A 1L water tank usually indicates its water capacity. The actual gas storage volume needs to be combined with the working pressure (commonly 200bar), which contains about 200L of gas at standard atmospheric pressure; during a calm dive (10 meters deep, breathing frequency 15 times/minute, tidal volume increases to 500ml×2=1000ml due to pressure), gas consumption is 1.5L per minute, so 200L can sustain for about 133 minutes, corresponding to about 2000 breaths; if on the surface (no extra pressure), consumption is only 0.75L per minute, allowing for nearly 4000 breaths, but diving mostly takes place in middle to deep water, and increased breathing frequency and tidal volume during exercise will shorten the duration.

Actual Gas Volume of a 1L Tank

The "1L" marked on a 1L scuba tank refers to "water capacity" (internal liquid volume). The actual gas storage volume is determined by the working pressure. Common tank working pressure is 200bar (some industrial tanks reach 300bar), meaning the internal gas is compressed to 200 times atmospheric pressure. According to gas laws, a 1L volume at 200bar is equivalent to 200L of gas at standard atmospheric pressure (1bar); 300bar corresponds to 300L. This means the tank does not hold 1L of gas, but stores more by high-pressure compression

Hidden Capacity of the Tank

When buying a scuba tank, the label always says "1L," but in actual use, this 1L can hold more than 1 liter of gas. Water capacity is the volume of water the tank can hold (1L means 1 liter of water), and working pressure is the maximum air pressure it can withstand (commonly 200bar or 300bar). A 200bar tank, with 1L of space, can fit 200 times the atmospheric pressure of gas, which would expand to 200L on the ground.

What 1L Refers To

The "1L" printed on a scuba tank, strictly speaking, is the water capacity (Water Capacity). It is the physical space inside the tank, and the volume of water is 1 liter.

A tank with a water capacity of 1L and a working pressure of 200bar means the internal gas is compressed to 200 times atmospheric pressure.

Take a specific example: Assume the tank working pressure is 200bar, and water capacity is 1L. According to the gas law, when this high-pressure gas is released to the standard atmospheric pressure of 1bar (such as on the ground), the volume will expand 200 times. Therefore, the actual gas storage volume is:

1L (Water Capacity) × 200bar (Working Pressure) = 200L (Gas Volume at Standard Atmospheric Pressure).

If it is a tank with a working pressure of 300bar and the same 1L water capacity, the gas storage volume is 300L (at standard atmospheric pressure).

Higher Working Pressure

The common working pressures for 1L scuba tanks on the market are 200bar (for recreational diving) and 300bar (for technical or industrial diving).

Here is a comparison table for clarity:

Tank Specification (Water Capacity × Working Pressure)	Gas Volume at Standard Atmospheric Pressure (L)	Applicable Scenario
1L×200bar	200L	Recreational diving, short-term use in shallow water
1L×300bar	300L	Technical diving, deep water or long-duration operations
0.6L×300bar	180L	Lightweight demand, small volume and large capacity

Some older tanks may be marked "Working Pressure 150bar," such a 1L tank can only store 150L of gas and is gradually being phased out.

Actual Usable Volume

A newly purchased tank has full pressure (e.g., 200bar), but after each fill, the pressure may not reach the nominal value. If filled to 190bar, the actual gas storage volume is 1L×190bar=190L (at standard atmospheric pressure).

For safety reasons, divers stop using the tank when the pressure drops to 50bar (reserved for emergency backup or surface breathing).

So, the actual usable gas volume for a 1L×200bar tank is: (200bar-50bar) × 1L = 150L (at standard atmospheric pressure).

This 150L of gas can support 200 minutes when breathing on the surface (pressure 1bar), consuming about 0.75L per minute (calm breathing, tidal volume 500ml, breathing frequency 15 times/minute); but if diving to 10 meters deep (pressure 2bar), 1L of gas is needed per minute (tidal volume increases due to pressure, 500ml×2=1000ml, breathing frequency may increase to 20 times/minute), 150L can only last for 150 minutes.

Standard Atmospheric Pressure

"Standard atmospheric pressure" is the average air pressure at sea level (about 1bar). The 200L (at standard atmospheric pressure) of gas in the tank, when released to 20 meters underwater (pressure 3bar), will be compressed to about 66.7L (200L÷3bar).

Working Pressure

Some people think 300bar tanks are better, but that's not necessarily true. High-pressure tanks require thicker steel, making them heavier (a 1L×300bar tank is about 20% heavier than a 200bar one), which is more strenuous to carry. A recreational diver diving to 30 meters and staying for 1 hour, a 200bar tank is sufficient; a technical diver who needs a longer bottom time or greater depth will choose 300bar.

EN 12245 mandates that tanks undergo a hydrostatic test every 5 years to check if the steel can still withstand the labeled pressure.

The test pressure for 300bar tanks is higher (usually 1.5 times the working pressure, i.e., 450bar), and the cost is also higher.

A 200bar tank stores 200L (150L usable), and a 300bar tank stores 300L (225L usable). The actual number of breaths depends on variables like diving depth and breathing frequency, but you now know that the 1L on the label is far more than just 1 liter of gas.

Gas Storage Comparison

When diving, for tanks both labeled "1L," why can some hold more gas? The secret lies in the working pressure. 200bar and 300bar tanks look similar in size, but the actual gas storage volume can differ by nearly half. When buying a tank, only looking at "1L" is not enough; you must look at the pressure value behind it.

200bar and 300bar

Here is a tangible example:

• 1L×200bar Tank: The internal gas is compressed to 200 times atmospheric pressure. According to Boyle's law (volume is inversely proportional to pressure at constant temperature), these gases will expand 200 times when released to the ground (1bar), so the gas storage volume at standard atmospheric pressure is 1L×200bar=200L.

• 1L×300bar Tank: Similarly, compressed to 300 times atmospheric pressure, when released to the ground it is 1L×300bar=300L.

Don't underestimate this 100L difference. During calm breathing on the surface, a person consumes about 0.75L of gas per minute (15 breaths/minute, 500ml per breath).

200L is enough for about 267 minutes of breathing (nearly 4.5 hours), and 300L can last 400 minutes (nearly 6.5 hours).

Weight Must Also Be Considered

A 1L×200bar tank, when empty, weighs about 1.5 kilograms; a 1L×300bar tank of the same model, due to thicker steel, will weigh about 1.8 kilograms, an increase of 0.3 kilograms.

Data refers to tests by the Compressed Gas Association (CGA) in the United States: for the same water capacity, an increase of 50bar in working pressure results in an average increase of 8%-10% in the empty tank weight.

So, before choosing a 300bar tank, you need to consider whether you can accept the extra weight.

Actual Usable Gas

A newly purchased tank has full pressure (e.g., 300bar), but the pressure will drop during use. Crucially, divers do not use the gas down to 0bar. The industry standard is to reserve 50bar for backup (for emergencies or surface breathing). Therefore, the actual usable gas volume needs to be discounted:

• 200bar tank: (200bar-50bar) × 1L = 150L (at standard atmospheric pressure)

• 300bar tank: (300bar-50bar) × 1L = 250L (at standard atmospheric pressure)

These 150L and 250L are the actual usable amounts. Let's calculate the number of breaths:

• Calm surface breathing (0.75L/minute): 150L can last 200 minutes, 250L can last 333 minutes (about 5.5 hours).

• Diving 10 meters deep (pressure 2bar, tidal volume doubled to 1L/breath, breathing frequency 20 times/minute): Consumption is 20L per minute, 150L can last 7.5 minutes, 250L can last 12.5 minutes? Incorrect—this is where mistakes are easily made!

Correction: The gas consumption underwater is calculated based on the "equivalent volume at standard atmospheric pressure." For example, at 10 meters deep (2bar), the amount of gas the body needs to inhale is 2 times that of the surface, so the consumption is 0.75L×2=1.5L per minute (at standard atmospheric pressure). Thus, 150L can last 100 minutes, and 250L can last 167 minutes (about 2.7 hours).

Tanks with Different Pressures

• 200bar Tank: Mainstream for recreational diving. Diving to 30 meters and staying for 1 hour, the 150L of usable gas is sufficient (underwater consumption is about 1.5L/minute, 150L lasts 100 minutes). It is lightweight (1.5 kilograms), suitable for beginners or short-distance diving.

• 300bar Tank: For technical diving or long-duration needs. For example, diving deep to 40 meters (pressure 5bar), consumption is 0.75L×5=3.75L per minute (at standard atmospheric pressure), 250L can last 66 minutes—20 more minutes than the 200bar tank. The drawback is the extra 0.3 kilograms of weight, and it requires a high-pressure compressor for filling (which ordinary dive stations may not have).

Data Sources

These calculations refer to the European standard EN 12245 (Safety requirements for compressed gas cylinders) and the gas management guide from the Professional Association of Diving Instructors (PADI).

The actual gas storage volume is based on the pressure stamped on the tank steel—some old tanks may be marked 150bar, with a storage volume of only 150L (100L usable), and these types of tanks are now gradually being phased out.

The difference in gas storage volume for 1L tanks with different pressures is in the pressure value multiplied by the water capacity: 200bar stores 200L (150L usable), and 300bar stores 300L (250L usable).

Standard Atmospheric Pressure

Breathing underwater is different from breathing on the ground. The atmospheric pressure on the ground is about 1bar (standard atmospheric pressure), and pressure increases by 1bar for every 10 meters underwater, for example, at 10 meters deep, the total pressure is 2bar; at 20 meters deep, it's 3bar.

For example: 1L (at standard atmospheric pressure) of gas in the tank, when released to 10 meters deep (2bar), will be compressed to a volume of 0.5L.

But when a diver takes a breath, the body does not feel 0.5L, but "how big this breath would be on the ground."

Unified Ruler

If the ground standard is not used, the calculation will be chaotic. For example:

• Diver A is at 10 meters deep (2bar), breathing 20 times per minute, inhaling 1L each time (compressed volume underwater), the actual consumed gas volume is 20 times × 1L = 20L (underwater volume).

• Diver B is at 20 meters deep (3bar), breathing 25 times per minute, inhaling 1L each time (compressed volume underwater), consuming 25L (underwater volume).

At this point, directly comparing 20L and 25L is meaningless—because the underwater volume is affected by pressure. But if both are converted to the volume at standard atmospheric pressure:

• A's 20L (2bar underwater) = 20L × 2bar = 40L (at standard atmospheric pressure).

• B's 25L (3bar underwater) = 25L × 3bar = 75L (at standard atmospheric pressure).

Using the Ground Standard

During calm breathing, each breath requires about 500ml of gas (ground standard volume). Underwater, although compressed gas is inhaled, the volume of lung expansion is the same as on the ground, which is equivalent to inhaling "500ml of ground volume" of gas, just compressed by the external pressure.

For example, at 10 meters deep, each breath requires 500ml×2bar=1000ml (ground standard volume), breathing 15 times per minute consumes 15×1000ml=1.5L (ground standard volume/minute).

What Happens Without the Standard

Suppose the consumption is calculated directly by the underwater pressure without using standard atmospheric pressure:

• At 10 meters deep, consumption is 20L per minute (underwater volume).

• The tank has 200L (at standard atmospheric pressure), converted to the volume at 10 meters underwater it is 200L÷2bar=100L (underwater volume).

• Theoretically, it can last 100L÷20L/minute=5 minutes.

But in reality? Calculated by the ground standard, 200L (ground) = 400L (10 meters underwater volume), it can sustain 400L÷20L/minute=20 minutes—a 4-fold difference!

This is the consequence of not using standard atmospheric pressure: confusing "compressed volume" and "actual consumption." The body actually needs the gas volume at the ground standard, and without using it for calculation, all data is inaccurate.

How to Calculate the Total Compressed Air Volume

The "1L" of a 1L scuba tank refers to water capacity (internal volume when filled with water). A common aluminum tank is labeled "1L 200bar," where 200bar is the filling pressure. To calculate the total compressed air volume, use "Pressure × Water Capacity": 200bar×1L=200 liters of gas at standard state (standard state refers to 1bar pressure, 25℃ normal temperature, similar to the normal pressure environment of daily breathing). If the tank is filled to 300bar, the total volume is 300 liters—higher pressure stores more gas.

Key Numbers of the Tank

To figure out how many times a 1L tank can be breathed, many people may only notice the "1L" marking when getting a new tank or renting one, but overlook another key parameter. Knowing only the tank's own volume is not enough; you also need to know the maximum pressure it can hold. Combining these two can determine the total gas volume.

The First Number

The first number to look at on the tank is the water capacity, usually in liters (L). This number is straightforward: how many liters of water the tank can hold when full. For example, a tank labeled "1L" holds 1 liter of water when full, which is about half the capacity of a large soda bottle (common soda bottles are 2L, 1L is about half of that).

Common scuba tanks on the market have water capacities of 1L, 1.2L, 1.5L, etc. 1L is a small capacity, suitable for divers with smaller body sizes or those diving in shallow areas.

The Second Number

The second key number is the maximum working pressure, in bar (bar). For example, a tank labeled "200bar" means it can be safely filled to 200 atmospheres of pressure (1bar≈1 atmosphere, sea level normal pressure is 1bar).

The maximum working pressure for aluminum tanks is commonly 200bar or 300bar, and steel tanks may be higher (e.g., 300bar or 345bar).

However, in actual use, to avoid metal fatigue or safety hazards, most divers do not fill to the absolute maximum value, but leave some margin, such as filling to 190bar or 290bar.

Two tanks with the same 1L water capacity, one labeled 200bar and the other 300bar, the latter can obviously hold more gas—because higher pressure squeezes more compressed air into the tank.

How to Use the Two Numbers

Knowing the water capacity and working pressure, you can calculate how much "standard state" gas is stored in the tank. "Standard state" here refers to an environment of 1bar pressure and 25℃.

The calculation is simple: Total Gas Volume (liters at standard state) = Water Capacity (L) × Maximum Working Pressure (bar).

For example, a 1L, 200bar tank has a total gas volume of 1×200=200 liters of standard state air.

If it is a 1L, 300bar tank, the total gas volume is 300 liters.

But if the actual filling pressure only reaches 180bar (e.g., the tank is nearing its inspection cycle, or there is concern about overpressure), then the total gas volume becomes 1×180=180 liters.

Where to See the Two Numbers

These two numbers are usually printed on the shoulder of the tank (near the valve), "1L 200bar" or "1.2L 300bar." Some tanks also mark the test date (e.g., "TT2025" means a hydrostatic test is required in 2025).

Why Must They Be Checked

Assume two divers, one using a 1L 200bar tank and the other a 1L 300bar tank, breathing at the same depth (e.g., 10 meters, pressure 2bar). The former has a total gas volume of 200 liters, and the latter 300 liters. The latter can take about 50 more breaths.

In addition, different tank brands may have different labeling conventions, some use imperial units (e.g., cubic feet), but metric liters and bar are common in China.

If the units are not confirmed, the total volume may be miscalculated. For example, "1L" and "1 cubic foot" (about 28.3 liters) are very different; the labeling must be clearly seen.

Using Simple Multiplication to Calculate Total Volume

Calculating the total volume only requires two steps: find the water capacity and working pressure on the tank, and then multiply them.

• Water Capacity: How much water the tank can hold, in liters (L). For example, a "1L" tank holds 1 liter of water when full, equivalent to the capacity of a medium-sized thermos cup (common thermos cups are 1.5L, 1L is slightly smaller). This is the tank's "physical space," fixed at the factory and engraved on the bottle body label.

• Working Pressure: The maximum air pressure the tank can safely withstand, in bar (bar). For example, "200bar" means it can hold 200 times the daily air pressure of gas (sea level daily air pressure is 1bar). This is like pumping air into a balloon; the higher the pressure, the more air is squeezed in.

Multiplying these two numbers gives the "total volume of standard state gas" stored in the tank, which is the 1bar pressure, 25℃ environment, similar to the normal pressure air we usually breathe.

Multiplication Formula

1L tank, working pressure 200bar. Total volume = 1L (Water Capacity) × 200bar (Working Pressure) = 200 liters of standard state gas.

What is the concept of these 200 liters? It is equivalent to all the high-pressure gas in the tank expanding to 200 liters when released into a normal pressure environment (1bar)—it is as "thin" as the air you usually breathe with your lungs.

If the tank's working pressure is 300bar? Total volume = 1×300=300 liters of standard state gas. It is the same 1L tank, but the pressure is 100bar higher, and the total gas volume is 100 liters more—this is the power of multiplication; every 1bar increase in pressure adds 1 liter to the total volume.

10 meters underwater, the pressure is 2bar (1bar atmospheric pressure + 1bar water pressure). When you take a breath at this depth, you don't need 1 liter of high-pressure gas, but the amount of gas that can inflate the lungs to balance with the external pressure.

For example, at 10 meters deep, each breath requires 500ml×2bar=1 liter (standard state) of gas. The 200 liters total volume can support 200 breaths (200÷1=200).

Actual Filling May Be Insufficient

For example, a tank labeled 200bar may actually only be filled to 190bar. At this time, the total volume = 1×190=190 liters of standard state gas, 10 liters less than the full pressure, and the number of breaths will also be 10 fewer (e.g., at 10 meters deep, it changes from 200 times to 190 times).

For example, a tank used for three years may only have an actual pressure of 180bar, total volume = 1×180=180 liters.

Total Volume Difference

For a more intuitive look, here is a table showing the total volume of a 1L tank at different pressures:

Tank Label	Water Capacity (L)	Working Pressure (bar)	Total Volume of Standard State Gas (L)	Equivalent Air Volume at Normal Pressure
1L 200bar	1	200	200	200 liters
1L 250bar	1	250	250	250 liters
1L Actual 180bar	1	180	180	180 liters

Looking at this table, it is clear: every 50bar increase in pressure adds 50 liters to the total volume; when the pressure is insufficient, the total volume is directly reduced.

Multiplication to Calculate Total Volume

A 1L 200bar tank has a total volume of 200 liters of standard state gas. At 10 meters deep (2bar pressure), each breath requires 1 liter (standard state) of gas, allowing for 200 breaths; at 20 meters deep (3bar pressure), each breath requires 1.5 liters, allowing for only about 133 breaths (200÷1.5≈133).

What is Standard State

When diving, you may have had this question: the tank says "1L 200bar," why can't you directly use 200 liters (high-pressure gas) to calculate how many times you can breathe? The actual calculation needs to use the gas volume at "standard state." Standard state is a "unified measure of gas volume," referring to 1bar pressure and 25℃ temperature.

The same 1 liter of gas, under high pressure (e.g., 200bar) and low pressure (e.g., 1bar), has the same number of molecules, but the volume is compressed.

The industry did not randomly choose 1bar and 25℃. 1bar is close to the atmospheric pressure at sea level (1.013bar), and 25℃ is normal temperature, which is close to the ambient temperature of most people's diving (tropical diving 28℃, cold water 18℃, but 25℃ is general enough).

Why Must It Be Used

Imagine you have two tanks: one is 1L 200bar, and the other is 1L 300bar. The former is 200 liters (high pressure), and the latter is 300 liters (high pressure).

When converted using the standard state, the former is 200 liters (normal pressure), and the latter is 300 liters (normal pressure).

When diving, what you breathe underwater is actually "expanded gas." For example, at 10 meters deep (pressure 2bar).

Suppose each breath requires 500ml (0.5 liters) of "normal pressure gas" (the same as breathing on the ground), but at 10 meters deep, this 0.5 liters of normal pressure gas will be compressed into 0.25 liters of high-pressure gas (because the pressure doubles, the volume halves).

The high-pressure gas from the tank flows out, expands back to 0.5 liters of normal pressure gas, which is enough for one breath.

At this time, the total volume at standard state (e.g., 200 liters of normal pressure gas) directly determines how many times you can breathe. 200 liters ÷ 0.5 liters/time = 400 times? Incorrect, this is easily confused.

For example, during calm breathing, each time is about 0.5 liters (normal pressure). The tank has 200 liters of standard state gas, which can support 200÷0.5=400 breaths—but this is on the surface (1bar).

At 10 meters deep (2bar), the external pressure is high, and the "standard state gas volume" required for each breath will increase. Specifically, at 10 meters deep, each breath requires 0.5 liters (normal pressure) × 2bar = 1 liter (standard state) of gas—the 200 liters total volume can only support 200 breaths.

Standard State

With the standard state, no matter how deep you are underwater, you can use the same formula to calculate the number of breaths:

Number of Breaths = Total Volume of Standard State Gas (liters) ÷ Standard State Gas Volume Required Per Breath (liters/time)

For example:

• Surface (1bar): Each breath requires 0.5 liters (normal pressure), 200 liters total volume can support 200÷0.5=400 breaths.

• 10 meters deep (2bar): Each time requires 0.5×2=1 liter, 200÷1=200 breaths.

• 20 meters deep (3bar): Each time requires 0.5×3=1.5 liters, 200÷1.5≈133 breaths.

How It is Labeled in Reality

The total gas volume (standard state) of the tank is usually not directly printed on the bottle body, but can be calculated from the water capacity and working pressure. For example, a 1L 200bar tank, total volume = 1×200=200 liters (standard state).

Some tank labels will state "Gas Volume: XX liters@1bar," which is the total volume at standard state. For example, "200 liters@1bar" means that at 1bar pressure, this gas can expand to 200 liters.

The gas consumption displayed on a dive computer or regulator (second stage), "Used 50 liters," refers to 50 liters of standard state gas.

Practical Tool

For example, you plan to dive to 30 meters (4bar), knowing the total tank volume is 200 liters (standard state), and each breath requires 0.5×4=2 liters (standard state), you can calculate 200÷2=100 breaths.

Actual Number of Breaths Possible

A 1L tank filled to 200 bar, after deducting 5-10 bar residual pressure, has about 190-195 liters of standard atmospheric pressure air usable. During a calm dive, a person breathes 15 times per minute, consuming 0.6-0.8 liters per breath, theoretically allowing for about 240-270 breaths; but in reality, increased movement or tension can increase single-breath consumption to 1 liter, reducing the number of breaths to around 190. Most divers use it at a moderate intensity, actually allowing for 150-200 breaths.

The Most Basic Calculation

A 1L tank refers to a tank with a water capacity of 1 liter, usually filled to 200 bar pressure, which is equivalent to compressing 1 liter of air to 200 times atmospheric pressure. The theoretical gas storage volume is 1 liter × 200 bar = 200 liters of standard atmospheric pressure air (standard atmospheric pressure refers to sea level pressure). However, the tank cannot be completely used up; 5-10 bar residual pressure is left to prevent water ingress, so the actual usable gas is about 190-195 liters.

During a calm dive, a person's breathing frequency stabilizes at 12-15 times per minute, and the actual consumption after inhaling and exhaling is the new air inhaled. Through breathing gas analysis, the single-breath consumption in a calm state is about 0.6-0.7 liters of standard air. Dividing the usable gas of 190 liters by 0.7 liters/breath results in about 271 times; if calculated by 0.6 liters/breath, it can reach about 325 times. In most cases, taking the middle value, it can theoretically allow for 270-290 breaths, corresponding to about 18-24 minutes underwater (because it uses 12-15 breaths per minute).

Air in the Tank

A 1L tank has a water capacity of 1 liter. When filling, air is compressed into the tank, and the pressure is usually pumped to 200 bar (1 bar ≈ 1 atmosphere).

At this point, the air volume in the tank is not 1 liter, but is compressed 200 times, equivalent to 200 liters of standard atmospheric pressure air (1 liter × 200 bar).

To prevent water from back-flowing into the tank, divers leave 5-10 bar residual pressure when closing the tank valve. This part of the gas cannot be breathed and must be deducted. Calculated by leaving 5 bar, 1 liter × 5 bar = 5 liters is deducted; leaving 10 bar deducts 10 liters.

So the actual usable air is 200 liters - 5 liters = 195 liters, or 200 liters - 10 liters = 190 liters, usable gas is about 190-195 liters of standard atmospheric pressure air.

During Calm Breathing

During a calm dive (e.g., hovering near the surface to watch fish), a person's breathing is stable: 12-15 breaths per minute, inhaling and slowly exhaling after each breath.

Measured by a breathing gas analyzer, the volume of air inhaled in this state at "standard atmospheric pressure" is about 0.6-0.7 liters.

You take one breath. At 10 meters underwater (pressure is 2 bar), the air in the tank is compressed more densely, but the actual "standard air volume" you inhale is still 0.6-0.7 liters, so the gas in the tank is consumed faster.

Theoretical Number of Breaths

With the total usable gas volume (190-195 liters) and the single-breath consumption (0.6-0.7 liters), the number of breaths can be calculated.

Calculated by the lowest usable gas of 190 liters and 0.7 liters/breath: 190÷0.7≈271 times; calculated by the highest usable gas of 195 liters and 0.6 liters/breath: 195÷0.6≈325 times.

Most divers' data for calm practice is concentrated around: usable gas of about 190 liters, single-breath consumption of around 0.65 liters.

The result is 190÷0.65≈292 times, about 270-290 times. Converted to time, breathing 12-15 times per minute, it can last 18-24 minutes underwater.

Is This Number Accurate

If descending to 10 meters deep, the air is compressed, the same movement requires more gas, and the number of breaths will decrease; if a beginner is nervous, breathing becomes fast and shallow, and single-breath consumption may rise to 0.8 liters, reducing the number of breaths.

After Increased Movement

A 1L tank can support about 270-290 breaths in a calm state. After increased movement, the breathing frequency will rise from 12-15 times per minute to 18-20 times, and the single-breath consumption will surge from 0.6-0.7 liters to 0.9-1 liter. Calculated with 190 liters of usable gas, the number of breaths directly drops to about 190 times (corresponding to 9-10 minutes of diving). If the movement is more intense, such as fast ascent, or carrying equipment, the breathing frequency can reach 20-25 times per minute, and single-breath consumption 1-1.2 liters, leaving only 160-190 times (6-8 minutes). Most fall between 150-200 times, depending entirely on how active you are, how deep the water is, and how cold it is.

How Much Gas the Body Consumes

The body's metabolic rate is 30%-50% higher than when calm, and oxygen consumption increases accordingly. When calmly hovering, oxygen consumption is about 0.8-1 liter of standard air per minute; but when swimming or moving things, oxygen consumption jumps directly to 1.2-1.5 liters/minute.

When calm, it is 12-15 times per minute. When moving, most people change to 18-20 times—you have to inhale the air faster to keep up with the body's demands.

The volume of a single inhale also increases: calmly inhaling 0.6-0.7 liters of standard air, but when moving a lot, you need to inhale 0.9-1 liter each time to suffice.

Take a case study: A 60 kg diver, when calm, consumes about 0.96 liters per minute (12 times × 0.8 liters); but when swimming, consumption is 1.4 liters per minute (18 times × 0.78 liters)—a difference of nearly 50%.

How Breathing Frequency Changes

Actual measurements show that during uniform swimming (speed of about 1 meter/second), the average breathing frequency is 18-20 times/minute, and single-breath consumption is 0.9-1 liter.

Calculated with 190 liters of usable gas: 190÷0.95≈200 times (taking the middle value of 0.95 liters/time), corresponding to 10-11 minutes of diving.

If fast swimming (e.g., chasing fish, speed above 1.5 meters/second), the breathing frequency can rush to 22-25 times/minute, and single-breath consumption is 1-1.1 liters.

At this time, the number of breaths drops directly to 190÷1.05≈181 times, only allowing for about 9 minutes of diving.

When Carrying Equipment

A dive team conducted a test: swimming 10 meters while carrying 5 kg of equipment, the breathing frequency increased from 15 times/minute to 25 times/minute, and single-breath consumption increased from 0.7 liters to 1.2 liters.

Calculated with 190 liters of usable gas: 190÷1.05≈181 times (taking the middle value of 1.05 liters/time), only allowing for 6-8 minutes of diving.

In more strenuous situations (e.g., carrying 10 kg of equipment ascending), the breathing frequency can reach 30 times/minute, and single-breath consumption 1.5 liters. At this time, 190 liters of gas can only last 127 times, less than 5 minutes.

Water Depth and Water Temperature

While movement increases, water depth and water temperature add fuel to the fire.

• Water Depth: Diving to 10 meters, the pressure is 2 times that of the surface, and the air volume required for the same movement also doubles. For example, swimming at 10 meters deep, the original 1.2 liters/minute oxygen consumption actually requires taking twice the amount of gas from the tank (because the gas in the tank is compressed).

• Water Temperature: When the water temperature is below 20℃, the body shivers, requiring extra oxygen to produce heat. In 10℃ cold water, gas consumption is 15%-20% more than in 25℃ water. When moving a lot, this may reduce your dive time by 2-3 minutes.

Which Factors Affect the Number of Breaths

The number of breaths from a 1L tank is not a fixed number. Single-breath consumption exceeding 1 liter, veteran divers can save 20% by breathing steadily; water depth is the second factor, descending 10 meters, the air is compressed, and the same movement requires 1 time more gas; water temperature is the third, 10℃ cold water burns 15%-20% more oxygen than 25℃ warm water; movement intensity is more direct, carrying equipment consumes 50% more gas than hovering. There is also the initial tank pressure. Filling to 200 bar and 180 bar, the usable gas differs by more than 10%. These factors combined can reduce the number of breaths from 270 times (ideal state) to 150 times (extreme situation).

The "First Brick"

20 times per minute, only inhaling a half-breath (standard air volume 0.5 liters), the single-breath consumption looks small, but the frequency is too high, and the total gas consumption is actually higher.

Measured data: A beginner calmly hovering, consumes 1.2 liters per minute (24 times × 0.5 liters); a veteran diver in the same state, breathing 12 times/minute, 0.8 liters each time, total consumption 0.96 liters/minute. The veteran diver saves 20%.

The Second Brick

Underwater, every 10 meters, the pressure increases by 1 bar (sea level is 1 bar). For example, at 10 meters deep, the air in the tank is compressed to 2 times the density. Although it feels as "full" as on the surface, the actual standard air volume taken away is 2 times that of the surface.

For example: When calmly hovering, single-breath consumption on the surface is 0.6 liters of standard air; at 10 meters deep, the same movement requires inhaling 1.2 liters of standard air (because the gas in the tank is compressed, more gas must be taken out to meet the body's needs).

If the usable gas is 190 liters, it can support 270 breaths on the surface, but only 158 breaths at 10 meters deep—a direct cut of 42%.

Diving 30 meters (pressure 4 bar), single-breath consumption becomes 0.6×4=2.4 liters, and the number of breaths plummets to 79 times, only allowing for 4 minutes of diving.

The Third Brick

The body "secretly burns more oxygen" when cold. When the water temperature is below 20℃, the human body's metabolic rate rises to maintain a body temperature of 37℃, and oxygen consumption increases.

Measured divers' core body temperature and gas consumption: In 25℃ warm water, calm consumption is 0.9 liters/minute; in 15℃ cold water, the consumption increases to 1.08 liters/minute in the same state—an increase of 20%.

It is more obvious when moving a lot. For example, swimming in 15℃ water, consumption increases from 1.4 liters/minute (warm water) to 1.68 liters/minute (cold water), and the number of breaths drops from 136 times (190÷1.4) to 113 times (190÷1.68)—a decrease of 17%.

The Fourth Brick

Movement intensity is the "accelerator" of gas consumption. Gas consumption varies greatly for different underwater activities:

• Hovering and Observing: Almost no movement, 12 breaths/minute, 0.6 liters per breath, consumption 0.72 liters/minute;

• Slow Swimming (1 meter/second): Kicking fins + paddling hands, 18 breaths/minute, 0.9 liters per breath, consumption 1.62 liters/minute;

• Fast Swimming (1.5 meters/second): Chasing fish or avoiding current, 25 breaths/minute, 1.1 liters per breath, consumption 2.75 liters/minute;

• Carrying Equipment: Lifting a 5 kg camera bag and swimming 5 meters, 30 breaths/minute, 1.3 liters per breath, consumption 3.9 liters/minute.

Calculated with 190 liters of usable gas, hovering can last 264 breaths (7.9 hours? No, it's minutes! 264 times ÷ 12 times/minute = 22 minutes), and carrying equipment can only last 49 breaths (1.6 minutes).

The Fifth Brick

Initial tank pressure and residual pressure also affect it. For example, some tanks are filled to 200 bar, and some are only 180 bar. The usable gas is 190-195 liters (200 bar) and 171-175 liters (180 bar × 95% usable), respectively.

With the same calm consumption of 0.65 liters/breath, the 200 bar tank can support 292 breaths, and the 180 bar tank can only support 263 breaths—a decrease of 10%.

Residual pressure is more concealed. Some tanks have 5 bar left after the valve is closed, and some have 10 bar.

A tank with 10 bar left has 5 liters less usable gas than one with 5 bar left (1 liter × 5 bar), which means 7-8 fewer breaths in a calm state (5 liters ÷ 0.65 liters/breath).

For example, a beginner carrying equipment at 10 meters deep in 15℃ water, breathing frequency 25 times/minute, single-breath consumption 1.2 liters—usable gas 190 liters (200 bar - 10 bar residual pressure), the number of breaths is only 190÷1.2≈158 times, less than 3 minutes.

Conversely, a veteran diver hovering in 5 meters shallow water in 25℃ warm water, breathing 12 times/minute, 0.6 liters per breath—the number of breaths can reach 190÷0.6≈317 times, diving for 26 minutes.