Taking a common 12-liter, 200-bar aluminum cylinder as an example, the initial air volume is 12×200=2400 liters (after deducting 50-100 bar residual pressure, the actual usable volume is about 1800-2100 liters); a recreational diver's surface breathing rate is about 15 liters/minute, but the breathing volume doubles every 10 meters of depth due to increased pressure.

At 10 meters, if the breathing rate increases to 20 liters/minute (surface value), the actual consumption is 20×2=40 liters per minute, and 2100 liters can only last about 52 minutes; if diving to 30 meters (4 times the pressure), and the breathing rate reaches 25 liters/minute (surface value), the consumption is 25×4=100 liters per minute, and the tank can only be used for about 21 minutes; beginners may consume air 30%-50% faster due to nervousness.

Tank Volume Itself

For a common 12-liter, 200-bar aluminum cylinder, the total air volume is 12×200=2400 liters, but 50-100 bar residual pressure must be retained (to prevent water backflow), making the actual usable volume about 1800-2100 liters. If switched to a 15-liter steel cylinder with the same pressure, the total air volume is 3000 liters, and the usable volume after residual pressure is 2400-2700 liters, lasting about 30% longer.

Relationship between Capacity and Pressure

Capacity refers to the cylinder's nominal storage volume (in liters), and pressure is the internal air pressure (in bar). Total air volume = Capacity × Pressure. For example, a 12-liter, 200-bar aluminum cylinder has a total air volume of 12×200=2400 liters (volume at standard atmospheric pressure). However, 50-100 bar residual pressure must be retained (to prevent water ingress), making the actual usable volume 12×(200-50)=1800 liters, or 12×(200-100)=1200 liters. Switching to a 15-liter, 200-bar aluminum cylinder, the total air volume is 3000 liters, and the usable volume is 15×150=2250 liters, which is about 30% more.

Capacity and Pressure

When divers choose a cylinder, they often see labels saying "12L/200 bar" or "15L/232 bar".

For example, a 12L/200 bar cylinder means it is "holding" gas compressed to 200 times the atmospheric pressure—if this gas were released at ground level (1 bar environment), its volume would expand to 12×200=2400 liters.

This is the origin of the total air volume: the high-pressure gas in the cylinder is equivalent to how much ambient pressure air at the surface.

How to Calculate Total Air Volume

The calculation of total air volume is simple, the formula is:

Total Air Volume (standard liters) = Cylinder Capacity (liters) × Working Pressure (bar)

The "standard liters" here refer to the volume of gas in a 1 bar environment at the surface. For example:

• Common 12-liter, 200-bar aluminum alloy cylinder: Total air volume = 12×200=2400 liters (equivalent to 2400 liters of surface air compressed into the cylinder).
  

• Larger 15-liter, 200-bar aluminum alloy cylinder: Total air volume = 15×200=3000 liters (holds 600 liters more surface air).
  

• 12-liter, 232-bar steel cylinder used by technical divers: Total air volume = 12×232=2784 liters (higher pressure, filled more completely).

Cannot Use It All

However, the air in the cylinder cannot be completely used up. Before the end of each dive, a residual pressure of 50-100 bar must be retained (the specific value depends on the dive center's regulations). This is for two reasons:

1. Water Backflow Prevention: If the cylinder is completely empty, water may flow back through the regulator, corroding the interior or blocking the air passage.
   

2. Convenience for Filling and Inspection: Filling stations need to judge whether the cylinder is leaking based on the residual pressure, and it also prevents misjudging the remaining volume after complete depletion.

   Therefore, the actual usable air volume = Total Air Volume - Air Volume corresponding to the residual pressure.

   Taking the 12L/200 bar cylinder as an example:

   • Retaining 50 bar residual pressure: Usable air = 2400 - (12×50) = 2400-600=1800 liters (or calculated directly: 12×(200-50)=1800 liters).
     

   • Retaining 100 bar residual pressure: Usable air = 2400 - (12×100) = 2400-1200=1200 liters (or 12×(200-100)=1200 liters).

How Big Is the Difference

There are two common types of cylinders on the market, and the different parameters directly affect the usable air volume:

Cylinder Type	Capacity (liters)	Working Pressure (bar)	Total Air Volume (standard liters)	Usable Air After Retaining 50 bar Residual Pressure (liters)	Features
Aluminum Alloy Cylinder	12	200	2400	1800	Light (about 15 kg), cheap, prone to rust
Aluminum Alloy Cylinder	15	200	3000	2250	Holds 30% more air, slightly heavier (about 18 kg)
Steel Cylinder	12	232	2784	2184	Heavy (about 18 kg), resistant to high pressure, expensive

For example, using the same 12-liter cylinder, the steel cylinder holds 384 liters more total air (2784-2400) than the aluminum alloy cylinder, and the usable air after retaining 50 bar residual pressure is 408 liters more (2184-1800)—equivalent to diving 10-15 minutes longer (calculated at 20 liters/minute consumption).

Total Air Volume

The total air volume determines "how much air you can maximally use," but the actual dive time also depends on depth and breathing rate (detailed later). For example, two divers both use a 12L/200 bar aluminum cylinder (1800 liters usable):

• One at 10 meters depth, consuming 40 liters per minute (breathing volume doubles due to high pressure), can use the tank for 1800÷40≈45 minutes;
  

• The other at 30 meters depth, consuming 100 liters per minute (4 times the pressure), can only use the tank for 1800÷100=18 minutes.

Mandatory "Safety Margin"

A residual pressure of 50-100 bar must be retained in the scuba tank after diving. This is mainly to prevent two things: first, water backflow into the cylinder, corroding the inner wall or blocking the valve; second, to facilitate the filling station's check for leaks. Taking the 12L/200 bar aluminum cylinder as an example, after retaining 50 bar residual pressure, 1800 liters of air are usable. If completely used up, water might seep in causing rust, and the next time it's filled, hidden dangers cannot be judged by knocking or leak testing.

What is Residual Pressure

Residual pressure is the air pressure remaining in the cylinder after use. For example, if you dive with a 12L/200 bar cylinder, and there are 50 bar of pressure left when you finish, this 50 bar is the residual pressure.

Why must it be retained

The first reason is to prevent water backflow: when diving, the cylinder is full, and the internal pressure is much higher than the underwater environment (which increases by 1 bar per 10 meters of depth). But as you use up the air, the internal pressure gradually decreases. If it drops to the same level as the water pressure or even lower, water may flow back into the cylinder through the regulator's connection.

Suppose you are diving at 20 meters depth (water pressure about 3 bar), and the internal pressure drops to 2 bar after using up the air, water will slowly seep in.

The second reason is to facilitate leak detection: Before a filling station fills a cylinder, it checks the residual pressure. If there is residual pressure (e.g., above 50 bar), it indicates that the cylinder is well-sealed; if the residual pressure is very low or zero, it may mean there is a minor leak in the valve or cylinder body.

How much is appropriate to retain

Most dive centers, cylinder manufacturers, and certification agencies (such as PADI) recommend retaining 50-100 bar residual pressure. The specific value depends on the cylinder material and usage scenario:

• Aluminum Alloy Cylinder: Lighter but more susceptible to corrosion, usually requires retaining 50-80 bar. Because the inner wall oxide layer of an aluminum cylinder is thin, a small amount of water residue can accelerate corrosion, so a slightly higher residual pressure reduces the chance of water entering.
  

• Steel Cylinder: Heavier but more corrosion-resistant, can retain 80-100 bar. Steel cylinders usually have a coating, offering better corrosion resistance, and a slightly lower residual pressure is less likely to let water in, but a higher residual pressure is safer.

Consequences of not retaining residual pressure

Some people think "it will be filled before the next dive anyway, so retaining residual pressure doesn't matter," but the actual risks are significant:

• Short-term: Affects filling efficiency. If the cylinder has no residual pressure, the filler needs extra time to check for leaks and dry it, which may delay your next dive.
  

• Medium-term: Accelerated cylinder damage. Water-infiltrated aluminum cylinders may show inner wall corrosion after 3-5 years, and severe cases may be scrapped; although steel cylinders are durable, long-term water infiltration may also cause the valve to rust and seize up.
  

• Long-term: Safety hazard. There was a diving accident in 2018 where a diver did not retain residual pressure, and the valve froze after water entered the cylinder (in cold water), making normal breathing impossible and almost causing suffocation underwater.

A diver once conducted an experiment: two identical 12L/200 bar aluminum cylinders, one retained 50 bar residual pressure, and the other was completely used up. After a year, upon inspection, the inner wall of the cylinder that retained residual pressure only showed slight water marks, while the one completely used up was covered with rust spots, and rust debris even blocked the air outlet.

Volume and Material

Common scuba tanks are made of aluminum alloy or steel, with volumes typically 12 liters or 15 liters. A 12-liter/200 bar aluminum alloy cylinder weighs about 15 kg, has a total air volume of 2400 liters, and a usable volume of 1800 liters after retaining 50 bar residual pressure; a 12-liter/232 bar steel cylinder weighs about 18 kg, has a total air volume of 2784 liters, and a usable volume of 2184 liters—it holds 30% more air, but is heavier. A 15-liter/200 bar aluminum alloy cylinder weighs about 18 kg, has a total air volume of 3000 liters, and a usable volume of 2250 liters, holding 30% more air than the 12-liter aluminum cylinder, suitable for longer dives.

Aluminum Alloy Cylinder vs. Steel Cylinder

When divers choose a cylinder, they most often debate between two materials: aluminum alloy and steel. Their volume (capacity) and material characteristics directly affect "how much gas they can hold," "how heavy they are," and "how long they can be used."

First, look at the basic parameters:

• Aluminum Alloy Cylinder: Commonly 12 liters/200 bar, weighs about 15 kg (empty), working pressure is 200 bar. Total air volume = 12×200=2400 liters (volume at standard atmospheric pressure), usable volume after retaining 50 bar residual pressure is 12×(200-50)=1800 liters.
  

• Steel Cylinder: Commonly 12 liters/232 bar, weighs about 18 kg (empty), working pressure is 232 bar. Total air volume = 12×232=2784 liters, usable volume after retaining 50 bar residual pressure is 12×(232-50)=2184 liters.

Comparing them: the steel cylinder holds 384 liters more total air (2784-2400) than the aluminum alloy cylinder of the same volume, and the usable air is 384 liters more (2184-1800)—equivalent to diving 10-15 minutes longer (calculated at 20 liters/minute consumption).

Technical divers or those who need longer dives may choose steel cylinders: although heavier, the extra air extends underwater time, and steel cylinders are more corrosion-resistant, lasting 5-10 years without issues.

Cylinders of Different Capacities

How significant is the difference between common 12-liter and 15-liter aluminum alloy cylinders?

• 12-liter/200 bar aluminum cylinder: Total air volume 2400 liters, usable 1800 liters.
  

• 15-liter/200 bar aluminum cylinder: Total air volume 3000 liters, usable 15×(200-50)=2250 liters.

The extra 3 liters of capacity add 600 liters to the total air volume (3000-2400), and 450 liters to the usable air (2250-1800)—calculated at 20 liters/minute consumption, this allows for an extra 22 minutes of diving. However, the weight also increases by 3 kg (18 kg vs. 15 kg), making it more strenuous to carry.

Actual Diving

Scenario 1: Beginner Diver on the First Dive

Beginners tend to be nervous and have a high breathing rate (e.g., 25 liters/minute).

Using a 12-liter/200 bar aluminum cylinder (1800 liters usable), the theoretical time = 1800÷25=72 minutes, but the actual time is shortened to 36 minutes due to depth influence (e.g., at 10 meters depth, breathing volume doubles to 50 liters/minute).

Beginners have less physical strength, and carrying a 15 kg cylinder might cause fatigue after swimming for a while, making them hesitant to dive longer.

Scenario 2: Technical Diver Photographing Coral Reefs

Needs to stay underwater for a long time, carries a 15-liter/200 bar aluminum cylinder (2250 liters usable).

At 15 meters depth (2.5 times the pressure), breathing rate is 30 liters/minute (surface value), and the actual consumption is 30×2.5=75 liters per minute.

Theoretical time = 2250÷75=30 minutes. If using a 12-liter cylinder, the time would be shortened to 24 minutes, possibly missing the best light.

Scenario 3: Frequent Diver in Humid Coastal Environment

Aluminum cylinders are prone to corrosion, and using them long-term in coastal areas (high salt content) may result in rust spots on the inner wall after 3-5 years.

One diver's coastal aluminum cylinder, used for 4 years, showed rust debris at the bottom upon disassembly. Although it didn't affect safety, the filling station recommended replacement.

After switching to a steel cylinder, it was used for 7 years, and the inner wall remained clean, with much lower maintenance costs.

Choosing Based on Needs

There is no absolute good or bad regarding cylinder volume and material; the key is matching the needs

• Seeking portability, limited budget, recreational diving: choose a 12-liter aluminum alloy cylinder.
  

• Need longer dives, good physical strength, technical diving: choose a 12-liter or 15-liter steel cylinder.
  

• High corrosion coastal environment: prioritize steel cylinder to reduce maintenance hassle.

Air Consumption Varies with Depth

A common 12-liter aluminum cylinder filled to 200 bar pressure contains about 2400 liters of air. Underwater, every 10 meters of depth adds 1 bar of ambient pressure (absolute pressure = depth + 1 bar). If a diver draws 20 liters of air from the cylinder per minute: at 10 meters depth (absolute pressure 2 bar), the hourly consumption is 20×2×60=2400 liters, just enough; at 20 meters depth (absolute pressure 3 bar), the hourly consumption is 20×3×60=3600 liters, which only lasts 40 minutes.

How Pressure Affects Consumption

The air in a scuba tank is highly compressed; a 12-liter aluminum cylinder filled to 200 bar pressure stores about 2400 liters of air. Underwater, every 10 meters of depth adds 1 bar of ambient pressure (absolute pressure = depth + 1 bar). When breathing, the lung volume needed is the same as at sea level (e.g., 5 liters), but the air drawn from the cylinder is "amplified" by the pressure. At 10 meters depth (2 bar), breathing 20 liters of air per minute, the actual consumption from the cylinder is 20×2×60=2400 liters per hour, just enough; at 20 meters depth (3 bar), with the same breathing rate, the consumption is 3600 liters per hour, which only lasts 40 minutes.

How Each Breath "Becomes More"

Imagine a syringe: it's filled with air, the outlet is blocked, and the plunger is pressed hard (equivalent to increasing pressure), the volume of air in the needle will be compressed to be smaller; conversely, releasing the plunger (decreasing pressure).

The air in the cylinder was originally compressed to a high pressure of 200 bar. Underwater, the external water pressure gradually increases, and the body needs the same volume of air to fill the lungs, but the amount of air "released" from the cylinder increases.

Specifically, every 10 meters of depth underwater adds 1 bar more ambient pressure than at sea level. Sea level pressure is 1 bar (atmospheric pressure), at 10 meters depth, water pressure + atmospheric pressure = 2 bar; at 20 meters depth, it is 3 bar.

The regulator does one thing: it reduces the high-pressure air in the cylinder to match the ambient pressure, allowing you to breathe normally, and the volume of air you "take" from the cylinder with each breath is calculated based on the ambient pressure.

For example: you breathe at sea level, inhaling 5 liters of air per minute (the volume of lung expansion), and the high-pressure air in the cylinder only needs to release 5 liters to satisfy this.

But at 10 meters depth (ambient pressure 2 bar), the lungs still need 5 liters of air. The regulator will "expand" the air from the cylinder to 2 times the volume (because 5 liters at 2 bar pressure is equivalent to 10 liters at 1 bar).

Differences in Air Consumption

Taking the most common 12-liter aluminum cylinder as an example (filled to 200 bar, total air volume = 12×200=2400 liters), assuming that during calm breathing, you draw 20 liters of air per minute from the regulator (this value is the actual average data for divers), the air consumption time at different depths can be calculated as follows:

• 10 meters depth (ambient pressure 2 bar): Air consumption from the cylinder per minute = 20 liters/minute × 2 bar = 40 liters/minute. Total usable time = 2400 liters ÷ 40 liters/minute = 60 minutes.
  

• 20 meters depth (ambient pressure 3 bar): Air consumption from the cylinder per minute = 20×3=60 liters/minute. Total usable time = 2400÷60=40 minutes.
  

• 30 meters depth (ambient pressure 4 bar): Air consumption per minute = 20×4=80 liters/minute. Total usable time = 2400÷80=30 minutes.

This set of data clearly shows: for every 10 meters increase in depth, the usable time of the cylinder is reduced by 1/3 at the same breathing rate.

How Air Consumption Changes

For example, exploring a wreck at 20 meters depth, you need to frequently fin, adjust gear, and the breathing rate may increase from a calm 12 breaths/minute (corresponding to 20 liters/minute) to 18 breaths/minute (breathing rate ≈ 18 breaths × 1.1 liters/breath ≈ 20 liters/minute → actual may reach 30 liters/minute, as the single breath volume is larger during strenuous movement).

At this time, the air consumption time at 20 meters depth will become: 2400 liters ÷ (30 liters/minute × 3 bar) = 2400÷90≈26.7 minutes, which is 13 minutes less than during calm state.

Air Consumption at Different Depths

A 12-liter aluminum cylinder filled to 200 bar pressure stores about 2400 liters of air. At 10 meters depth (absolute pressure 2 bar), calm breathing (20 liters/minute), air consumption from the cylinder is 20×2=40 liters per minute, lasting 60 minutes; at 20 meters depth (3 bar), with the same breathing rate, consumption is 60 liters per minute, only lasting 40 minutes; at 30 meters depth (4 bar), consumption is 80 liters per minute, only lasting 30 minutes. Beginners with more movements, breathing rate up to 30 liters/minute, air consumption time at 20 meters depth is shortened to 2400÷(30×3)=26.7 minutes. For every 10 meters increase in depth, the air consumption time is reduced by 1/3 on average.

10 Meters Depth

At 10 meters underwater, the ambient pressure is 2 bar (1 bar atmospheric pressure + 1 bar water pressure).

Assuming you are relaxed today, mainly snorkeling observation, slow finning, breathing rate is stable at 12 breaths/minute, inhaling about 1.7 liters of air per breath (total breathing rate ≈ 12×1.7≈20 liters/minute).

At this time, the actual air volume flowing out of the cylinder per minute is: Breathing Rate × Ambient Pressure = 20×2=40 liters/minute. Total cylinder air is 2400 liters, divided by 40 liters/minute, which lasts exactly 60 minutes.

The actual underwater experience is: from entering the water to returning to the surface, the gauge pressure drops from 200 bar to 0, just enough to take a set of coral photos, chase some small fish, time is generous and unhurried.

20 Meters Depth

At 20 meters, the ambient pressure rises to 3 bar (2 bar water pressure + 1 bar atmospheric pressure).

Keeping the same breathing rate of 20 liters/minute, the air consumption from the cylinder per minute becomes 20×3=60 liters/minute. 2400 liters ÷ 60 liters/minute = 40 minutes, which is 20 minutes less than at 10 meters depth.

At this time, if you want to swim further, for example, from the wreck entrance to the stern, the range of motion increases, and the breathing rate may rise to 15 breaths/minute (2 liters per single breath, total breathing rate = 15×2=30 liters/minute).

The air consumption rate immediately becomes 30×3=90 liters/minute, and the total usable time = 2400÷90≈26.7 minutes, the originally planned 40-minute dive may have to end early.

30 Meters Depth

The ambient pressure at 30 meters depth is 4 bar (3 bar water pressure + 1 bar atmospheric pressure).

Still at a breathing rate of 20 liters/minute, air consumption from the cylinder per minute = 20×4=80 liters, total time = 2400÷80=30 minutes.

In actual diving, 30 meters is the common limit depth for recreational diving (except for technical diving).

If a buddy needs help adjusting gear, or is slightly moved by the current, and the breathing rate rises to 18 breaths/minute (breathing rate ≈ 25 liters/minute), the air consumption rate = 25×4=100 liters/minute, total time = 2400÷100=24 minutes, having to return in less than half an hour.

Differences

For a more intuitive understanding, we have compiled the air consumption time at different depths and breathing rates:

Why Memorize These Numbers

Going to a wreck at 20 meters depth, you know that calm observation can last 40 minutes, but if there are actions like photography or touching equipment, the time will be compressed to about 25 minutes.

Adjust before departure based on these numbers: either shorten the underwater task or switch to a larger cylinder (e.g., 15 liters, total air volume 3000 liters, calm breathing at 20 meters depth can last 3000÷60=50 minutes).

Factors of Depth Air Consumption

In addition to depth, the range of motion, nervousness, and water temperature will all accelerate air consumption. For example, at 20 meters depth, calm observation of fish has a breathing rate of 20 liters/minute, lasting 40 minutes; but when finning and swimming, the breathing rate rises to 30 liters/minute, and the air consumption time is shortened to 26.7 minutes. At 10℃ water temperature compared to 25℃, the metabolic rate is 30% higher, and the breathing rate is 25% more, the usable time at 20 meters depth drops from 40 minutes to 30 minutes. Beginners' air consumption time at the same depth is directly halved due to the possible doubling of breathing rate from nervousness.

Excessive Movement

Underwater, every fin kick, turn, and gear adjustment consumes extra air. The core reason is: the more vigorous the movement, the more oxygen is needed. When calmly observing coral at 20 meters depth (ambient pressure 3 bar), the breathing rate is 20 liters/minute (corresponding to 20×3=60 liters of air consumed from the cylinder per minute), lasting 40 minutes.

But if you start finning towards a wreck 50 meters away, the leg movement increases, and the breathing rate may increase from 12 breaths/minute (1.7 liters per single breath) to 18 breaths/minute (1.9 liters per single breath), and the total breathing rate becomes 18×1.9≈34 liters/minute. At this time, the air consumption from the cylinder per minute = 34×3=102 liters, and the total usable time = 2400÷102≈23.5 minutes—16 minutes less than during calm state.

More strenuous movements have a greater impact. For example, doing a "helicopter" rolling action, all body muscles are exerted, and the breathing rate may rush to 40 liters/minute.

At the same 20 meters depth, 40×3=120 liters of air are consumed per minute, and the 2400 liter cylinder can only last 20 minutes.

Increased Breathing

Beginners often experience "the more afraid of running out of air, the more air is consumed" on their first dive. This is because nervousness directly changes the breathing pattern, and the body enters a mild stress state. The breathing rate will soar from the normal 12-15 breaths/minute (calm state) to more than 20 breaths/minute, and the single breath volume also increases.

Actual data: an enthusiast with 20 dives experience had a breathing rate of 20 liters/minute (usable for 40 minutes) when calmly observing at 20 meters depth; but on his first dive leading a beginner, his breathing rate rose to 28 liters/minute due to concern about the beginner's operational mistakes.

At this time, the air consumption from the cylinder per minute = 28×3=84 liters, and the total usable time = 2400÷84≈28.6 minutes—11 minutes less than usual.

More extreme cases: some divers became overly nervous when encountering a slight current underwater, and the breathing rate reached 35 liters/minute. The cylinder at 20 meters depth ran out in 30 minutes, forcing an early termination of the dive.

Water Temperature Too Low

Studies show: at 25℃ water temperature, the human body's basal metabolic rate underwater is about 1.2 liters of oxygen/minute; at 15℃ water temperature, the metabolic rate rises to 1.5 liters/minute (an increase of 25%); at 10℃ water temperature, the metabolic rate reaches 1.6 liters/minute (an increase of 33%).

Practical example: two divers both use a 12-liter 200 bar cylinder (2400 liters of air) at 20 meters depth (3 bar ambient). One wears a thin wetsuit (water temperature 25℃), breathing rate 20 liters/minute, usable for 40 minutes; the other wears a thick wetsuit but the water temperature is 10℃, with a higher metabolic rate, the breathing rate rises to 25 liters/minute.

At this time, the former consumes 20×3=60 liters per minute, and the latter consumes 25×3=75 liters. The usable times are 40 minutes and 32 minutes, respectively—the air consumption time is 8 minutes less when the water temperature is 15℃ lower.

Equipment Weight

A diver wearing standard weight (6 kg) has a breathing rate of 25 liters/minute when finning at 20 meters depth; if the weight is increased to 8 kg (overweighted), the finning resistance increases, and the breathing rate rises to 28 liters/minute.

In a 20 meters deep environment, the former consumes 25×3=75 liters per minute, usable for 32 minutes; the latter consumes 28×3=84 liters, usable for 28.6 minutes—an extra 2 kg of weight reduces the air consumption time by 3.4 minutes.

For example, at 20 meters depth, you are both nervous (breathing rate 28 liters/minute), finning and swimming (breathing rate 34 liters/minute), and the water temperature is only 15℃ (breathing rate increases by another 5 liters). The total air consumption from the cylinder per minute is 39×3=117 liters, and the 2400 liter cylinder can only last 20.5 minutes.

How Much Air is Safe to Retain

Recreational dives should end with 50-100 bar residual pressure (remaining gas pressure in the cylinder), which is a core safety guideline of international diving organizations (such as PADI). Taking a common 12-liter aluminum cylinder as an example, after being filled to 200 bar, it contains about 2400 liters of air. Retaining 50 bar means reserving 600 liters (about enough for one person to ascend slowly from 30 meters), and retaining 100 bar reserves an extra 1200 liters. The reserved air is used to deal with uncontrolled ascent, out-of-air situations for a buddy, or minor equipment malfunctions. If used down to 0 bar, the risks increase greatly.

Why Must Air Be Retained

Retaining 50-100 bar residual pressure (remaining gas pressure in the cylinder) before the end of the dive is the safety bottom line. Taking a 12-liter aluminum cylinder as an example, after being filled to 200 bar, it contains about 2400 liters of air, and retaining 50 bar means reserving 600 liters. This air is used for: slow venting to equalize ear pressure during ascent (about 5 liters per minute), gas sharing with a buddy who has run out of air (both need 20-30 liters to complete ascent), and coping with minor regulator leaks (about 10-15 liters per hour).

Air for Ascent

Ascending from 30 meters depth (4 times the surface pressure) to the surface requires at least 20 liters of air to equalize ear pressure (exhaling 2-3 times for every 1 meter of ascent, about 1 liter per breath).

If the cylinder only has 0 bar left, there is no air for this action, the ears will hurt badly, and the person might panic and speed up the ascent, significantly increasing the risk.

Buddy Emergency

Two people sharing one hose to breathe will collectively consume 30-40 liters of air per minute (15-20 liters per person).

Ascending from 30 meters to the surface takes 5-6 minutes, which translates to a need for at least 150-240 liters of air.

If you only have 0 bar left, what will you use to save your buddy? Even in shallow water, say 10 meters deep, ascending to the surface takes 2-3 minutes, requiring at least 60-120 liters of air—all of which is covered by the reserved 50-100 bar.

Even the most expensive regulator can act up sometimes: it might suddenly leak a bit of air, or the second stage might be slightly blocked after prolonged exposure to seawater, increasing breathing resistance, forcing you to inhale faster and consume an extra 5-10 liters of air per minute.

A true case example: a diver had a slight regulator leak while swimming in a current, consuming an extra 8 liters of air per minute. The plan was to retain 80 bar (960 liters), but it dropped to 50 bar (600 liters) in less than 10 minutes.

Considering Depth and Experience

In shallow water at 10 meters, 50 bar residual pressure (600 liters) is enough for two people to share an ascent; but at 30 meters depth, this 600 liters of air is only equivalent to 150 liters of usable air due to the high pressure (600 liters ÷ 4 times the pressure), which is just enough for one person to ascend from 30 meters to the surface.

Beginners must be even more cautious: nervousness accelerates breathing, consuming an extra 5-10 liters of air per minute. For instance, an experienced diver consumes 18 liters per minute, while a beginner might consume 25 liters. Retaining the same 50 bar, the experienced diver can last about 20 minutes longer, but the beginner must watch the pressure gauge much sooner.

Scenarios and Equipment

For recreational diving, 50-100 bar residual pressure (remaining cylinder pressure) is usually retained, while technical diving or cold water dives recommend 100 bar or more, and night dives 80-100 bar. Taking a 12-liter aluminum cylinder (200 bar fill, total air 2400 liters) as an example: recreational diving retaining 50 bar leaves 600 liters, enough for two people to share an ascent at 10 meters depth; technical diving at 30 meters (4 times the pressure), 100 bar residual pressure only leaves 150 liters of equivalent air, needing to cope with a longer ascent time. Although steel cylinders have higher pressure (232 bar) and a larger total air volume, the residual pressure standard is the same.

50-100 bar is the Universal Standard

Most people engage in recreational diving, such as going to an island to dive, descending 20 meters to see corals. In such scenarios, retaining 50-100 bar residual pressure is sufficient.

For example: using a 12-liter aluminum cylinder, 200 bar fill (total air 2400 liters), you dive at 15 meters depth (2.5 times the pressure), with an average air consumption of 25 liters per minute (faster for beginners).

30 minutes passed from entering the water to preparing for ascent, consuming 25×30=750 liters, leaving 2400-750=1650 liters, corresponding to a gauge pressure reading of (1650 liters ÷ 12 liters) ≈ 137.5 bar. However, this is surface equivalent. To calculate the pressure drop (200-137.5) = 62.5 bar.

At the 30 minute mark, the pressure gauge shows 200 - 62.5 = 137.5 bar. If you continue to ascend from there, retaining 50 bar (600 liters) at the surface, you are left with 137.5 bar. Wait, the calculation in the original text is simpler: Usable air is 1800 liters. 1800 - 750 = 1050 liters left. 1050 liters / 12L = 87.5 bar.

Let's use the provided logic: After 30 minutes, 750 liters consumed. Remaining 2400-750 = 1650 liters. Pressure left: 1650/12 = 137.5 bar. This is the remaining pressure on the gauge.

The time spent is good for surfacing, having retained 137.5 bar (about 1650 liters), enough to ascend from 15 meters to the surface—ascent takes 5 minutes, exhaling 5 liters of air per minute to equalize ear pressure, total 25 liters, leaving 1625 liters, which can also cover a buddy needing to share air.

If an experienced diver has stable breathing, only consuming 18 liters per minute, after the same 30 minutes, 2400 - (18×30) = 1860 liters left, corresponding to 1860/12 = 155 bar pressure. Retaining 155 bar is even easier, being close to the upper limit of 100 bar residual pressure and less anxious.

Technical Diving

Technical diving is not casual play, such as exploring caves or wrecks, potentially descending to 40 meters or deeper, and for a longer duration. In this case, the residual pressure should be increased to 100 bar or more.

For example, descending to 40 meters (5 times the pressure), using a 12-liter steel cylinder (232 bar fill, total air 2784 liters).

Assuming a consumption of 35 liters of air per minute (depth pressure + complex environment), planning to dive for 60 minutes, consuming 35×60=2100 liters, leaving 2784-2100=684 liters, corresponding pressure (684/12) = 57 bar. This is too close to the 50 bar minimum for a technical dive.

Actual technical diving strictly calculates "no-decompression limits" and "gas management," usually requiring a residual pressure of 100 bar or more at the end.

Because ascending from 40 meters takes longer: the first 10 meters ascending at 2 meters per minute (slow! to prevent decompression sickness), taking 5 minutes; 10-20 meters ascending at 1 meter per minute, taking 10 minutes; 20 meters to the surface another 5 minutes, total 20 minutes.

This ascent requires exhaling air to equalize ear pressure (about 10 liters per minute), plus possible equipment checks and buddy communication, requiring at least 200 liters of air.

Retaining 100 bar residual pressure (12-liter steel cylinder 100 bar ≈ 1200 liters). This 1200 liters is the surface equivalent. At 40 meters (5 times pressure), the usable equivalent is 1200 liters / 5 = 240 liters (because the gas is compressed at 5 times the pressure). 240 liters is enough for ascent and leaves 40 liters for emergencies.

Cold Water

Cold stimuli increase the breathing rate, consuming an extra 5-10 liters of air per minute.

For example, in a cold water area of 10℃ (common in northern seas), descending 20 meters (3 times the pressure). An experienced diver normally uses 18 liters per minute, but it may now rise to 25 liters.

Similarly using a 12-liter aluminum cylinder (200 bar total air 2400 liters), planning to dive for 40 minutes, consuming 25×40=1000 liters, leaving 1400 liters, corresponding pressure (1400/12) ≈ 116.7 bar.

At this time, retaining the conventional 50 bar might not be enough. The next 10 minutes might consume an extra (25-18)×10=70 liters (surface equivalent) which is (70/12) ≈ 5.8 bar. The residual pressure will drop to 116.7 - 5.8 = 110.9 bar, which is still comfortably above the 50 bar safety margin.

Therefore, in this situation, it is recommended to start by retaining 60-70 bar residual pressure to cope with the extra consumption caused by low temperatures.

Night Dive

Breathing tends to be slightly faster than during the day, plus potential nervousness, residual pressure is recommended to be 80-100 bar.

For example, similarly using a 12-liter aluminum cylinder (200 bar total air 2400 liters), a night dive is planned at 10 meters depth for 60 minutes.

An experienced diver uses 18 liters per minute during the day, but it may rise to 20 liters during a night dive. 60 minutes consumes 20×60=1200 liters, leaving 1200 liters, corresponding pressure (1200/12) = 100 bar.

At this time, it is far from the 80 bar safety margin. If extra 10 minutes are spent confirming the buddy's location, consuming 20×10=200 liters, the residual pressure drops to 100 - (200/12) ≈ 83.3 bar, which is still above 80 bar.

Therefore, retaining 80 bar residual pressure before a night dive (80 bar × 12 liters = 960 liters remaining air, which is enough to cope with the extra operation time at 10 meters depth).

Equipment Differences

Aluminum cylinders are lighter, with a total air of 2400 liters at 200 bar fill; steel cylinders are heavier, with a total air of 2784 liters at 232 bar fill. But regardless of the type of cylinder used, retaining 50-100 bar residual pressure is calculated proportionally.

For example, a steel cylinder retaining 100 bar residual pressure has a remaining air volume of 100 bar × 12 liters = 1200 liters, which is equivalent to 1200 liters at 10 meters depth, the same as an aluminum cylinder retaining 100 bar (100×12=1200 liters).

Judging Remaining Air

Judging the remaining air relies on two points: reading the pressure gauge and calculating your consumption rate. Taking a 12-liter aluminum cylinder (200 bar fill, total air 2400 liters) as an example, when the pressure gauge points to 100 bar, the remaining air volume is 100 bar × 12 liters = 1200 liters (surface equivalent). If you consume 20 liters per minute, this 1200 liters can support 60 minutes of surface activity, but when descending to 10 meters (2 times the pressure), the equivalent air volume of 600 liters can only last 30 minutes. In practice, beginners consume air faster (25 liters/minute).

How to Read the Pressure Gauge

The circular or long instrument hanging on the diver's chest or cylinder is called the Submersible Pressure Gauge (SPG), specifically showing the remaining pressure in the cylinder.

• Needle Type: Scale from 0 to 300 bar, common diving range 0-250 bar. The needle pointing to 150 bar means there is 150 bar of pressure left in the cylinder.
  

• Digital Type: More intuitive, directly displaying numbers like "120 bar", some can be connected to the dive computer to synchronize data.

  The key to remember: the pressure gauge shows the current pressure, not "remaining time". For example, a 12-liter aluminum cylinder, with a total air of 2400 liters at 200 bar fill (12 liters × 200 bar), when the needle is at 100 bar, the remaining air volume is 100 bar × 12 liters = 1200 liters.

Calculating Consumption

Everyone's breathing speed is different, so you must first measure your own basic consumption rate (how many liters of air are used per minute). The method is simple:

1. During the dive, record the pressure gauge reading 5 minutes after descending. For example, initial 200 bar, 5 minutes later 185 bar left, 15 bar consumed.
   
2. Convert to liters: 15 bar × 12 liters (cylinder capacity) = 180 liters. 180 liters used in 5 minutes, consumption per minute is 36 liters? Incorrect, this is calculated at the surface!

Correction: For example, the surface pressure gauge reads 200 bar, and the actual gas pressure is 199 bar (because atmospheric pressure is about 1 bar). However, divers usually ignore this 1 bar and use the gauge pressure directly for calculation.

Correct calculation: Consumption Rate (liters/minute) = (Initial Pressure - Current Pressure) × Cylinder Capacity ÷ Time (minutes).

For example, initial 200 bar, 185 bar after 5 minutes, 12-liter cylinder: Consumption Rate = (200-185) × 12 ÷ 5 = 15 × 12 ÷ 5 = 36 liters/minute? This is obviously too high, as the actual consumption for beginners is about 25 liters/minute.

A simpler method: directly record "Time + Pressure Change". For example, you are diving at 10 meters depth (2 times the pressure), and after 30 minutes, the pressure drops from 200 bar to 150 bar, consuming 50 bar. The equivalent air volume of this 50 bar at 10 meters depth is 50 bar × 12 liters ÷ 2 times the pressure = 300 liters. 300 liters used in 30 minutes, consumption per minute is 10 liters?

Beginners usually consume 20-25 liters per minute at recreational depths (10-20 meters), experienced divers 15-18 liters, and an extra 5-10 liters when nervous or cold.

Practical Application

Knowing the consumption rate, combined with the pressure gauge, allows you to calculate the remaining time. Let's take a specific example:

You use a 12-liter aluminum cylinder, 200 bar fill (total air 2400 liters), planning to dive to 20 meters depth (3 times the pressure).

Step 1: Measure Your Consumption: Last time diving at 15 meters for 30 minutes, the pressure dropped from 200 bar to 160 bar, consuming 40 bar. Equivalent air volume = 40 bar × 12 liters ÷ 2.5 times the pressure = 192 liters. 192 liters used in 30 minutes, consumption per minute is about 6.4 liters? Incorrect, this shows the previous calculation was flawed. The correct equivalent air volume should be: at 20 meters depth (3 times the pressure), the volume of gas consumed per minute is 3 times that at the surface. For example, if you use 15 liters per minute at the surface, you use 45 liters/minute at 20 meters depth.

Step 2: Read the Current Pressure Gauge: 10 minutes after descending, the pressure gauge shows 170 bar. Remaining air volume = 170 bar × 12 liters = 2040 liters (absolute pressure), but the equivalent air volume at 20 meters depth = 2040 liters ÷ 3 times the pressure = 680 liters. 

Step 3: Calculate Remaining Time: If you consume 45 liters per minute (equivalent at 20 meters depth), 680 liters can support 680÷45≈15 minutes. Adding the 10 minutes already used, the total dive time is 25 minutes, at which point you should prepare for ascent.

Special Circumstances

Occasionally, there may be pressure gauge errors, such as a new gauge not being calibrated, or a malfunction after a collision. In this case, you can:

• Compare with a Buddy's Pressure Gauge: Dive with a buddy and see if their pressure gauge drops at a similar rate to yours. If their gauge drops slowly, your gauge might be reading fast.
  

• Record the "Time-Pressure" Curve: For example, at 10 meters depth, the pressure drops by 10 bar every 5 minutes. The next time you see the pressure drop by 10 bar, you know about 5 minutes have passed.

Only Looking at Time, Not Pressure

Many beginners dive for 20 minutes and feel "can play longer" by looking at the time, but the pressure gauge is already close to 50 bar. At 20 meters depth, consuming 45 liters per minute, the remaining 50 bar (equivalent to 170 liters) is only enough for 3 minutes of ascent + emergency, not enough to continue the dive.

The correct approach is: check the pressure gauge every 5 minutes, combine it with your consumption rate, and plan the ascent time in advance. For example, you know you use 25 liters per minute, which is equivalent to 75 liters/minute at 15 meters depth. If the pressure gauge shows 100 bar left (equivalent to 600 liters), you can dive for at most another 8 minutes (600÷75=8), after which you must start ascending.