicker wall of approximately 11 mm to withstand the internal pressure. Aluminium cylinders are typically bulkier and heavier than steel cylinders when comparing internal volumes. For example, the common 11.1 litre (80 cubic foot), 207 bar, aluminium cylinder weighs 14.3 kg vs the 12.2 litre (100 cubic foot), 232 bar, steel cylinder weighing in at just 12.9 kg. So the steel cylinder has 25% more air and weighs 1.4 kg less.

The aluminium cylinders also have a significant fluctuation of weight between slightly negatively buoyant when full (they sink), to positively buoyant when empty (they float). Thus divers have to carry extra weight to compensate for this. That's not aproblem in the tropics where because or thin or no wetsuits divers don't carry much weight, but in temperate/cold waters divers don't like having to carry the extra weight.

Aluminium cylinders are usually used for technical diving as stage or decompression cylinders. Cylinders are available in a mixed range of sizes including 0.9 litre (S6), 1.9 litre (S13), 2.7 litre (S19), 5.7 litre (S40), and 11.1 litre (S80) with varying working pressures of between 207 and 240 bar.

What Size Cylinder Do I Need?

While steel and aluminium scuba tanks come in a wide range of sizes (see above), the following are the most commonly used sizes:

12.2 litre (100 cubic foot), 232 bar, steel cylinder
Faber Steel Cylinder - 232 bar - 12.2 litre std - DIN/K Valve RRP: $699, Our Price: $495, You Save $204 (29%).
— This cylinder size is used by the vast majority of divers. They come in two size options 'Compact' and 'Standard’. The thing to consider is body length. A Standard is around 625 mm long with 178 mm (7 inch) diameter and 12.9 kg weight. The Compact cylinder has an overall length of about 515 mm with 204 mm (8 inch) diameter and 14.2 kg weight, so it weighs 1.3 kg more. The vast majority of divers go for the Standard size, but the Compact can be suitable for people with shorter spines. The Standard sized 12 litre is available in either 232 bar or 300 bar, but it's rare that scuba divers opt for 300 bar cylinders. A pair of the Standard 12.2 litre cylinders is the preferred choice of most technical and sidemount divers.

11.1 litre (80 cubic foot), 207 bar, aluminium cylinder
Catalina S80 Aluminium Cylinder - 11.1 litre (80 cuft) - 207 bar RRP: $449, Our Price: $389, You Save $60 (13%).
— The S80 is the benchmark 'Standard Aluminium 80' cylinder used by the majority of recreational sport divers worldwide, especially in warmer waters. A pair of these S80 size cylinders is also a popular sidemount choice. They are also used as stage cylinders by technical divers doing very long run times.

10.5 litre (85 cubic foot), 232 bar, steel cylinder
Faber Steel Cylinder - 232 bar - 10.5 litre - DIN/K Valve RRP: $639, Our Price: $495, You Save $144 (23%).
— This shorter cylinder with 560 mm length with 178 mm (7 inch) diameter and 11.3 kg weight, is preferred by many women, and blokes with a shoter frame, because it's lighter and easier to handle. If you are a diver with a very good surface air consumption rate, as many women are, then this cylinder has enough air capacity for most dives. A pair of these steel tanks is also popular with sidemount divers, and women technical divers.

15.0 litre (125 cubic foot), 232 bar, steel cylinder
Faber Steel Cylinder - 232 bar - 15 litre - DIN/K Valve RRP: $781, Our Price: $665, You Save $116 (15%).
— This larger size, 610 mm long with 204 mm (8 inch) diameter and 16.5 kg weight, cylinder is the choice of divers with a poor surface air consumption rate. However, it's larger size and weight makes it awkward for many divers to use.

5.7 litre (40 cubic foot), 207 bar, aluminium cylinder
Catalina S40 Aluminium Cylinder - 5.7 litre (40 cu ft) - 207 bar RRP: $399, Our Price: $359, You Save $40 (10%).
— The S40 is a popular choice as bailout cylinders for rebreather divers. The S40 is also popular as stage cylinders by new technical divers just starting out. However, if they progress to deeper depths, they'll typically need the extra capacity of S80s as stage cylinders, so we recommend that size.

3 litre (25 cubic foot), 232 bar, steel cylinder
Faber Steel Cylinder - 232 bar - 3 litre - DIN/K Valve RRP: $463, Our Price: $416, You Save $47 (10%).
— This small cylinder is used as 'pony bottle' or 'pony cylinder.' It is a small reserve cylinder that would generally attach to the main cylinder with a clamp and used as a reserve in emergencies. Available with 232 bar or 300 bar working pressures. A pair of these cylinders is a common choice for rebreather divers.

2.7 litre (19 cubic foot), 207 bar, aluminium cylinder
Catalina S19 Aluminium Cylinder - 2.7 litre (19 cu ft) - 207 bar RRP: $349, Our Price: $319, You Save $30 (9%).
— The S19 makes an excellent choice as an emergency air source tank (pony bottle) for recreational sport divers, and as bailout cylinders for rebreather divers. It has also become popular as an oxygen deco tank as its capacity is plenty for all but the most extreme dives.

Should I Buy a 232 Bar or 300 Bar Cylinder?

232 bar or 300 bar? More is better, right? Sounds like a no-brainer, but there are some drawbacks. Higher pressure requires more metal to contain it, so the higher pressure 300 bar tank is usually as big as the lower pressure 232 bar one, but much heavier (16.7 kg vs 12.9 kg) as the walls have to be thicker to contain the extra pressure.

Getting a 300 bar cylinder filled can be tricky, so always check your local air filling source to make sure they can fill to 300 bar. Not all dive shops can. If they can, they usualy charge much more, even double, for 300 bar fills, as they take much longer to do properly.

You also must have DIN (Deutsches Institut fur Normung which is a German manufacturing standard) connection on your regulator to attach to a 300 bar cylinder as these higher pressure cylinders are only available with DIN300 valves.

What Type of Cylinder Valve Do I Need?

Cylinder valves are usually made from chrome plated brass and are available in two connection types, DIN and International A-Clamp, also called Yoke.

DIN vs Yoke Regulators

DIN Valves

DIN valves (Deutsches Institut fur Normung which is a German manufacturing standard) are available for either 232 or 300 bar cylinders. The 232 bar connectors are shorter than the 300 bar version and will not seal in a 300 bar valve. 300 bar connectors will fit in either 232 or 300 bar cylinder valves. There are two distinct benefits to the DIN connection:

The O-ring is fitted to the regulator thread rather than the cylinder valve face. This means that when the first stage it fitted the O-ring is trapped inside the valve outlet, preventing the O-ring from being forced out under pressure and its also protected. The whole connection is much more compact, secure and safer. That's why technical divers use DIN vales and DIN is the mandatory standard in Europe.

What is the difference between DIN 200 Bar and DIN 300 Bar valves?

The Deutsches Institut Für Normung (DIN) is a German standards setting organisation similar to the American National Standards Institute (ANSI) and Compressed Gas Association (CGA). DIN 477 is a specification that recommends cylinder valve outlet and connector designs for specific types of gases and pressures based upon safety considerations. These various designs have deliberate incompatibilities to preclude the possibility of errors when handling different types of compressed gases at differing working pressures.

The two valve outlets and connectors of interest for divers are the DIN 477 No. 13 and the DIN 477 No. 56 (formerly No. 50), both designated for use with compressed air. The DIN 477 valve and regulator fittings are most widely used outside the U.S. The regulator first stage DIN connector is a male screw type, and instead of clamping on to the outside of the valve as does the yoke, it screws directly into the female DIN outlet of the valve. The sealing O-ring is held in the end of the regulator connector rather than in the face of the outlet. The DIN 477 system, with it's captured o-ring design, has proven to be very reliable for use with SCUBA.

Outlet/Connector #13 is from DIN 477 part 1 - for cylinders with test pressure ratings up to 300 bar and is commonly referred to in the SCUBA industry by the slang term "200 bar", probably because most cylinders with 300 bar test pressures have working pressures in the 200 bar range.

Outlet/Connector #56 is from DIN 477 part 5 - for cylinders with test pressure ratings up to 450 bar and is commonly referred to in the SCUBA industry by the slang term "300 bar". The two designs are nearly identical, but the #56 valve outlet is deliberately deeper so the shorter #13 connector will not be long enough to seat properly. This is a safety feature to prevent connecting a low pressure device to a high pressure supply.

It's important to understand that the '200 bar' or '300 bar' descriptions are just slang terms that have nothing to do with the pressure ratings of the outlets and connectors themselves!

Yoke / A-Clamp Valves

Yoke (A-Clamp) valves are still widely used in many countries, including Australia, but as DIN valves become more popular sales of Yoke connection regulators have fallen. A-Clamp connectors have a maximum pressure rating of 232 bar and are typically only used in recreational diving (i.e. non-technical diving). The Yoke valves are also referred to as A-Clamp or International (INT) valves.

Most modern 232 bar cylinder valves are now convertible between A-Clamp and DIN using an insert that is supplied with new cylinders. The insert is left in for A-Clamp regulators, and removed using an 8 mm Allen key to allow a DIN regulator to screw into the cylinder valve. These changeable valve are typcially called DIN/K valves.

Valve Threads and Cylinder Neck Threads

!!! IMPORTANT !!! Cylinders and valve threads come in a variety of sizes and great care must be taken to ensure when fitting a valve to a cylinder that matching threads are used. The valve thread specification must exactly match the neck thread specification of the cylinder. Improperly matched neck threads can fail under pressure and can have fatal consequences if someone is hit by the flying valve or cylinder. Only qualified scuba servicing professionals should be installing this equipment. Please ensure you have the training and experience required before purchasing this item if you intend to fit it yourself. If you are unsure, please call your local scuba technician to provide appropriate information for your system.

Australian Standard Diving Cylinder Valve Thread

The Australian Standard says that aluminium and steel tanks/cylinders/bottles must have a standard 3/4" NPSM (NGS) parallel thread, sealed by an O-ring, which has a 60° thread form, a pitch diameter of 0.9820 to 0.9873 in (24.94 to 25.08 mm), and a pitch of 14 threads per inch (5.5 threads per cm). It is torqued to 40 to 50 Nm (30 to 37 lbf·ft) on aluminium cylinders.

European Diving Cylinder Valve Threads

On most steel and aluminium diving cylinders in Europe the M25X2 ISO parallel thread, which is sealed by an O-ring and torqued to 100 to 130 N·m (74 to 96 lbf·ft) on steel, and 95 to 130 N·m (70 to 96 lbf·ft) on aluminium cylinders, is now commonly used. (M25 means the diameter of the thread is 25 mm and the X2 defines the 2 mm pitch of the thread.) On smaller cylinders where the neck isn't large enough for the M25X2 thread a M18x1.5 parallel thread, which is sealed by an O-ring, and torqued to 100 to 130 N·m (74 to 96 lbf·ft) on steel cylinders, and 85 to 100 N·m (63 to 74 lbf·ft) on aluminium cylinders is used.

Please Note: The M25X2 metric thread valves are only for use with metric M25X2 threaded cylinders. While they can be wound into a cylinder/tank with a 3/4 inch NPSM threaded neck, they will not seal properly and the combination can be very extremely dangerous. Do not try and use these M25X2 valves on typical Faber, Catalina, Luxfer etc. cylinders purchased in Australia.

When someone tries to fit a M25 valve into a 3/4 BSP cylinder thread, they will get it to seal, but only after damaging the brass threads on the valve. This produces an unstable fitting which can result in the valve coming out with incredible force that could seriously injure someone, or even kill. You have been warned.

In recent times some companies in Australia have been importing cylinders and valves with M25X2 threads, contarary to Australian Standards. The owners of the M25X2 thread cylinders and valves are typically unaware of this. Thus it can be easy for a very dangerous mix up to oocur.

Do I Need A Cylinder Boot?

In the real world yes, you need a cylinder boot on a steel scuba cylinder. Most steel cylinders have a rounded bottom. Thus a cylinder boot gives your otherwise wobbly steel cylinder something to stand in to keep it upright. It also gives it a layer of protection against bumps and bangs. A worthwhile investment. Most of the steel cylinders we sell come with a boot.

Most aluminium scuba tanks have a flat bottom and thus can stand upright without a cylinder boot. Some people put cylinder boots shaped for aluminium tanks on them for added protection of the cylinder base.

Important Cylinder Accessories You May Want To Consider

Mesh protector sleeves that slide over the cylinder to protect from exterior damage.

  • Trident Tank Mesh Protector - 9-12.2 lt - 6.9-7.25" Trident Tank Mesh Protector - 9-12.2 lt - 6.9-7.25"
    RRP: $16, Our Price: $14, You Save $2 (13%).
    For years suppliers have been reducing the quality of tank mesh protectors. What's commonly available now isn't anywhere near the quality most mesh protectors once were. So we searched the world to find this high quality, super durable cylinder mesh protector.

Valve covers to help keep water and dust out of the valve opening.

  • Sonar Vinyl DIN/Yoke Valve Cap Protector Cover Sonar Vinyl DIN/Yoke Valve Cap Protector Cover
    RRP: $11, Our Price: $9, You Save $2 (18%).
    Keep one of these protective covers on the top of your cylinder/tank Yoke valve to prevent loss of your O-ring and keep debris out of the valve when not in use.

Handles and carriers these make hauling your tank a little easier.

  • Tank Holder - Foam Double Cylinder Holder Tank Holder - Foam Double Cylinder Holder
    RRP: $60, Our Price: $50, You Save $10 (17%).
    This lightweight cylinder holder design works great in car boots, trucks, and boats. It will hold two of most tanks and stop them from rolling around.

Compressed gas stickers should be displayed on all vehicles carrying diving cylinders.

Cylinder Inspections

Australian Standards (AS2030.5) require that your scuba diving tank be visually inspected and pressure tested at a certified testing station once per year.

A Visual Test is a detailed visual inspection of the entire cylinder and includes the following checks:

  • Thread Go and No Go testing for imperfections
  • Bulges
  • Dents
  • Wear
  • General corrosion
  • Area corrosion
  • Isolated pitting
  • Thread damage
  • Weight check

A Hydrostatic Test is also performed. During the hydrostatic test the cylinder is filled with water and then placed inside a high pressure chamber that is also filled with water. The cylinder is then pressurised to its test pressure which is high enough to slightly expand the cylinder. The expansion displaces the surrounding water which is channelled into a measurement tube. If the cylinder expansion is with acceptable limits and returns to an acceptable range it successfully passes the test.

Cylinders that do not pass either of these tests are generally destroyed to prevent continued unsafe use. Test centres may return the cylinder if requested, but the cylinder must be rendered unusable before the cylinder is collected.

The Scuba Doctor Service and Repair Centre can do both types of tank testing. See Scuba Cylinder Testing for more details.

Cylinder Care

It's important that you look after your dive cylinder properly. For details about the care, storage and transportation of steel and aluminium dive tanks, please see Scuba Cylinder Care.

By taking proper care of your scuba cylinder you'll be rewarded with many years of safe, trouble-free service.

Recommended Scuba Cylinders

When purchasing scuba cylinders, the long-term advantages of steel's excellent buoyancy characteristics and long life make it the best choice for most divers, but especially those in cold and temperate waters. Choose a 232 bar steel tank size that meets your needs when it is under filled, putting an end to short fill concerns. For most divers this will be a 12.2 litre (100 cubic foot) cylinder, but some may prefer the smaller 10.5 litre (85 cubic foot) cylinder.

If your budget is tight, then an aluminium cylinders initially costs significantly less. If going with an aluminium cylinder, avoid paints, and choose the brushed finish. (The paint used on aluminium cylinders flakes off and ends up everywhere.)

For both steel and aluminium cylinders you should choose a convertible valve having a DIN outlet with K (yoke) insert, often described as a 'DIN/K' valve. (We provide this as our standard offering.)

Remember, the standard 207 bar aluminium 11.1 litre (80 cubic foot) capacity cylinder with a K valve is not a 'one-size-fits-all' tank. Making the right cylinder choice can significantly improve your diving enjoyment. Please use this guide when reviewing scuba diving cylinders and you should be able to find the right cylinder/tank to last you for years of diving.

You can see our full range of available Scuba Diving Cylinders in our online dive shop.

The Scuba Doctor Service and Repairs