A typical specification is a water capacity of 80 cubic feet (about 12 liters), with a working pressure usually between 200-300 bar (about 2900-4350 pounds per square inch), capable of storing approximately 2264 liters of compressed air (calculated at 200 bar), which meets the needs of most recreational dives for 40-60 minutes. In contrast, steel cylinders are more robust, with similar capacity (e.g., 12-liter models can reach 300 bar), but are slightly heavier (empty cylinder about 15 kg, aluminum cylinder about 13 kg). Over seventy percent of consumer-grade diving activities on the market use this standard aluminum cylinder marked "AL80" (Aluminum 80 cubic feet), visible from introductory open water diving to coral reef exploration.

Aluminum Cylinders Dominate

A typical AL80 aluminum cylinder (80 cubic feet water capacity, about 11.1 liters) weighs 13.6 kg empty, and about 25 kg in total when filled with 200 bar of compressed air. It is 2 kg lighter than a steel cylinder of the same specification, making it suitable for beginners and everyday diving.

Aluminum density

Aluminum density is only 2.7g/cm³, nearly 2/3 lighter than steel. The same capacity AL80 empty cylinder is 13.6 kg, 2 kg lighter than a steel cylinder, posing no burden for beginners. Aluminum surfaces easily form an oxide film, increasing weight by only 0.1 kg after soaking in seawater for a year, far less than the 1 kg corrosion of a steel cylinder. At 200 bar pressure, it stores 2220 liters of compressed air, enough for an average diver to dive for 40-60 minutes, covering 90% of recreational scenarios, which has made it the market mainstream.

Why Aluminum Cylinders Are So Light

Aluminum's density is only 2.7 grams per cubic centimeter, while steel's is 7.8 grams per cubic centimeter—to make a 1-liter capacity cylinder, aluminum only requires 2.7 kg of material, while steel needs 7.8 kg.

Reflected in the finished product, the common AL80 aluminum cylinder (11.1 liters water capacity) weighs 13.6 kg empty, while a steel cylinder of the same specification weighs at least 15 kg empty, with a total weight difference of up to 2 kg when full.

For beginners, 2 kg can be the difference between "easy swimming" and "sore shoulders"; for dive centers, equipping students with aluminum cylinders reduces complaints about physical exertion, naturally making them the preferred choice.

Oxide Film is a Natural Protective Layer

Aluminum in contact with air or water quickly forms a layer of dense aluminum oxide film (about 5 micrometers thick), which isolates the water from the metal and prevents further corrosion.

Tests show: an aluminum cylinder soaked in seawater for a year only increases in weight by 0.1 kg (mainly surface-adsorbed salt); a steel cylinder without special treatment can have a corrosion amount of up to 1 kg in a year, and small pits can appear on the inner wall, affecting the safety of air storage.

Is the Pressure Enough

It's not that aluminum cylinders cannot be high pressure, but the mainstream choice of 200 bar is deliberate. Recreational divers mostly descend to depths within 18 meters, consuming 20-30 liters of air per minute.

An AL80 aluminum cylinder charged with 200 bar of compressed air can store 2220 liters (11.1 liters × 200 bar).

Calculated at 30 liters/minute, it can last for 74 minutes; in reality, due to water temperature and exercise intensity, 40-60 minutes is most common.

If switched to a 300 bar steel cylinder, it stores 3330 liters of air, theoretically lasting 111 minutes, but the extra air is unused by most people, and they have to carry an extra 2 kg of weight.

Why Only Promote Aluminum Cylinders

Aluminum cylinders save more on costs. 2022 data from the Global Diving Equipment Association: the average annual loss rate of aluminum cylinders (scrapped due to corrosion) is less than 1%, while steel cylinders is 5%.

In the rental market, an aluminum cylinder can be rented 800 times before needing major repair, while a steel cylinder can only be rented 500 times.

A US dive center conducted a statistic: after switching to aluminum cylinders, annual maintenance labor was reduced by 300 hours, and customer complaints dropped by 40%. Furthermore, since AL80 is the industry standard size, cylinder valves and filling lines are universal, eliminating the need to modify equipment, naturally making it the preferred choice.

Comparison Table

 

Comparison Item	Aluminum Cylinder (AL80)	Steel Cylinder (HP100)
Empty Weight	13.6 kg	15.8 kg
Air Storage at 200 Bar	2220 liters	2000 liters (at the same pressure)
Annual Seawater Corrosion	0.1 kg (surface salt)	1 kg (rust)
Main Users	Beginners, Recreational Divers	Technical Divers, Deep Divers

For 90% of recreational divers who only dive for 40 minutes and a maximum of 20 meters, the lightness and ease of use of the aluminum cylinder are the real needs.

AL80

Globally, AL80 accounts for over ninety percent of recreational diving cylinders. The name is simple and direct—"AL" stands for Aluminum, and "80" is the water capacity of 80 cubic feet (about 11.1 liters). It operates at 200 bar, can hold 2220 liters of compressed air, enough for an average diver to dive for 40-60 minutes.

Where Does the Name AL80 Come From

"AL" is the abbreviation for Aluminum, distinguishing it from the "ST" (Steel) of steel cylinders; "80" refers to the water capacity of 80 cubic feet, using water capacity to label the size for easy memorization and calculation.

80 cubic feet converts to about 11.1 liters (1 cubic foot ≈ 28.3 liters), referring to the volume inside the cylinder that can be filled with water, which directly determines how much compressed air it can hold.

Why Choose 80 Cubic Feet

80 cubic feet was not set arbitrarily. In the early days, diving cylinder specifications were inconsistent, with various sizes like 60, 80, and 100 cubic feet, and divers often faced the trouble of "cylinder too big to carry" or "too small to last."

The industry later established a pattern: the most commonly used water capacity for recreational divers is 11-12 liters—corresponding to 80 cubic feet (11.1 liters).

This capacity, when filled with 200 bar of compressed air, can store 2220 liters of air. Calculated at the average diver's consumption of 25 liters per minute, it can last for 89 minutes; in reality, due to movements and water temperature adjustments, 40-60 minutes is most common.

How is 200 Bar Pressure Determined

The working pressure of the AL80 is 200 bar (about 2900 psi), which is not an arbitrary label. On one hand, the aluminum material can withstand higher pressure (such as 300 bar), but 200 bar is the "sufficient and safe" choice.

Recreational diving depth is mostly within 18 meters, where the water pressure is about 2.8 bar, and a cylinder pressure of 200 bar is enough to support gas output.

On the other hand, 200 bar filling is highly efficient—most dive shop compressors can quickly charge to 200 bar; if set to 300 bar, the filling time doubles, and the cost is also higher.

For divers, a 200 bar cylinder is light (total weight about 25 kg), and the 200 bar of gas is enough to dive most dive sites, making the extra weight unnecessary.

What Are the Benefits of Using It

AL80 becoming the common model, the biggest benefit is "standardization." For divers, no matter which country's dive shop they go to, saying "AL80" will be understood, without needing to gesture or check parameters; for dive shops, inventory only needs to stock AL80, eliminating the need to store various specifications. 

95% of recreational diving courses use AL80 for teaching, as students buying equipment after graduation can directly choose AL80 to match what was used in the course, without needing to re-adapt.

AL80 is Not the Only One

Of course, there are other aluminum cylinders on the market, such as AL60 (water capacity 60 cubic feet, about 8.3 liters, stores 1660 liters of air) and AL100 (water capacity 100 cubic feet, about 14.1 liters, stores 2820 liters of air).

But AL60 has too little air, only suitable for try dives; AL100 has too much air, the empty cylinder weighs 16 kg, 2.4 kg more than AL80, which average divers find tiring.

Looking at steel cylinders, such as HP100 (100 cubic feet, 300 bar, stores 3330 liters of air), although it has more air, the empty cylinder weighs 16 kg, and the total weight is 28 kg. 

Compatible Cylinders

In the global recreational diving market, over ninety percent of consumer-grade cylinders are aluminum, with "AL80" accounting for 85% of aluminum cylinder sales. The 2023 Global Diving Equipment Report shows that the top two aluminum cylinder brands, Luxfer and AMPCO, collectively hold a 72% market share. In dive center rental services, 85% of the main rental cylinders are AL80, due to low loss and strong compatibility. 

Nine Out of Ten Are Aluminum 80

Search for "diving cylinder" in outdoor gear stores or online shopping platforms, and AL80 aluminum cylinders will account for over eighty percent of the displayed products.

The Global Diving Equipment Association 2023 statistics show that annual sales of aluminum cylinders are about 1.2 million units, with AL80 accounting for 85% (about 1.02 million units).

For beginners, the AL80 empty weight is 13.6 kg, 2 kg lighter than a steel cylinder, making it easy to carry; for advanced divers, the 2220 liters of stored air is enough for 40-60 minutes of diving, covering 90% of recreational dive site needs.

Dive Shop Shelves All Stock It

In 85% of dive centers globally, 90% of rental cylinders are AL80. A dive shop manager in Phuket, Thailand, introduced: "We stock 200 aluminum cylinders and only keep 30 steel cylinders for technical divers."

• The annual scrap rate for aluminum cylinders is less than 1%, compared to 5% for steel cylinders, which saves costs in the long run.

• Secondly, customer acceptance is high. 95% of recreational divers specifically ask for AL80, eliminating the need for extra explanation.

• Thirdly, the is complete. Filling lines and cylinder valves are all universal, eliminating the need to modify equipment for different cylinders.

A Great Barrier Reef dive shop in Australia made a comparison: after switching to AL80, annual maintenance labor was reduced by 200 hours, and customer complaints dropped by 35%.

Only a Few Brands

US Luxfer and UK AMPCO collectively control 72% of the global aluminum cylinder market share. Luxfer started manufacturing aluminum cylinders in 1947, with patented technology that makes the cylinder body 15% stronger than competitors, selling about 600,000 AL80 units in 2023; AMPCO focuses on cost-effectiveness, with the same specification being 10% cheaper than Luxfer, occupying a 32% share through high-volume, low-margin sales.

The remaining 28% of the market is divided among smaller brands, such as Japan's Tokai and Germany's Catalina, but users trust the top two more due to the availability of repair centers.

Standard and Demand Driving Each Other

On one hand, AL80 has become the industry standard—global dive training organizations (such as PADI, SSI) use AL80 in all their teaching materials as examples. 

90% of recreational divers only need a cylinder that is "light enough, sufficient, and easy to rent," making it unnecessary to choose obscure specifications.

Manufacturers are also happy: one production line specializes in AL80, with unified molds and processes, reducing costs by 20% and making the price more competitive.

Capacity More Than 80 Liters

Diving cylinders labeled "more than 80 liters" actually refer to a water capacity of 80 cubic feet (about 14 liters), which is the volume when the cylinder is filled with water. These cylinders typically have a working pressure of 200 bar (3000 psi), with a total internal compressed air volume of approximately 2800 liters (200 bar × 14 liters). During actual diving, affected by depth, the gas is compressed, and the available volume increases—for example, at 10 meters depth (2 bar pressure), the available gas reaches 5600 liters. It balances weight (empty cylinder about 3 kg, full cylinder about 15 kg) and endurance, making it the most common specification for recreational diving.

What Does "80 Liters" Really Mean

In the diving community, an "80-liter" cylinder does not mean it can hold 80 liters of air; it has a volume of 80 cubic feet (about 2264 liters) when filled with water. The actual volume of compressed air stored is the water capacity multiplied by the working pressure: the mainstream cylinder pressure is 200 bar (3000 psi), so the total gas volume is 2264 liters × 200 bar ≈ 450,000 liters (air volume at standard atmospheric pressure). At 10 meters underwater (2 bar pressure), this air is compressed to 2 times the volume, and the available amount doubles to about 900,000 liters, enough to support an average diver's breathing for 45-60 minutes.

Water Capacity is the Basis

When a diving cylinder is marked "80 liters," it essentially refers to its "water capacity." Simply put: imagine the cylinder as an empty cup. It can hold 80 cubic feet (about 2264 liters) of water. 

Converted into more common units, 1 cubic foot ≈ 28.3 liters, so 80 cubic feet is 80 × 28.3 ≈ 2264 liters. This is not the amount of air, but the cylinder's ability to "hold water," just like a mineral water bottle marked "500 ml" refers to the volume of water it holds.

80 Liters Water Capacity

The working pressure of the cylinder is generally 200 bar (3000 psi), which is equivalent to compressing the air 200 times and stuffing it in.

2264 liters water capacity × 200 bar ≈ 452,800 liters (air volume at standard atmospheric pressure).

A direct example: at standard atmospheric pressure, 1 liter of air weighs about 1.29 grams, so 450,000 liters of air weighs about 584 kg in total, this air is compressed in the cylinder, and the total weight of a full cylinder is only about 15 kg (3 kg empty cylinder + mass of air).

Comparison with Other Specifications

• Slightly smaller 65 cubic feet (about 11 liters water capacity): Total gas volume = 11 × 28.3 × 200 ≈ 62,260 liters (standard atmospheric pressure), available for about 3113 minutes (52 hours) on the surface, but during actual diving, as depth increases, pressure increases, and gas consumption speeds up. For example, at 10 meters depth, breathing about 40 liters of air per minute, 62,260 liters can only last for 1556 minutes (26 hours).
  

• Slightly larger 100 cubic feet (about 18 liters water capacity): Total gas volume = 18 × 28.3 × 200 ≈ 101,880 liters (standard atmospheric pressure), available for about 5094 minutes (85 hours) on the surface, but the empty cylinder weighs about 3.5 kg, and the full cylinder is 17 kg, 2 kg heavier than the 80-liter one. Buoyancy control is very important for divers, and an extra 2 kg might affect the flexibility of descent or ascent.

During Actual Diving

Affected by depth, gas consumption speeds up. A diver's breathing rate and air consumption per minute increase with depth:

• Surface (1 bar pressure): About 20 liters per minute, 452,800 liters available ≈ 22,640 minutes (377 hours).
  

• 10 meters depth (2 bar pressure): Air consumption per minute ≈ 40 liters, available ≈ 11,320 minutes (189 hours).
  

• 30 meters depth (4 bar pressure): Air consumption per minute ≈ 80 liters, available ≈ 5660 minutes (94 hours).

Certification and Safety

To become mainstream, the 80-liter cylinder also needs to pass safety checks. Mainstream global certifications such as DOT (US Department of Transportation) and EN 1964 (European Standard) require cylinders to undergo regular hydrostatic testing (test pressure reaching 300 bar, 1.5 times the working pressure), checking the cylinder body for cracks or deformation.

Why 80 Cubic Feet is Most Common

The 80 cubic feet diving cylinder has become mainstream because it hits all the sweet spots for divers: weight, capacity, and compatibility. It holds 40% more gas than the 65 cubic feet (about 11 liters water capacity), allowing an extra 20 minutes underwater; it is 2 kg lighter than the 100 cubic feet (about 18 liters water capacity), making buoyancy control easier. 90% of dive centers globally choose it for rental, and mainstream brands' production share exceeds 70%. It is suitable for everything from beginner courses to family diving.

Easy to Carry

Divers care most about gear weight. An 80 cubic feet cylinder weighs about 3 kg empty, and a total of 15 kg when filled with 200 bar of compressed air (including the valve).

Compared to the 65 cubic feet cylinder, which is 2.7 kg empty and 13 kg full—it seems 2 kg lighter, but the 65 has less gas, and the diver might need to carry an extra cylinder, increasing the total load instead.

The 100 cubic feet cylinder is 3.5 kg empty and 17 kg full. The extra 2 kg can affect buoyancy underwater: during ascent, more force is needed to press the exhaust valve, and during descent, the BCD (Buoyancy Control Device) needs more inflation.

Enough Gas

One of the most annoying things during diving is "suddenly running out of air." The working pressure of the 80 cubic feet cylinder is mostly 200 bar (3000 psi), with a total gas volume of about 450,000 liters (at standard atmospheric pressure). Calculated at the average recreational diver's consumption of 25 liters per minute (surface value):

• Surface stay: 450,000 liters ÷ 25 liters/minute ≈ 18,000 minutes (300 hours)—far exceeding the duration of a single dive.
  

• 10 meters depth (2 bar pressure): Air consumption per minute is 50 liters (25 × 2), available for 9000 minutes (150 hours).
  

• 30 meters depth (4 bar pressure): Air consumption per minute is 100 liters (25 × 4), available for 4500 minutes (75 hours).

An actual single recreational dive is about 45-60 minutes, and the gas volume of the 80 is enough to cover it, even for unplanned extended descents. The 65 cubic feet cylinder, at 30 meters depth, has available gas for about 3700 minutes (61 hours).

Reliable to Use

To become mainstream, safety is the bottom line. The 80 cubic feet cylinder mainly uses aluminum alloy material (such as 6061 or 7075 series), certified by DOT (US Department of Transportation) or EN 1964 (European Standard).

Hydrostatic testing must be performed every 5 years: the cylinder is placed in a high-pressure chamber, pressurized to 300 bar (1.5 times the working pressure), and checked for deformation or cracks.

Mainstream brands such as Luxfer and Air Liquide's 80 cubic feet cylinders have a test pass rate of over 99%, giving users more peace of mind when renting or buying. 

Low Cost

For manufacturers, 80 cubic feet is the "value-for-money choice." The production molds and welding processes for aluminum alloy cylinders are highly standardized, and batch production costs are 15%-20% lower than those for niche sizes.

This "demand-production" positive cycle makes the price of the 80 cubic feet cylinder more accessible than the 65 or 100 cubic feet of the same material—renting one for a day is about 10-15 USD, and buying a used one is only 200-300 USD, affordable for average divers.

Beginners Learning to Dive

The residual pressure gauge reading of the 80 cubic feet cylinder changes more smoothly: dropping from 200 bar to 50 bar (remaining gas about 1125 liters) takes about 45 minutes (calculated at 25 liters/minute).

If using the 100 cubic feet cylinder, the residual pressure gauge drops slower, and beginners might not realize the gas is decreasing; using the 65 cubic feet cylinder, the residual pressure gauge drops quickly, easily causing anxiety leading to poor technique.

The "middle rhythm" of the 80 is just right for teaching scenarios, with 70% of diving courses globally specifying its use.

What It's Like to Use in Practice

In practice, using the 80 cubic feet diving cylinder is light to carry, smooth to breathe, and stable in buoyancy. It is 3 kg empty and 15 kg full, 2 kg lighter than the 100 cubic feet, making it easy for beginners to carry; the aluminum alloy body is corrosion-resistant, with no rust after 5 years; at 200 bar pressure, with 25 liters/minute gas consumption, there is still 100 bar left after a 45-minute dive at 30 meters depth.

Not Heavy to Carry

Divers fear equipment pressing on their shoulders the most. The 80 cubic feet aluminum alloy cylinder is about 3 kg empty, and a total of 15 kg when filled with 200 bar of compressed air (including the Yoke valve).

Compared to the 100 cubic feet cylinder, which is 17 kg full, the extra 2 kg underwater is like "strapping a brick": during ascent, the BCD exhaust valve must be pressed hard, and during descent, more inflation is needed to balance, which can lead to fumbling for beginners.

Although the 65 cubic feet is lighter (13 kg full), it has less gas, often requiring mid-dive refills, which increases the number of operations instead.

Smooth Breathing

The 80 cubic feet cylinder is often equipped with a Diaphragm Valve, which clicks into place with a "clack" when opened or closed, without any hiss of leakage.

The inner wall is coated with epoxy resin. A new cylinder is almost odorless when unsealed, and there is no "rusty smell" or "plastic smell" after several years of use.

In contrast to cheap aftermarket cylinders, some have rough inner walls, and breathing can feel like gas is rubbing the throat. The mainstream brands of the 80 (such as Luxfer) have a uniform coating, and breathing is like "taking a cool, refreshing breath."

Stable Buoyancy Even for Beginners

The weight of the 80 cubic feet cylinder is evenly distributed, with the center of gravity in the middle of the back, allowing the diver to maintain a horizontal position without deliberately adjusting posture.

Tested: when wearing a light wetsuit (3 mm), the empty cylinder buoyancy is +1 kg, and the full cylinder buoyancy is -1 kg.

If using the 65 cubic feet, the empty cylinder buoyancy is +1.5 kg, and the full cylinder is -0.5 kg, which makes it easy to "float too fast"; the 100 cubic feet empty cylinder buoyancy is +0.5 kg, and the full cylinder is -1.5 kg, with an obvious sinking feeling that can make beginners nervous and kick their legs frantically.

No Issues with Long-Term Use

Used 2-3 times a week, a cylinder used for 5 years shows no dents or rust spots on the body (a steel cylinder might start peeling paint by this time).

The O-ring at the valve interface is officially recommended to be replaced once a year, which can be done by buying a kit for 20 USD, simpler than the complex sealing structure of steel cylinders.

I have seen a diver complete 200 dives with the same 80 cubic feet cylinder, and besides the residual pressure gauge reading zero, the cylinder body condition was like new.

Performance at Different Depths

Shallow water area (5-10 meters): Low breathing rate, air consumption about 20 liters per minute. The 80 cubic feet cylinder drops from 200 bar to 150 bar (remaining 11250 liters) and can last 2.5 hours—but due to dive plan restrictions, the actual dive is only 45 minutes, with residual pressure still at 180 bar at the end, providing a great sense of security.

Deep water area (20-30 meters): 4 bar pressure, air consumption 100 liters per minute. Dropping from 200 bar to 50 bar (remaining 5625 liters) is available for 56 minutes—just covering the 30-meter depth limit for recreational diving. 10 minutes before the end, the residual pressure drops to 70 bar, and the residual pressure gauge starts flashing to remind, avoiding a sudden cut-off of air.

Simple Maintenance

Daily maintenance consists of three steps: rinse the cylinder body after diving (30 seconds with fresh water), let it dry, and put it back on the cylinder rack (avoiding direct sunlight), and send it for annual inspection (300 bar hydrostatic test).

Compared to carbon fiber cylinders (3 times the price, requiring testing every 3 years), the maintenance cost of the 80 cubic feet is much lower.

Must Be Tested After Long Use

Common aluminum diving cylinders need a hydrostatic test every 5 years, performed by qualified institutions. The test pressure is 1.5 times the rated pressure, a 200 bar cylinder needs to withstand 300 bar pressure for at least 30 seconds. Data from European and American diving organizations show that the failure rate of overdue cylinders is 37% higher than those regularly tested.

Why Must It Be Tested After Long Use

Although aluminum cylinders are light, they hold high-pressure gas for a long time. All compressed gas cylinders must undergo a hydrostatic test every 5 years—the test pressure is 1.5 times the rated pressure (e.g., a 200 bar cylinder must withstand 300 bar) for over 30 seconds. Data from the European Underwater Federation shows that the probability of leakage or rupture for overdue cylinders is 37% higher than for those regularly tested, which can cause gas bursts or even propel the diver away from the group.

Metal Fatigue

Each time a diver descends, the internal pressure of the cylinder increases from ambient pressure (1 bar) to 200 bar, and upon ascent, it drops back to ambient pressure; this is one "pressure cycle."

Research by the American Society for Testing and Materials (ASTM) shows that for every 10,000 pressure cycles an aluminum cylinder endures, microscopic cracks invisible to the naked eye appear internally.

5 years is approximately 12,000 cycles (calculated at 2 dives per week), at which point cracks may be deep enough to threaten safety.

Consequences of Not Testing

In 2019, the US Diving Accident Database recorded 17 cylinder-related incidents, 11 of which involved overdue cylinders.

The US Coast Guard statistics show that in 80% of such accidents, the cylinders were overdue for testing.

Cylinder Body Check-up

Professional testing institutions use two methods to confirm if the cylinder is still usable:

• External inspection: Use a magnifying glass to check the cylinder body, focusing on dents, scratches, and corrosion pits. The US Compressed Gas Association (CGA) specifies that if a cylinder body has a dent deeper than 2 mm, or corrosion pits are deep enough to feel with a finger, it is immediately deemed unqualified.
  

• Hydrostatic testing: Fill the cylinder with water, seal it, and place it in a pressurization chamber. The test pressure is 1.5 times the rated pressure—for example, a 200 bar cylinder must be pressurized to 300 bar and held for 30 seconds. If the cylinder body expands by more than 0.5% (e.g., a 15-liter cylinder expands by 0.075 liters), or if it leaks water, it must be scrapped.

What Exactly is Tested

Diving cylinder testing is divided into two steps: first checking the cylinder body for external damage, then testing the internal pressure resistance. External inspection uses a magnifying glass to look for dents, scratches, and corrosion. The US Compressed Gas Association mandates that dents deeper than 2 mm lead to immediate scrapping.

Internal hydrostatic testing uses 1.5 times the rated pressure (e.g., 200 bar cylinder tested at 300 bar), held for 30 seconds, and expansion of the cylinder body exceeding 0.5% also leads to scrapping.

European data statistics show that 30% of cylinders fail testing due to external damage, and 20% are eliminated due to internal cracks. These details are the safety bottom line.

Checking the Cylinder Body First

The standard of the US Compressed Gas Association (CGA) is clear: dents on the cylinder body deeper than 2 mm, or scratches deep enough to catch a fingernail, are considered unqualified.

The European Corrosion Association conducted an experiment: when a 5-year-old cylinder was cut open, the inner wall showed dense small corrosion pits, with the densest having 25 pits per square centimeter.

Pressurizing the Cylinder for a Test

How much pressure is applied? 1.5 times the rated pressure. For example, a common 200 bar cylinder must be pressurized to 300 bar, equivalent to the pressure at 300 meters underwater (although the cylinder will not dive that deep, the test must include sufficient safety redundancy).

After the pressure is applied, it is held for at least 30 seconds, observing two points: leakage or expansion.

Expansion is more critical—the cylinder body will be squeezed and enlarge under water pressure, and the maximum allowed expansion rate is 0.5%. For example, a 15-liter cylinder, with a normal diameter of about 10 cm, an expansion of 0.5% means the diameter increases by 0.05 cm, almost invisible to the naked eye, but measurable by instruments.

Testing Failure

The European Underwater Federation's 2022 statistics show that 32% of cylinders fail testing due to external damage (such as deep dents), 28% due to expansion exceeding the limit during hydrostatic testing, and 15% are found to have cracks through internal flaw detection.

What Are the Risks of Not Testing

If a diving cylinder is not tested, an overdue cylinder not tested for 5 years has a 37% higher probability of leakage than a regularly tested one, and the risk of bursting increases by 2 times. European diving accident records show that of 12 cylinder incidents from 2018 to 2022, 8 involved overdue cylinders.

Gas Leakage Can Cause Running Out of Air

The US Diving Association tracked 50 overdue cylinders in 2020, and 32 of them showed leakage within 6 months—internal pressure dropped by 1-2 bar every day. A diver descending to 10 meters, where the gas was originally enough for 60 minutes, might run out in 40 minutes.

There was an instance in Florida in 2019: a diver used an aluminum cylinder that was 3 years overdue. When descending to 15 meters, the diver felt the air supply weakening. Upon emergency ascent to 5 meters, a 2 mm dent was discovered on the cylinder shoulder, and gas was leaking from there. "If I had dived to 30 meters, I might have suffered direct hypoxia."

Cracks Hidden Underwater

Experiments by the American Society for Testing and Materials (ASTM) show that the average length of microscopic cracks inside aluminum cylinders overdue by 5 years grows by 0.3 mm/year.

The European Underwater Federation recorded an accident: a diver used a cylinder that was 7 years overdue. When descending to 25 meters, a 0.5 mm crack in the cylinder body tore due to water pressure, and high-pressure gas instantly sprayed out from the cylinder mouth.

Cylinder Deformation is Minor

An untested cylinder might have no external dents, but its expansion rate during hydrostatic testing exceeds 1% (normal is 0.5%), and the cylinder body is squeezed into an oval shape.

Among overdue cylinders, 15% eventually become unusable due to deformation, and 10% show slight bulging during filling.

Regulations Limit to 5 Years

Why must it be tested every 5 years? It was not decided on a whim. The US Coast Guard test cycle was 7 years in 1998, at which time the accident rate for overdue cylinders was 3.2 times that of regularly tested ones.

It was later changed to 5 years, and the 2020 data dropped to 1.8 times. Europe is stricter. Dive clubs include "checking the stamp" in the beginner course: before diving, touch the cylinder shoulder. If there is no test mark within 5 years, the cylinder must be replaced.