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In the UK 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 exposure suit.
However diving cylinders can be made from either steel or aluminium. Even though both types are metal the two cylinders are manufactured differently, look slightly different, have different working pressures and behave very differently in the water.
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. 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 cylinders are negatively buoyant (more so when the cylinder is full) and are the typical dive cylinder to be found in the UK. Cylinders are available in a range of sizes include 3, 5, 7, 12 and 15 litres and 232 or 300 bar specifications
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. Aluminium is a softer metal and requires a thicker wall of approximately 11mm to withstand the internal pressure. Aluminium cylinders are typically bulkier than steel when comparing internal volumes and have a significant fluctuation of weight between slightly negatively buoyant when full to positively buoyant when empty. Aluminium cylinders are usually used for technical diving as stage or decompression cylinders. Cylinders are available in a mixed range of sizes including 1.5, 3, 5.74, 7 and 11.1 litres with varying working pressures of between 200 and 232 bar.
In the UK steel cylinders come in a variety of sizes:
3l 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 232bar or 300bar working pressures.
7l cylinders are used in pool sessions for children / small people who find the larger cylinders to heavy to handle, they are also used in technical diving as stage cylinders. Available with 232bar or 300bar working pressures.They can also be twinned (clamp together) to form a twin set.
10l cylinders are also used in the pool but also in open water for people who don't use a lot of air or have a smaller frame. Like 7lt cylinders these can be twinned
12l cylinders These are the most commonly used size. They have designed them to come in two size options 'dumpy' and 'standard’, the thing to consider is body length, the Dumpy cylinder has an overall length of about 515mm and a standard is around 625mm. Most people would go for the standard size, but the dumpy is ideal for people with shorter spines such as smaller ladies. The standard sized 12lt is available in either 232bar or 300bar options. The standard 12lt again can be twinned.
15l cylinder. This has the largest capacity of air and is, therefore, the heaviest option - it is quite heavy! Only available with 232 bar working pressure.
232bar or 300bar, More is better, right? Sounds like a no-brainer, but there's are some drawbacks, higher pressure requires more metal to contain it, the high-pressure tank is usually as big as the lower pressure one, but heavier (the walls have to be thicker to contain the extra pressure). Also getting it filled can be tricky so always check your local air filling source to make sure they can fill 300bar. You also must have DIN (Deutsches Institut fur Normung which is a German manufacturing standard) connection on your regulator to attach to a 300bar cylinder as these higher pressure cylinders are only available with DIN300 valve.
Cylinder valves are usually made from chrome plated brass and are available in two connection types, DIN and A-Clamp.
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, preventing the o-ring from being forced out under pressure and it also protected. The whole connection is much more compact and secure.
A-Clamp valves are still widely used in many countries but as DIN valves become more popular sales of A-Clamp regulators have fallen. A-Clamp connectors have a maximum pressure of 232 bar and are typically only used in recreational diving (i.e. non-technical diving).
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 to allow a Din regulator to screw into the cylinder valve.
In the real world yes you need a cylinder boot. A cylinder boot gives your otherwise wobbly cylinder something to stand in and keep it upright. It also gives it a layer of protection against bumps and bangs. A worthwhile investment. Most steel 10,12 and 15l cylinder we sell come with a boot, if it is included you will see it in the image.
Under the current European EN standards a diving cylinder must be tested every two and a half years from the cylinders initial test date that is stamped on the cylinder by the manufacturer. The cylinder alternates between two types of test, a visual and a hydrostatic test. The first test will be a visual test followed by a hydrostatic…visual…hydrostatic, etc, throughout its entire life unless you exceed the 2.5yr schedule or any significant damage is done to the cylinder in which case a hydrostatic test is mandatory.
A visual test is essentially that, a detailed visual inspection of the entire cylinder and includes the following checks:
A hydrostatic test is performed every five years and includes a visual inspection as well as a water pressurisation test. 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.
Handling – Keep your cylinder safe, the more hits they take the shorter their lifespan. Never leave a cylinder standing in an upright attitude, since it can easily be knocked or tipped over. Merely laying the cylinder on its side is inadequate, especially in a van, car, truck or boat. Always block or secure your tank so it can’t fall over easily or roll around, which can damage it, other equipment or you. Always face cylinders valves towards the rear of your vehicle.
Keep moisture out – Essentially if moisture gets in you are much more likely to get cylinder corrosion. The easiest way to avoid this is to keep your cylinder pressurised. Keeping some pressure in the cylinder will help to avoid this. When filling ensure that the compressor has at least two moisture filters. This should keep moisture to a minimum. Chipped paint to the outside should be repaired to cover exposed metal to prevent rusting also as water can migrate under the enamel paint and damage the cylinder during storage unseen.
The markings found on cylinders can vary depending on the region they were made and their layout can differ from manufacturer to manufacturer.
Below are two examples of cylinder markings you might find in the UK and the meaning of each marking