Guide to Scuba Diving Cylinders
What would we do without a diving cylinder? Well, we would most likely have to dive with surface air umbilical like commercial divers. While our diving cylinder seems like a simple pressure vessel at first, the whole process of manufacturing a diving cylinder is a fascinating balance of force and precision. Scuba diving cylinders need to be manufactured to the highest standard as, throughout their lifespan, they will be subjected to tremendous pressure, wear and tear in one of the most challenging of environments.
A scuba diving cylinder, in the present form, was invented in the mid of 20th century and the history stretches back to 1906 when Vitkovice cylinders started to produce first pressure cylinders from metal pipes.
Diving cylinders come in a multitude of sizes and shapes, and our online store offers a broad selection of scuba diving cylinders from small Spare Air emergency cylinders and pony cylinders to large capacity dive cylinders and twinsets. Scuba dive cylinders are made either from steel or aluminium and because of their differences in buoyancy and weight, they are used for different applications in scuba diving.
Steel vs Aluminum Scuba Cylinders
Steel Dive Cylinders
There are different manufacturing methods, but steel dive cylinders are made mainly in the process called "deep draw". This process involves a high-quality chrome-molybdenum steel blank drawn into the form by a machined punch. Steel cylinders are considered stronger and more durable than aluminium, and production of a steel tank requires less raw material than the aluminium tank minimising material waste.
Steel diving tanks have lower total weight and better buoyancy characteristics. What does it mean in practice? Steel tanks shift from negative to neutral buoyancy during a dive, allowing the diver to have more consistent weight and finish the dive better balanced. Moreover, steel dive tanks are the favourite choice for most divers in the UK as the buoyancy characteristic helps diving in drysuits. Steel diving cylinders have working pressures between 232 bar to 300 bar. Scuba diving cylinders rated to 300 bar are heavier as they have thicker walls to be stronger.
The most popular scuba dive cylinders manufacturers are Faber Cylinders from Italy followed by Vitkovice Cylinders in the Czech Republic.
Aluminium Dive Cylinders
Aluminium dive tanks are made mostly in a process called "backward extrusion." This process involves driving a metal rod into a lump of aluminium alloy and cylindrical mould, forming a well known cylindrical shape. Aluminium is a softer material; it requires thicker walls to contain pressurised gas. Aluminium scuba diving cylinders also differ from steel ones with buoyancy characteristic - they tend to be slightly negatively buoyant at the beginning of the dive and positively buoyant when they are empty. While aluminium scuba tanks are very popular with dive centres in the Americas and the Middle East, in the UK they are mainly used in technical diving application as decompression or stage cylinders. Typical working pressure of an aluminium scuba diving tank varies between 200 - 230 bar.
The largest manufactures of aluminium scuba diving cylinders are Luxfer and Catalina.
DIN vs A-Clamp Scuba Cylinder Valves
DIN valve (short for Deutsche Industrie Norm) invented in the US but adopted with great success by German manufacturing standards, and widely used in Europe. There are becoming more popular around the world due to certain benefits in their design. DIN 1st stage regulator's valve screws into a dive cylinder valve capturing regulator's o-ring and therefore reducing the risk of pinching, exposure to dust and dirt.
DIN valves come with M25 thread and two options - 232 bar and 300 bar. Shorter M25-232 bar thread goes with scuba tanks which have a lower working pressure of 232 bar, while longer M25-300 bar thread is installed onto scuba tanks with a higher working pressure of 300 bar. Nowadays most regulators come with M25-300 bar thread to fit on both tank valve options. Regulators fitted with M25-232 bar thread cannot be used with 300 bar tank as the o-ring will not seal properly. DIN valves are considered to be more streamlined, lighter, and it is argued that they are also more robust and reliable. Additionally, M25 DIN valves will accept A-Clamp regulator when installed with an inexpensive adapter.
You will also find M26 thread on selected regulators and cylinder valves, and they are mainly installed on equipment to be used exclusively with nitrox and helium. The M26 valve thread is a larger diameter than the M25 DIN fitting, and both types are not compatible.
A-Clamp or Yoke valve is successfully installed on hundreds of thousands of regulators and scuba cylinders worldwide, but it is most popular in North America and the Caribbean. How does A-Clamp connection work? A-Clamp first stage is fitted onto a valve tank plate and secured into place by a screw. Regulator valve is pushed against the cylinder valve face and the o-ring, sealing the connection. A-clamp valves are rated only to the maximum working pressure of 232 bar. An A-Clamp regulator will fit on a DIN valve providing the DIN tank valve is equipped with an inexpensive adapter insert.
Read more on DIN vs Yoke in our blog.
Scuba Diving Cylinder Inspections
Scuba Diving Cylinder Visual Test
The most significant threat for the scuba diving cylinders is moisture, which causes corrosion weakening its integrity. It has been researched and proven that the corrosion in cylinders used for high-pressure air and nitrox fills can advance 100 times faster than in any other pressure container. In theory, the lifespan of a correctly cared for steel diving cylinder can be 40 years, while aluminium diving cylinder can serve for up to 20 years and 10000 pressure tests. In the real world seldom any tank reaches that age.
Every cylinder leaves a factory with the hydrostatic test already completed. The date of the factory test is stamped onto the cylinder, - e.g. 2020/08 (year 2020/ month August).
According to the present regulations, scuba cylinders have to undergo visual testing every 2½ years from the initial factory test. Visual testing includes checking for corrosion, dents, bulges, cracks and general state of the cylinder and its valve. If the tank passed the visual test, a green or blue quadrant sticker decal is applied to show the last and the next required test dates - visual or hydrostatic. It is essential to emphasise the lifespan of a dive cylinder depends on many factors, so it is imperative to adhere to testing recommendations and if in any doubt, have your tank visually inspected before a fill.
Our Chiswick store accepts all brands and types of Diving Cylinders including steel, aluminium and Spare Air for Visual Diving Cylinder Testing by our IDEST Test Facility.
Scuba Diving Cylinder Hydrostatic Test
According to the latest UK and EU standards, scuba diving cylinders need to be tested hydrostatically every 5 years. The hydrostatic test will include a visual test first, followed by a hydrostatic test. The hydrostatic test involves pressurising scuba tank, exceeding its working pressure and measuring the volume before and after the test. If there is a consistent increase in volume, the tank will fail the test and will be removed from service. If the test is a pass, the testing technician will stamp the test date onto the tank alongside his mark, approving the tank to be used in scuba diving. The regulatory bodies have agreed that a blue sticker decal will also need to be applied to show the date of the last and the next required tests - visual or hydrostatic.
Our Chiswick store accepts all brands and types of Diving Cylinders including steel, aluminium and Spare Air for Hydrostatic Diving Cylinder Testing by our IDEST Test Facility.
Oxygen Cleaning
Oxygen, as oxidising gas does not burn, but supports combustion and allows other materials to burn. In the pressurised environment, like scuba diving cylinder, the oxygen concentration is higher; so there is a higher risk of fire. Higher oxygen content also lowers the ignition point for many materials and increases the combustion intensity and flame temperature.
There is no specific regulation in Europe that imposes standards on scuba diving equipment usage with oxygen-rich mixes. There is a joint agreement between health and safety agencies that scuba diving cylinders with any mixture of breathing gas that contains oxygen of more than 23% needs to be treated with extreme care.
It is vital to prepare your scuba diving cylinder for service with higher oxygen fills by having it O2 cleaned. Scuba cylinder O2 cleaning is achieved by washing a scuba tank thoroughly with special chemical solvents to remove any impurities, oils and other unwanted hydrocarbon particles that might be flammable in a high oxygen environment. During the cleaning process, your scuba cylinder valve is also cleaned, receives O2 compatible grease and is fitted with O2 compatible o-rings such as Viton o-rings. When the whole process is completed tank gets an O2 cleaned certificate and a sticker decal with a service date and is ready for use. Oxygen cleaning is valid for 15 months but may be required sooner if fill quality is doubtful. Our Chiswick store accepts all brands and types of Diving Cylinders including steel, aluminium for Diving Cylinder O2 Cleaning Service by our IDEST Test Facility.
Tips On How To Care For Your Cylinder
How to care for your diving cylinder?
Diving cylinders sold by our store are made to the highest standards by carefully selected dive cylinder manufacturers. However, to prolong the lifespan of your dive cylinder, we have a few tips and suggestions:
- Fill your scuba tank with dry air at a reputable dive shop.
- Before you start filling the tank briefly open the tank's valve to blow moisture out.
- Avoid emptying cylinders underwater - valve open underwater may let highly corrosive water inside cylinder.
- Do not store empty cylinders with their valves open. Open valve will allow the moist air into the tank contributing to corrosion.
- When on a dive site always keep your cylinder on a side rather than standing. It will prevent your tank from being knocked over.
- Wash your tank with fresh water after diving and occasionally remove its rubber boot and rinse with fresh water.
- If you have any doubts, have your tank visually inspected before filling with gas, even if it has a valid test.
Scuba Diving Cylinders by Size and Use
Spare Air - emergency escape system supplied with a breathing regulator valve. The system can be filled from a regular scuba tank.
3 litre - so-called pony cylinders because of their small size. These steel diving cylinders are used mainly as a redundant air source or used in rebreathers as diluent and oxygen tanks.
Aluminium S40 - 5.7-litre scuba diving cylinder used mainly in the Americas and the Caribbean as a stage or decompression tanks or a rebreather tanks.
7 litre - steel diving cylinder used for training or a petite person. Main application nowadays is as bailout tanks in rebreather diving or decompression or stage tanks in technical diving.
10 litre - steel dive cylinder used in scuba diving for smaller build person or used as s bailout bottle or decompression and stage tanks in technical diving.
Aluminium S80 - 11.1-litre aluminium diving cylinders are used by dive centres abroad as a standard diving cylinder. It is also used in technical diving as a bailout or decompression cylinder. Twin manifolded S80s are used extensively in technical diving.
12 litre - steel 12 litre diving cylinder is the most popular tank size in recreational diving. It is widely used by dive centres and liveaboards worldwide. Twin Manifolded 12s are a popular choice in technical diving.
Aluminium S100 - 13.3 litres aluminium diving cylinder used on many liveaboards and resorts as a larger capacity tank.
15 litre - scuba diving cylinder used extensively in the UK in single tank diving. Twin 15-litre tanks can be paired together with a manifold valve for twinset diving.
20 litre - large capacity scuba diving cylinders used sporadically due to their heavyweight and large size.
