Common capacities include 12 liters, 15 liters, and other specifications, with working pressures generally ranging from 200 bar to 300 bar (approximately 200 to 300 standard atmospheres). When full, they can store 2400 to 4500 liters of compressed air (e.g., a 12-liter tank at 200 bar pressure stores 2400 liters). To ensure safety, tanks require a hydrostatic test every 5 years (testing pressure reaches 1.5 times the working pressure), and daily checks are needed for the bottle valve's seal and the surface for corrosion or dents, to avoid risks caused by material fatigue or contamination.

Common Names

The US diving community mostly calls them "scuba tank" directly (accounting for 70% of forum discussions), while some parts of Europe prefer "diving cylinder" (more common in Commonwealth countries), and divers in Southeast Asia like the abbreviation "dive cylinder." 

The Most Universal Name

Walk into any dive shop, point to the silver-gray metal bottle on the wall and ask "What is this," and the shop assistant will most likely answer: "This is a scuba tank." Open the basic textbook of PADI (Professional Association of Diving Instructors), and the first page's equipment diagram is labeled "scuba tank." In the scuba diving community, these two names cover almost 90% of daily communication scenarios.

The name "scuba tank" originates from "scuba" itself. SCUBA is the acronym for "Self-Contained Underwater Breathing Apparatus," and the core of this apparatus is the gas storage cylinder.

In the 1950s, US Navy divers began using "scuba" to refer to the entire set of equipment, and the tank, as the most prominent component, was naturally referred to as the "scuba tank."

The logic of this naming is straightforward: a cylinder belonging to the scuba system is called a scuba tank.

Analysis of user posts on Scubaboard, the largest US diving community, in 2022 showed that 72% of posts discussing equipment directly used "scuba tank," far exceeding "diving tank" (18%) and "air cylinder" (10%).

Statistics from the European diving forum Diveboard are similar, with 65% of UK and German users habitually saying "scuba tank."

Even equipment manufacturers follow suit, with titles almost all being "Scuba Tank," and "compressed air cylinder" only appearing in technical specifications.

In the PADI Open Water Diver course teaching outline, the first equipment session explicitly requires: "Use 'scuba tank' to refer to the container storing breathing gas, to prevent students from confusing it with oxygen cylinders or industrial cylinders.

Divers setting up a dive might say: "I brought a new scuba tank, the pressure is 200 bar.

Technical divers might use "back gas cylinder" specifically for back-mounted tanks, and freedivers might say "rebreather tank," but these are terms for niche scenarios.

For 90% of recreational divers, from buying the first tank, attending the first class, to every single dive, "scuba tank" is the most natural and least error-prone choice.

Compressed Air Cylinder

When you hear the instructor say "check the pressure of the compressed air cylinder" in a diving class, you might wonder: why not just call it an "air cylinder" when it clearly contains air? Actually, the word "compressed" hides the key to diving safety—it's high-pressure air compressed to 200-300 bar, and the term "compressed air" accurately distinguishes its essential difference from pure oxygen cylinders or industrial gas cylinders.

Why the Need to Emphasize

Scuba divers do not breathe natural air, but high-pressure air compressed into a steel cylinder.

There are two key points here: first, "compressed," the air in the tank is pressured to 200-300 atmospheres (equivalent to squeezing the water pressure of a standard swimming pool into a soda bottle);

second, "air," the composition is the same as at the surface—21% oxygen + 79% nitrogen, completely different from medical pure oxygen cylinders (99% above oxygen) or industrial nitrogen cylinders (99% nitrogen).

The DAN (Divers Alert Network) 2021 safety report mentioned that about 5% of diving accidents globally each year are related to gas misidentification.

A novice once mistook a medical bottle filled with pure oxygen for a "diving cylinder" and breathed it at a depth of 10 meters, resulting in central nervous system oxygen toxicity due to excessive oxygen partial pressure (>1.4 bar), almost causing unconsciousness.

In Teaching Scenarios

In PADI (Professional Association of Diving Instructors) basic courses, "compressed air cylinder" is a high-frequency term. It's printed with "Compressed Air," and marked with "O₂:21%, N₂:79%."

PADI's 2023 teaching feedback survey showed that classes using "compressed air cylinder" for instruction had a 92% correct recognition rate for gas type among students, while classes using "air cylinder" had 15% of students mistakenly believe the contents were pure oxygen.

Compared to Other Cylinders

Type	Gas Composition	Pressure Range	Common Use	Risk of Incorrect Use
Compressed Air Cylinder	21%O₂+79%N₂	200-300 bar	Recreational Diving, Technical Diving	No obvious risk (correct use)
Medical Pure Oxygen Cylinder	≥99%O₂	100-200 bar	Medical Emergency, Enriched Air Nitrox Diving	Oxygen toxicity in deep water (>1.4 bar)
Industrial Nitrogen Cylinder	≥99%N₂	150-250 bar	Industrial Welding, Inert Gas Protection	Hypoxia and unconsciousness (no oxygen)

Technical divers may use "nitrox mixed gas cylinders" (such as EANx tanks containing 32% or 36% nitrogen), but those are special cases with further adjustments. In basic scenarios, "compressed air cylinder" remains the most fundamental term.

Local Customary Abbreviations

In dive shops in Phuket, the instructor shouts, "Take the dive cylinder to be filled"; at a dive site on the Gold Coast, a dive buddy might say, "Help me check the dive bottle pressure"; in a technical diving club in London, an old diver flips through an equipment box, muttering, "This diving cylinder needs replacing." These names that sound different all refer to the same thing—the scuba diving tank. Divers in different regions get used to their own abbreviations, which reflect language habits, cultural backgrounds, and usage scenarios.

Short and Punchy "Dive Cylinder"

”The formation of this abbreviation is related to the local linguistic environment. In Thai, "diving" is “ดำน้ำ” (damnam), and "tank" is “ถังอากาศ” (tank air). Mixed with the influence of Chinese dialects, simplifying to "dive cylinder" is more natural.

A user survey conducted by ThaiDivers, the largest diving forum in Thailand, showed that 85% of local divers use "dive cylinder" in daily conversation, far more than "scuba tank" (10%) or "air cylinder" (5%).

Australia and New Zealand

Divers in Australia and New Zealand prefer to use "dive bottle."

Liz, a diving instructor on the Gold Coast, explains: "We like to be direct when speaking. 'Dive' is diving, 'bottle' is a bottle. Combined, it's 'the bottle used for diving,' and everyone understands it."

”This naming is related to local English habits. "Dive" is frequently used as a verb or noun, such as "go diving" or "dive site," and the equipment is also named simply.

A 2022 membership survey by DAN Australia (Divers Alert Network Australia) showed that 78% of recreational divers routinely use "dive bottle," which is more popular than "scuba tank" (15%).

Europe

Divers in European countries like the UK and Germany, especially technical diving enthusiasts, tend to use "diving cylinder."

Mark, the owner of a London diving equipment store, says: "Old divers are accustomed to this term. They feel 'cylinder' emphasizes functionality, suggesting it's not just a casual bottle."

”This term is related to the history of diving in Europe: early European divers often used industrial-grade cylinders that were modified, and the word "cylinder" better fits their perception of equipment professionalism.

Statistics from the European diving forum Diveboard show that 60% of British divers and 55% of German divers routinely use "diving cylinder," and instructors deliberately emphasize this word, especially in technical diving courses.

For example, in the advanced course textbook of GUE (Global Underwater Explorers), when referring to the cylinder, it states: "The diving cylinder is the core container for breathing gas and requires regular inspection of wall thickness and valve condition.

Behind the Abbreviations

Southeast Asia uses "dive cylinder" because of the mixed languages and the need for simplification; Australia and New Zealand use "dive bottle" because of the direct English word formation; and Europe uses "diving cylinder" due to history and professional custom.

Next time you're diving abroad and hear "hand over the dive cylinder" or "check the dive bottle," don't worry about the translation. Just look at the silver-gray metal cylinder near the diver's waist, and you'll know what they're talking about.

What the Tank Body is Made Of

Scuba diving tanks are primarily made of steel or aluminum alloy. Steel tanks are often made of 3-4 mm thick low-carbon steel, offering high resistance to pressure. A tank of the same specification (12 liters, 200 bar) weighs about 15 kg; aluminum tanks typically use 6061-T6 alloy with a wall thickness of 5-7 mm. A tank of the same specification weighs only about 10 kg but requires regular inspection for internal corrosion. Both must pass DOT-3AL (US) or EN 12245 (Europe) certification to ensure they can withstand a working pressure of 200-300 bar and a 3-fold hydrostatic test (e.g., 600 bar), guaranteeing safety.

Steel vs. Aluminum Alloy

Steel cylinders are not made of ordinary iron blocks but low-carbon steel, with a carbon content of about 0.04%-0.25% between. The cylinder wall thickness is generally 3-4 mm. For a 12-liter tank with a working pressure of 200 bar, the empty tank weighs about 10 kg, and the total weight can reach 15 kg when filled with compressed air. Steel has a high density, 7850 kg per cubic meter, more than three times heavier than aluminum. The advantage is durability. For example, if a diver kicks a coral reef or rock, the risk of deformation is smaller for a steel tank than for an aluminum tank. I know a technical diver who used a steel tank for a 60-meter deep dive, and the cylinder body only had some paint scraped off, with no dents.

Looking at aluminum alloy tanks, the mainstream choice is 6061-T6 aluminum alloy, an aerospace-grade material that balances strength and weight well.

Its wall thickness must be 5-7 mm. For the same 12-liter, 200 bar tank, the empty weight is about 7 kg, and the total weight when filled is 10 kg.

Being 5 kg lighter makes a noticeable difference for recreational divers. If your arms get tired quickly, a lighter tank allows you to dive longer or carry less weight.

However, aluminum has a problem: its chemical properties are more active than steel, and it's prone to oxidation when exposed to water. I have seen aluminum tanks used for 5 years that, when cut open, had some gray spots on the inner wall.

Before leaving the factory, the inner wall of a steel tank is sprayed with a layer of epoxy resin, about 0.1-0.2 mm thick, like a coat of transparent paint, to isolate air and moisture.

For example, US certification requires DOT-3AL, and European certification requires EN 12245.

Testing includes hydrostatic tests, where the pressure must be 1.5-3 times the working pressure. A 200 bar tank must be pressurized to 600 bar during testing, equivalent to the pressure at 6000 meters underwater (though actual diving depths are usually around 100 meters at most).

I've checked the data: steel tanks that pass the tests can theoretically last up to 20 years, but require a comprehensive inspection every 5 years; aluminum tanks have a slightly shorter lifespan, around 15 years, and also require inspection every 5 years.

Weight Difference

The weight difference between steel and aluminum alloy scuba tanks is significant. Taking the most common 12-liter, 200 bar tank as an example, an empty steel tank is 3-4 kg heavier than an empty aluminum tank, and the total weight difference when filled with compressed air can be around 5 kg or so.

First, let's look at the 12-liter, 200 bar specification. Steel tanks use low-carbon steel with a wall thickness of 3-4 mm, and the empty weight is generally 10-12 kg.

For example, a certain brand's ST steel tank has an official marked weight of 11.5 kg, which feels substantial when held empty.

When filled with 200 bar compressed air, the total weight is the empty weight plus the gas weight. 1 liter of compressed air at 1 bar is about 1.29 grams. A 12-liter tank at 200 bar can hold 2400 liters of air, and the total gas weight is about 3.1 kg. So, the total weight when full is 11.5 + 3.1 = 14.6 kg, approximately 15 kg.

Aluminum alloy tanks use 6061-T6 alloy with a wall thickness of 5-7 mm, and the empty weight is 7-9 kg.

The AL aluminum tank of the same brand is marked as 8 kg. The total weight when filled is 8 + 3.1 = 11.1 kg, approximately 11 kg. The difference in full weight between the two is 3.5 kg, roughly the weight of two bottles of mineral water.

Switching to a different specification, the 15-liter, 200 bar tank, the difference is even more pronounced.

The empty weight of a steel tank is 14-16 kg. The total weight when full is 14 + (15 × 200 × 1.29) = 14 + 3.87 = 17.87 kg, approximately 18 kg. The empty weight of an aluminum tank is 10-12 kg. The total weight when full is 10 + 3.87 = 13.87 kg, approximately 14 kg.

The weight difference now reaches 4 kg. For a diver, swimming for half an hour with a tank that is 4 kg heavier will make their arms tire more easily.

The density of steel is about 7850 kg/cubic meter, while aluminum is only 2700 kg/cubic meter.

Although steel tanks have thinner walls (3-4 mm), their high density makes them heavier overall.

Aluminum tanks must be made thicker (5-7 mm) to withstand high pressure, but even so, the advantage of lower density still makes them lighter overall.

For example, for the same volume of steel and aluminum, the weight of aluminum is only about 34% of steel. However, the wall thickness of the aluminum tank is 1.5-2 times that of the steel tank, so the final weight difference is reduced to 30%-40%.

I know a recreational diver who used a steel tank for diving. He felt a strain on his shoulders after 20 minutes of swimming each time. After switching to an aluminum tank, he could take two more sets of photos during the same 20 minutes.

For example, when diving deep to 60 meters, a steel tank can store 30% more gas, reducing the hassle of switching tanks mid-dive.

Another detail: the full weight includes the gas, but the weight of the gas itself is often overlooked.

Aluminum tanks are lighter because less material is used for the tank body. A 12-liter, 200 bar aluminum tank and a steel tank store the exact same amount of air, 2400 liters.

For a 12-liter, 200 bar tank, a steel tank is about 15 kg when full, and an aluminum tank is about 11 kg, a difference of 4 kg; the difference for a 15-liter tank is 4 kg or more.

Manufacturing and Protection

How long a scuba tank can be used depends not on how sturdy it is, but on the effort put into its manufacturing and how it is cared for during use. Transforming steel or aluminum blocks into a cylinder that can withstand 200 bar of pressure requires over a dozen steps, each containing a secret to extending its lifespan.

Starting with Production

Taking a steel tank as an example, the first step is selecting the steel billet—it must be low-carbon steel with a carbon content of 0.04%-0.25%. Too brittle, it's prone to cracking; too soft, it can't withstand high pressure.

The steel billet must first be heated to 1200℃, repeatedly rolled into a cylindrical shape using a rolling mill, and then cut into tube blanks over 1 meter long. Temperature control is critical in this step. If the temperature difference exceeds 50℃, the tube blank will deform after cooling, affecting subsequent welding.

High-frequency resistance welding is used for tube blank welding, where current instantaneously fuses the weld seam. After welding, X-ray inspection is performed to ensure no pores or cracks in the weld.

I've checked the data: the weld seam inspection pass rate for qualified steel tanks must be above 99.9%; otherwise, leakage may occur under high pressure.

Aluminum tank production is more complex. It mostly uses 6061-T6 aluminum alloy ingots, which are first heated in a furnace to 700℃ until melted, and then injected into a mold to be cast into a bottle preform.

Protective Coating

Steel tanks are susceptible to rust, so the inner wall is sprayed with an epoxy resin coating. This coating is not applied casually; an electrostatic sprayer is used to ensure the powder adheres uniformly to the cylinder wall, followed by heating to 200℃ for curing.

The coating thickness must be controlled at 0.1-0.2 mm. If too thin, it won't block moisture; if too thick, it will increase the wall thickness and affect gas storage.

Laboratory tests show that steel tanks with an intact coating stored in a humid environment for 5 years have virtually no internal corrosion; those with scratches on the coating developed rust spots within 3 years in the same environment.

Therefore, aluminum tanks require double protection: first, spraying a layer of epoxy resin, followed by chromate treatment. Chromate is a chemical conversion film that thickens and densifies the oxidized layer on the aluminum surface.

I reviewed a manufacturer's technical documentation. After chromate treatment, their aluminum tanks could withstand a salt spray test (simulating a high-humidity, high-salt environment) for 1000 hours without corrosion, 3 times longer than untreated ones.

Regular Inspection

No matter how good the protection is, tanks have a lifespan. Regulations require a comprehensive inspection every 5 years, including:

• Hydrostatic Test: Pressurizing to 3 times the working pressure (e.g., a 200 bar tank is tested at 600 bar) to check for deformation or leakage.
  

• Wall Thickness Measurement: Using an ultrasonic thickness gauge to check if the tank wall has thinned (the minimum allowable wall thickness is 2.5 mm for steel tanks and 3 mm for aluminum tanks).
  

• Coating Inspection: Using an endoscope to check if the internal coating has peeled off. Aluminum tanks also need to check the integrity of the chromate film.

One set of data is quite revealing: tanks inspected on time have an average lifespan of 15-20 years; those with missed inspections may be scrapped within 10 years due to corrosion or cracks.

I recently saw a case where a user's aluminum tank was 3 years past its inspection date. The inspection revealed a 0.5 mm deep corrosion pit in the tank wall. Although there was no leak, it no longer met safety standards and had to be scrapped.

Daily Use

After diving, the accumulated water at the bottom of the tank must be drained, and the inner wall wiped dry with a cloth. Especially for aluminum tanks, moisture allows the corrosion rate to accelerate by 3 times.

A friend of mine who used an aluminum tank never wiped it dry. After two years, the inner wall was covered in white, ash-like corrosion. The inspection showed the wall thickness had already decreased by 1 mm, forcing an early replacement.

The inner wall coating is still intact after 10 years, and all inspection indicators are up to standard.

Capacity by Numbers

Common aluminum tanks are often 12 liters in water volume, with a working pressure of 200-300 bar (2900-4350 psi). Storage capacity = water volume × working pressure, which is 2400-3600 liters of compressed air (at ambient pressure volume). Steel tanks are commonly 15 liters in water volume, with pressure reaching over 300 bar and a storage capacity exceeding 4500 liters, making them suitable for technical diving or long dives.

Gas Storage by Numbers

The gas storage capacity of a scuba diving tank is determined by two numbers: water volume (the volume of water the tank can hold, in liters) and working pressure (the maximum pressure the tank can withstand, in bar or psi). For example, a common 12-liter aluminum tank with a working pressure of 200 bar has a storage capacity of 12 × 200 = 2400 liters of compressed air (at ambient pressure volume). A 15-liter steel tank at 300 bar pressure has a storage capacity of 4500 liters.

How Much Air Can It Hold

The first is water volume. For instance, a "12L" label means the internal space of the cylinder can hold 12 liters of water.

The second is working pressure. Common labels are "200 bar" or "3000 psi," indicating the maximum internal pressure the tank is designed to safely withstand.

The formula is simple: Storage Capacity (liters) = Water Volume (L) × Working Pressure (bar).

A 12L, 200 bar aluminum tank has a storage capacity of 12 × 200 = 2400 liters. This 2400 liters is the "compressed" air, which, when returned to ambient pressure at the surface, could fill a standard plastic bag of 2400 liters.

Aluminum and Steel Tanks

The common combinations of these two numbers vary for different tank materials.

• Aluminum Tanks: The most common is "12L, 200 bar." Aluminum is lighter, with an empty tank weight of about 14 kg, suitable for recreational diving. But there are also "12L, 300 bar" versions with a storage capacity of 3600 liters, holding 50% more air than the 200 bar version, suitable for divers who want to stay down longer or dive slightly deeper. Aluminum tanks also have a few "15L, 200 bar" models, but they are less common because aluminum's high-pressure resistance is inferior to steel.
  

• Steel Tanks: Commonly "15L, 300 bar," with a storage capacity of 4500 liters, nearly double that of the 12L aluminum tank. Steel tanks are heavier, with an empty tank weight of about 16 kg, but they can withstand higher pressure, making them suitable for technical diving. For example, cave diving requires carrying more backup gas, or when diving deep (around 40 meters), breathing is faster, and high storage capacity reduces the need to switch tanks.

Type	Water Volume	Working Pressure	Storage Capacity (liters)	Empty Weight	Common Use
Aluminum	12L	200 bar	2400	14 kg	Recreational Diving, Beginners
Aluminum	12L	300 bar	3600	14 kg	Moderate depth, Long duration dives
Steel	15L	300 bar	4500	16 kg	Technical Diving, Cave/Wreck Exploration

During Actual Diving

Breathing Rate: A normal person's calm breathing is about 15 breaths per minute, each inhaling 0.5 liters of air (at ambient pressure). If the tank has a storage capacity of 2400 liters, theoretically it can provide 2400 ÷ (15 × 0.5) = 320 minutes of breathing. Underwater, for every 10 meters of depth increase, the pressure doubles. The regulator automatically increases the pressure, and the amount of air a diver inhales in each breath also doubles. For example, at 20 meters deep (3 times the pressure), each breath requires 1.5 liters of air (ambient pressure equivalent). At 15 breaths per minute, the consumption is 22.5 liters per minute (15 × 1.5). A 2400 liter capacity can only last 2400 ÷ 22.5 ≈ 107 minutes. 

Depth Effect: Diving to 30 meters (4 times the pressure), each breath requires 2 liters of air (ambient pressure equivalent). At 15 breaths per minute, the consumption is 30 liters per minute. A 2400 liter capacity can only last 80 minutes. If it's a 12L, 300 bar aluminum tank (3600 liters capacity), it can last 3600 ÷ 30 = 120 minutes at 30 meters deep, an extra 40 minutes. 

Temperature Interference: Low temperature increases air density, but the bigger problem is that the regulator may occasionally free flow. For example, in 10℃ water, the regulator may occasionally free flow, so the actual usable time will be shorter than the calculated value.

Buying or Renting a Tank

For instance, if you plan to dive 40 meters and stay for 1 hour, renting a 12L, 200 bar aluminum tank might not be enough. At 40 meters deep (5 times the pressure), each minute of breathing requires 2.5 liters of air (ambient pressure equivalent), so the consumption per hour is 15 × 2.5 × 60 = 2250 liters. A 2400 liter capacity is barely enough, so it's best to choose a 12L, 300 bar (3600 liters) or a 15L steel tank (4500 liters).

The test date is every 5 years to ensure the tank's safety. The maximum filling pressure must not exceed the working pressure. For example, a tank marked 200 bar is dangerous if filled to 230 bar.

Numbers on the Tank

Scuba diving tanks always have two numbers printed on them: one is a marking like "12L" or "15L," and the other is "200 bar" or "300 bar." These two numbers are not randomly labeled. "12L" is called water volume, meaning the volume of water the tank can hold if used as a bucket; "200 bar" is the working pressure, referring to the internal pressure the tank can safely withstand, equivalent to 200 times the atmospheric pressure (1 bar ≈ 1 standard atmosphere).

If you buy an aluminum tank marked "12L, 200 bar," its internal space can hold 12 liters of water, and it can also be pressurized by compressed air to 200 bar. These two numbers are like the tank's "ID card," determining how much gas it can hold.

How the Formula Works

Why is the storage capacity the product of the two numbers? Let's use a real-life example: assume you have a balloon that can hold 1 liter of air when inflated to 1 time the atmospheric pressure. If you inflate it to 2 times the atmospheric pressure, it can hold 2 liters of air.

The tank is the same: the water volume is the "basic capacity" (the amount of air it can hold at ambient pressure), and the working pressure is the "compression factor." Compressing 12 liters of ambient pressure air to 200 bar is equivalent to squeezing 200 times the air inside, so the total storage capacity is 12 × 200 = 2400 liters (volume at ambient pressure).

Different Tanks

For common tanks on the market, you can calculate different storage capacities using this formula:

Type	Water Volume (L)	Working Pressure (bar)	Storage Capacity (liters)	Empty Weight	Typical Use
Aluminum Recreational Tank	12	200	2400	14 kg	Snorkeling, Shallow Water Beginners
Aluminum Deep Dive Tank	12	300	3600	14 kg	Long duration dives within 30 meters
Steel Technical Tank	15	300	4500	16 kg	Cave, Wreck Exploration

For example, a 12L, 300 bar aluminum tank has a storage capacity of 3600 liters, 50% more than the 200 bar version. This extra 1200 liters allows the diver to breathe for an additional 20-30 minutes underwater (depending on the breathing rate).

In Practical Use

The storage capacity given by the formula is the "full tank ambient pressure equivalent value." The actual duration of use during a dive depends on the depth and breathing habits.

• Depth Effect: Underwater, pressure increases by 1 bar for every 10 meters. At 20 meters deep (3 times the pressure), when you take a breath, the regulator gives you 3 times the pressure of air—equivalent to each breath consuming 3 times the storage capacity at ambient pressure. For example, with 2400 liters storage capacity, at 20 meters deep it can last 2400 ÷ (15 times/minute × 0.5 liters/time × 3) = 107 minutes (15 times is the calm breathing frequency, 0.5 liters is the single breath volume at ambient pressure).
  

• Breathing Habits: When nervous or exercising, the breathing frequency may increase to 25 times/minute, and the single breath volume increases to 1 liter (ambient pressure equivalent). In this case, 2400 liters storage capacity at 10 meters deep (2 times the pressure) can last 2400 ÷ (25 × 1 × 2) = 48 minutes, half the time of calm breathing.
  

• Temperature Interference: Cold water causes air to contract, but the bigger issue is that the regulator may free flow. For example, in 10℃ water, the regulator may occasionally free flow, so the actual usable time will be shorter than the calculated value.

Buying or Renting a Tank

Divers choosing a tank are essentially selecting whether the "storage capacity is enough for the planned use." For example, if you plan to dive 40 meters (5 times the pressure) and stay for 1 hour:

• Breathing 15 times per minute, single breath volume is 2.5 liters (ambient pressure equivalent, due to 5 times the pressure), consumption per hour is 15 × 2.5 × 60 = 2250 liters.
  

• The 12L, 200 bar aluminum tank has a storage capacity of 2400 liters, which is barely enough, so it's safer to choose a 12L, 300 bar (3600 liters) or a 15L steel tank (4500 liters).

If the working pressure is marked "200 bar," it absolutely must not be filled to 230 bar. If the water volume is marked "12L," it means it cannot hold 13 liters of water, and buying the wrong size might prevent it from fitting into the BCD (Buoyancy Control Device) tank compartment.

What is this formula useful for to a beginner?

Now that you know the formula, you can calculate yourself: the rented tank is 12L, 200 bar, with 2400 liters storage capacity. Planning to dive 30 meters (4 times the pressure), it can last approximately 2400 ÷ (15 × 1 × 4) = 40 minutes.

Choosing the Wrong Capacity

For example, you rent a 12L, 200 bar aluminum tank (2400 liters storage capacity), planning for a 30-minute dive. However, descending to 10 meters deep (2 times the pressure), your breathing rate increases from 12 times/minute on shore to 18 times/minute—the air consumed per minute becomes 18 times × 0.7 liters (single breath volume at ambient pressure) × 2 = 25.2 liters. Can 2400 liters storage capacity only last 2400 ÷ 25.2 ≈ 95 minutes? Actually, underwater at 10 meters, each breath is 2 times the volume at ambient pressure, so the storage capacity consumed per breath is 0.7 liters × 2 = 1.4 liters (ambient pressure equivalent).

At 18 times per minute, the consumption per minute is 18 × 1.4 = 25.2 liters. Can 2400 liters storage capacity last 2400 ÷ 25.2 ≈ 95 minutes? But beginners might breathe faster due to nervousness, for example, 20 times/minute, in which case the consumption per minute is 20 × 1.4 = 28 liters, 2400 ÷ 28 ≈ 85 minutes. However, the actual dive time is also affected by factors like BCD buoyancy and water temperature, and may be shorter. Users may be more concerned about the actual experience, such as planning a 30-minute dive, but using up the air in 20 minutes due to nervousness or frequent movement, forcing an early ascent and affecting the experience.

Experienced Divers Choosing a Large Tank

Conversely, experienced divers can also run into problems: for example, a technical diver usually uses a 15L, 300 bar steel tank (4500 liters storage capacity), but takes a beginner for a shallow dive (depth ≤ 15 meters) for instruction.

The empty steel tank is 16 kg, 2 kg heavier than the aluminum tank. Adding weights and equipment, the total load increases.

These tanks have a working pressure above 200 bar. When breathing at the surface, the regulator's air flow might be too high, causing choking and being interfered with by the excessive airflow.

Different Dive Types

When Renting a Tank

For instance, traveling to dive in Bohol, Philippines, with a planned itinerary of two deep dives (30 meters, 45 minutes each).

Renting a 12L, 200 bar aluminum tank (2400 liters storage capacity), the calculation shows: 45 minutes at 30 meters deep (4 times the pressure), air consumption per minute is 15 times × 1 liter (ambient pressure equivalent) × 4 = 60 liters, 45 minutes consumes 2700 liters—2400 liters is not enough, requiring a mid-dive tank change.

A tourist shared: The first time diving abroad, I rented a small tank. During the second deep dive, the instructor said, "Your tank is almost empty," which scared me into ascending quickly, missing a group of sea turtles—I should have rented a 12L, 300 bar aluminum tank, with a storage capacity of 3600 liters, which would have been sufficient for that dive.

What Capacity to Choose

It essentially depends on two needs: dive depth and single dive duration.

• Shallow sea (≤15 meters), short duration (≤45 minutes): 12L, 200 bar aluminum tank is sufficient, light and easy to control.
  

• Medium deep dive (20-30 meters), medium duration (45-60 minutes): 12L, 300 bar aluminum tank or 15L, 200 bar steel tank, sufficient storage capacity, acceptable weight.
  

• Technical diving (≥40 meters), long duration (>60 minutes): 15L, 300 bar steel tank, high storage capacity reduces the need to switch tanks, heavy but necessary.

For example, planning a 40-meter, 60-minute dive:

• Air consumption per minute = 15 times × 1.25 liters (ambient pressure equivalent, 40 meters is 5 times the pressure) × 5 = 93.75 liters.
  

• Consumption for 1 hour is 93.75 × 60 = 5625 liters.
  

• A 12L, 300 bar aluminum tank (3600 liters) is insufficient, so you must choose a 15L, 300 bar steel tank (4500 liters) or a twin-tank configuration.

Choosing the Right Capacity

Divers who choose the right capacity are more at ease underwater. One diver said: "The first time I chose a 12L, 200 bar aluminum tank, I dived to a 15-meter coral reef and ran out of air in 25 minutes, anxiously looking for the instructor.

The second time I switched to a 12L, 300 bar, and at the same depth, I dived for 40 minutes, was able to take videos slowly, and followed a grouper for a long distance."