How to Choose the Right Scuba Diving Tank Size: 0.5L, 1L, or 2.3L

Selecting a scuba tank depends on the usage scenario: 0.5L (200bar, approximately 100 liters of gas) is only enough for a short experience in shallow water (e.g., 3-5 minutes), suitable for snorkeling or children; 1L (200bar, approximately 200 liters of gas) can support standard diving within 10 meters for 7-10 minutes, satisfying beginner needs; 2.3L (200bar, approximately 460 liters of gas) can extend the time to 15-23 minutes, suitable for diving around 15 meters depth where longer exploration is needed. Calculating based on a breathing rate of 20-30 liters/minute (under pressure) is more accurate.

Tank Capacity and Gas Volume

The "capacity" of a scuba tank refers to its water volume (the volume when filled with water). The actual usable gas volume must be calculated based on the working pressure. A 0.5L tank at 200bar pressure stores about 100 liters of standard gas (0.5L×200bar), only enough for a short experience in shallow water (e.g., 3-5 minutes); a 1L tank stores 200 liters of gas, supporting standard diving within 10 meters for 7-10 minutes; a 2.3L tank stores 460 liters, usable for 15-23 minutes at 15 meters depth (calculated based on a resting breathing rate of 15-20 liters/minute).

How to Calculate Gas Volume

The "0.5L," "1L," and "2.3L" printed on a scuba tank refer to its water volume, which is how many liters of water it can hold. However, what divers truly care about is "how much breathable compressed air" it can hold, which is different from the water volume. To convert the water volume into the actual usable gas volume, one must use the basic physics principle, Boyle's Law.

Boyle's Law states that at a constant temperature, the pressure and volume of a fixed amount of gas are inversely proportional. The more severely a gas is compressed (the higher the pressure), the smaller the volume; conversely, when the gas returns to a low-pressure environment (such as underwater), the volume expands back to its original size.

A tank with a water volume of 0.5L has a working pressure of 200bar (bar is a unit of pressure; 1bar is approximately equal to 1 kg-force pressing on 1 square centimeter).

According to Boyle's Law, the volume of the high-pressure air in the tank at surface standard pressure (1bar) is the water volume multiplied by the working pressure.

The formula is: Standard Liters (SL) = Water Volume (L) × Working Pressure (bar).

Thus, 0.5L×200bar=100 liters of standard gas (SL).

These 100 liters represent the "total air volume equivalent at surface pressure." Regardless of the underwater depth, this total volume remains constant, but the air consumed per minute by the diver will "appear" greater due to the increase in water pressure.

Calculating the 1L tank: at the same 200bar pressure, it is 1L×200bar=200 liters of standard gas.

The 2.3L tank is 2.3L×200bar=460 liters of standard gas.

If the tank is a 300bar high-pressure version (such as some steel tanks), the 0.5L tank can hold 0.5×300=150 liters of standard gas, the 1L tank can hold 300 liters, and the 2.3L tank can hold 690 liters.

Here is a detail: the actual working pressure of the tank may be marked on the tank body, such as "200bar" or "300bar." You should confirm this value before purchasing a tank, as different pressures will affect the final gas volume.

For example, some might think that "a 0.5L tank can only hold 100 liters of gas," but if it is a 300bar high-pressure tank, it can actually hold 150 liters. The extra 50 liters can almost double the dive time.

For instance, a novice at 10 meters depth, with a breathing rate of approximately 20 liters/minute (Standard Liters/minute), using a 1L×200bar=200 liters standard gas tank, the theoretical usage time is 200÷20=10 minutes.

If they kick their fins fast and their breathing rate increases to 25 liters/minute, the time becomes 200÷25=8 minutes. This is why novice instructors always say, "Leave a 5-minute safety margin."

Comparing with the 0.5L tank, if it is 200bar, 100 liters of standard gas, at a 20 liters/minute breathing rate, it can only be used for 5 minutes, basically only enough to take a couple of photos near the surface before returning to the boat.

A 2.3L×200bar=460 liters tank, at 20 liters/minute, can be used for 23 minutes, which is sufficient to circle a coral reef twice at 15 meters depth, take some videos, and ascend slowly.

Different Breathing Rates

During a dive, how long the tank lasts is determined by the breathing rate, which is a critical variable. With the same 1L tank containing 200 liters of standard gas, some people can use it for 10 minutes, while others can only use it for 8 minutes.

First, clarify the unit of the breathing rate: Standard Liters per minute (SL/min). For example, at 10 meters deep (2bar pressure), if you inhale 20 liters of gas per minute, you are actually consuming 40 liters of high-pressure gas, but in terms of standard liters, it is still 20SL/min (because the regulator automatically reduces the pressure, so we only focus on the total volume equivalent at the surface).

When Resting

If you are just floating in the water watching fish and not moving much, the breathing rate is at its lowest. In this case:

0.5L tank (100L standard gas): 100÷15≈6.7 minutes (about 7 minutes), 100÷20≈5 minutes. Generally enough to descend from the boat, take a couple of photos of the coral, and slowly ascend back to the surface.
1L tank (200L standard gas): 200÷15≈13 minutes, 200÷20≈10 minutes. Enough to float for a bit longer, watch clownfish in the sea anemones, or wait for a buddy to ascend together.
2.3L tank (460L standard gas): 460÷15≈30 minutes, 460÷20≈23 minutes. Sufficient to circle a large part of a reef at 15 meters depth, observing fish schools in different areas.

During Light Activity

Real diving is rarely completely static. Kicking fins to swim forward, occasionally adjusting the BCD (Buoyancy Control Device), or gently touching a starfish on a rock, the breathing rate will rise to 20-25SL/min:

0.5L tank: 100÷20=5 minutes, 100÷25=4 minutes. At this point, there is not even enough time to shoot a video, it is more suitable for trying out in shallow water, not for deep exploration.
1L tank: 200÷20=10 minutes, 200÷25=8 minutes. Novice instructors often say, "Leave 5 minutes of safety time," so the actual planned dive time of 10 minutes will be compressed to 5 minutes of activity + 5 minutes of ascent, just enough for a basic dive.
2.3L tank: 460÷20=23 minutes, 460÷25=18 minutes. At this point, you can do more: for example, follow the dive guide to find a clownfish nest, or stop to shoot a 5-minute video of the coral reef, and still have a few minutes left to ascend slowly.

During Strenuous Activity

If you encounter strong currents and need to swim back to the boat quickly, or help a buddy retrieve a dropped camera, the breathing rate might spike to 30SL/min or more:

0.5L tank: 100÷30≈3.3 minutes. This is definitely not enough; you might run out of air halfway and must return immediately.
1L tank: 200÷30≈6.7 minutes. Only enough to support an emergency situation, not suitable for a regular dive plan, as it can easily cause anxiety.
2.3L tank: 460÷30≈15 minutes. Although the time is shortened, 15 minutes is enough to handle most unexpected situations—such as a quick ascent from a deeper 15 meters to a 5-meter safety stop, and then a slow swim back to the boat.

The Effect of Depth

For every 10 meters deeper underwater, the pressure increases by 1bar. The body requires more oxygen, and you might unconsciously speed up your breathing.

However, the depth limit for recreational diving (usually not exceeding 40 meters) has a limited impact on the breathing rate.

For example, at 30 meters deep (4bar pressure), your breathing rate might increase from 20SL/min to 22-23SL/min, which is not a significant change.

What truly limits the time is the total gas volume. Even if the breathing rate only increases by 10%, the 23 minutes of the 2.3L tank will turn into 20 minutes, a noticeable impact.

A real example: a diver took a 1L tank (200L standard gas) to a 12-meter deep area, planning to photograph coral.

For the first 5 minutes, he swam slowly, breathing rate 18SL/min; for the next 3 minutes, he knelt down to photograph an octopus, breathing rate increased to 22SL/min; finally, he wanted to chase a school of fish, kicking faster, breathing rate reached 25SL/min.

Total consumption: 5×18+3×22+2×25=90+66+50=206L, which exceeded 200L, leaving 2 liters of gas for ascent. This shows that the actual breathing rate fluctuates, and a safety margin must be included in the plan.

Looking at the 0.5L tank, assuming it is used near the surface (1bar pressure), with a breathing rate of 20SL/min, it can only be used for 5 minutes.

But for a child's discovery dive, they might just float on the surface, breathing lighter, at 15SL/min, which allows for 6-7 minutes, just enough for a fun attempt.

The advantage of the 2.3L tank is more apparent during longer activities.

For example, if someone wants to draw a dive log marker underwater, or teach a novice to adjust weight, these stationary actions consume more gas, but the total volume of 460L can support a longer duration.

How to Choose Based on Actual Scenarios

Snorkeling usually takes place near the surface, at a depth not exceeding 2 meters, with minimal activity, mainly for sightseeing. In this case, a 0.5L tank (holding 100 liters of standard gas at 200bar pressure) is the most suitable.

Lightweight: An aluminum 0.5L tank weighs about 2 kg in total, 0.5 kg lighter than a 1L one. Children or novices who are afraid of bearing weight can wear it without pressure.
Sufficient Time: The breathing is very light during snorkeling, with a breathing rate of about 15SL/min. 100 liters of gas can last for 6-7 minutes. This is enough to jump off the boat, float and watch the coral reef, take a few surface photos, and slowly swim back to the boat.
Low Cost: The price of a 0.5L tank is about 30% lower than a 1L one, suitable for experience classes or occasional use.

But be careful not to use a 0.5L tank for deep diving. A novice once mistakenly thought "a small tank saves effort," and went down to 5 meters deep with a 0.5L tank. After kicking a couple of times, the breathing rate rose to 20SL/min, and the air ran out in 5 minutes. They ended up ascending in a panic, holding the regulator—this kind of experience is frustrating.

1L is the Most Reliable

Novice courses are usually within 10 meters, requiring the learning of basic actions such as regulator use, BCD inflation, and slow ascent. Time must be allowed for each step of practice. The 1L tank (200 liters of standard gas) is the most frequently recommended by instructors.

Enough Time to Practice Actions: Novices are not skilled in their movements, and their breathing is heavier than experienced divers, with a breathing rate of about 20SL/min. The 200 liters of gas can theoretically last for 10 minutes, but the instructor will require a 5-minute safety margin, leaving 5 minutes of actual activity time. This 5 minutes is enough to complete the sequence: descend → adjust BCD → observe small fish → practice exhaling through the mouth → slow ascent, ensuring a steady process.
Less Psychological Pressure: Novices easily get nervous, and nervousness leads to heavy breathing. The gas volume of a 1L tank is twice that of a 0.5L tank. Even with heavy breathing, the air will not run out in 3 minutes, which helps alleviate anxiety.
Strong Versatility: 1L tanks are the most common in dive shops, and they are usually available when renting equipment. Buying one later will seamlessly fit in.

A true case: Novice A first used a 0.5L tank, and just as they learned to adjust the BCD, they found only 2 minutes of air left, which made them panic. The instructor had to change their tank. Later, when they switched to a 1L tank, they could calmly complete the same course process and even had time to watch the coral.

2.3L Offers More Freedom

If you can already easily dive to 15 meters and want to explore the reef for a longer time, or use a GoPro to film fish schools, the 2.3L tank (460 liters of standard gas) solves the pain point of "not enough time."

Extended Exploration: Assuming a breathing rate of 20SL/min, 460 liters can last for 23 minutes. What can be done in these 23 minutes? Swim for 10 minutes from the boat to a new reef, kneel down to film clownfish entering and exiting a sea anemone (5 minutes), then follow a Napoleon wrasse for 3 minutes, and finally, leave 5 minutes for a slow ascent.
Handling Unexpected Situations: Underwater, you might encounter a stronger current, requiring an extra 5 minutes of swimming back to the boat, or helping a buddy retrieve a dropped light, consuming an extra 2 minutes. The gas volume of the 2.3L tank can withstand these accidents without having to ask the dive guide for air midway.
Suitable for Teaching Novices: When a dive guide takes a novice, they need to control their speed and provide more explanations. The 2.3L tank can support longer stationary time. For example, when a novice is hesitating whether to touch a sea star, the dive guide can explain a few more words without having to rush them to "keep up."

Divers who have compared 1L and 2.3L tanks report: when using a 1L tank to shoot a video, after 10 minutes, they start worrying about running out of air, and the camera shakes; with a 2.3L tank, they can focus on composition and even dare to dive to 20 meters deep (of course, staying within the no-decompression limits), capturing more details.

Special Circumstances

Sometimes the weather is good before heading out, and the plan is to dive to 15 meters; but upon arrival, the waves are strong, and the dive is changed to 5 meters shallow water. The flexibility of the tank size is important here:

A person with a 1L tank can explore for a longer time in the shallow water. With the breathing rate dropping to 15SL/min, 200 liters can last for 13 minutes, which is enough to circle a small reef twice.
A person with a 2.3L tank has more flexibility. In shallow water, they can dive for 23 minutes (15SL/min), or even bring a buddy along as a "mobile air source" (of course, ensuring the buddy has a backup tank).

Choosing a tank size is about matching your diving goals: choose 0.5L for a relaxed experience, 1L for learning basic skills, and 2.3L for more exploration or video shooting.

Gas cylinders suitable for beginners

Novices who practice breath-holding or primarily dive in pools/shallow water will find the 0.5L tank a suitable choice. Its nominal capacity is 0.5 liters, and at the common 200bar pressure, the total gas volume is about 100 liters (standard pressure equivalent). For a novice's static breath-holding practice, the gas consumption is about 0.5-1 liter per minute, so 100 liters of gas can support 100-200 minutes of practice. The weight is only about 1.5 kg (aluminum tank), much lighter than a standard 12L tank (about 15 kg).

Core Parameters of Small Tanks

The "0.5L" marked on a scuba tank refers to its internal actual volume. But this is not the whole story; what truly determines how long a diver can use it is the "gas volume," which must be calculated in conjunction with the filling pressure.

First, clarify two basic concepts: tank capacity is the physical volume, in liters (L); gas volume is the total liters of compressed air inside the tank after filling (standard pressure equivalent).

For example, if a 0.5L tank is filled to 200bar pressure (a common diving pressure), the actual stored gas volume is 0.5L×200bar=100 liters (the "liters" here is the air volume at standard pressure, similar to the volume after the high-pressure gas is released to normal atmospheric pressure).

If filled to 300bar (some high-pressure tanks), the gas volume becomes 0.5×300=150 liters, but 0.5L tanks are rarely used at 300bar due to material and design limitations; most are 200bar.

During static breath-holding practice (holding breath and not moving in the water), the gas consumption of an adult using a regulator is approximately 0.5-1 liter per minute (slower than normal breathing, because it is coordinated with breath-holding).

Calculated based on 100 liters of gas at 200bar, it can support 100-200 minutes of practice.

If it is dynamic practice (slow movement in the water), the gas consumption will rise to 1-1.5 liters per minute, and 100 liters of gas can also last for 67-100 minutes.

0.5L tanks are mostly aluminum, with an empty weight of only 1.2-1.5 kg. When fully filled, the total weight (including the 100 liters of high-pressure air inside the tank) is about 1.5-1.8 kg. Compared to the common 12L standard tank (empty tank 13.6 kg, full tank 15 kg), the small tank is light, as if carrying nothing.

Some might ask, "Is 0.5L too little gas to be enough?" It depends on the purpose. If the novice only wants to practice breath-holding in a pool, 100 liters of gas is enough for dozens of breath-holding repetitions; if going for a shallow dive in open water (e.g., 2-5 meters), 20-30 minutes of swimming is also sufficient.

However, if diving to 10 meters deep, the gas consumption will rise to 2-3 liters per minute (for every 10 meters deeper, the pressure doubles, and the gas consumption roughly doubles). At this point, 100 liters of gas can only last for 33-50 minutes, and one might need to surface midway.

But for a novice's first open water experience, 30-40 minutes is enough to complete basic practice, and there is no need to pursue a long duration.

In addition, the "smallness" of the 0.5L tank is also reflected in its size. Its diameter and height are usually smaller than 1L or 2.3L tanks. When a novice wears a wetsuit, the small tank is more flexible when attached to the waist or BCD (Buoyancy Control Device) and is less likely to be caught by equipment.

The capacity of the 0.5L tank (0.5 liters) and the gas volume (100 liters at 200bar) are specially designed for novice basic training: the light weight reduces the burden, the limited capacity forces focus on breathing control, and the sufficient duration covers the initial practice needs. This

Characteristics of Gas Consumption During Practice

The scene of a novice entering the water with a wetsuit for the first time is very common: standing by the pool, taking a deep breath, biting the regulator, slowly sinking, body straight, breathing lighter and slower than usual.

First, look at a set of basic data: an adult's daily breathing on land is about 12-16 breaths per minute, inhaling about 500 milliliters of air each time, with a total gas consumption of about 6-8 liters/minute.

When a novice practices static breath-holding (staying still in the water, only breathing and not swimming), the breathing frequency will drop to 8-12 breaths per minute (about 30% slower than on land), and the tidal volume will increase to about 600-700 milliliters each time (due to the need for more complete gas exchange).

Based on this calculation, the gas consumption during static breath-holding is 8 times/minute×0.65 liters/time≈5.2 liters/minute? Incorrect. This needs to be corrected: the air output by the regulator is the standard pressure equivalent, and the gas consumption calculation is the "volume of compressed air released from the tank."

Simply put, when a novice practices static breath-holding, the actual tank gas volume consumed per minute is approximately 0.5-1 liter (calculated based on a tank at 200bar pressure).

Take a specific example: a novice practices static breath-holding in a pool, lasting 2 minutes on the first descent, using 1 liter of gas (0.5 liters/minute);

The second time, adjusting the breath, lasting 3 minutes, using 2.5 liters (about 0.83 liters/minute).

The entire process consumes less than 4 liters of gas, and the 0.5L tank at 200bar has 100 liters of gas, enough for 25-50 repetitions of such breath-holding practice.

When a novice swims, the range of motion is small, the speed is slow (about 0.5-1 meter/second), the body remains horizontal, and the breathing frequency is maintained at 10-15 breaths per minute, with a tidal volume of 700-800 milliliters each time.

The gas consumption at this time is about 1-1.5 liters/minute (tank release volume). Similarly, calculated based on the 100 liters of gas in the 0.5L tank, it can support 67-100 minutes of swimming.

There are three main reasons:

First, when first entering the water, a novice may breathe slightly faster due to fear, but they will gradually relax after 10-15 minutes, and their breathing will stabilize;
Second, the range of motion is small. Experienced divers might swim and observe the surroundings, with a large range of body rotation, increasing gas consumption;
Third, no extra work is needed.

In contrast, a person with 100 dives experience, the gas consumption for static breath-holding might drop to 0.3-0.6 liters/minute (more efficient breathing control), and the dynamic swimming is also controlled at 0.8-1.2 liters/minute.

If a novice starts with a 1L or 2.3L tank, they might encounter two problems:

One is too much gas, leading to a "don't care about gas consumption" attitude during practice, thus neglecting breathing control training;
The other is the heavy weight of the large tank (1L aluminum tank empty is about 1.8 kg, full is 2 kg; 2.3L aluminum tank empty is about 4 kg, full is 4.6 kg). The novice's underwater load increases, and their movements are more likely to deform, indirectly leading to increased gas consumption.

However, the "limited gas" of the 0.5L tank actually becomes a training tool.

Looking at the actual scene: a diving club statistics showed that among students who dived for the first 5 times, 80% used the 0.5L tank, with an average underwater duration of 25-40 minutes and a remaining gas volume of 30-50 liters.

If they switch to a 1L tank, the remaining gas might exceed 70 liters, and the students tend to develop a complacent mentality of "plenty of time left," which, in turn, affects the practice results.

The Lightness of the Weight

Lifting a 0.5L tank, one can clearly feel the lightness. The aluminum empty tank weighs only 1.2-1.5 kg. When filled to 200bar pressure, the total weight (tank body plus high-pressure air inside) is also only 1.5-1.8 kg. What does this weight mean? It is equivalent to a 1.5-liter bottle of mineral water or the weight of a 3-year-old child.

Water is 800 times denser than air. The human body will naturally float or sink underwater, requiring adjustment by the BCD (Buoyancy Control Device) and hand/foot movements.

If the tank carried is too heavy, the novice has to use extra shoulder, arm, or waist strength to "support" it, causing the body to unconsciously lean forward or backward.

For example, with a 1.8 kg 0.5L tank, the weight is evenly distributed on the back, and the novice can easily balance by adjusting the BCD; but switching to a 4 kg 2.3L tank (aluminum empty tank about 4 kg, full tank 4.6 kg), the weight is concentrated in the lower back, and the novice may need to frequently adjust the BCD inflation volume or tense their shoulders to counteract the weight, which disrupts body balance.

The frog kick efficiency of a novice carrying a 4 kg tank is about 20% lower than that of a novice carrying a 1.5 kg tank.

The light weight of the 0.5L tank allows the novice's leg strength to be primarily used for propulsion, rather than "fighting the equipment."

Novices using the 0.5L tank usually master the basic breathing rhythm by the 3rd dive; those using the 2.3L tank may need 5-6 dives to achieve the same result.

For example, in a shallow area of 2-5 meters, where the current is slight, the novice needs to maintain their position to observe coral or small fish.

When carrying the 0.5L tank, the body only needs slight adjustments to stay fixed; when carrying the 2.3L tank, a slight rush of water might push the body off target, and the novice has to expend more effort to fight the current and equipment weight, which negatively affects the practice experience.

The 0.5L tank is small and lightweight, fitting snugly on the BCD. The novice will not be bothered by the tank bumping their waist or back during activities, and they will feel more relaxed psychologically.

Compared to other size tanks, the lightness of the 0.5L tank is "just right."

The 1L aluminum tank is about 2 kg when full, 20%-30% heavier than the 0.5L one, which a novice might still adapt to, but they might feel shoulder soreness during long practice sessions; the 2.3L aluminum tank is nearly 5 kg when full.

Dive Duration Reference for Different Scenarios

A 0.5L tank at 200bar pressure stores about 100 liters of gas (0.5L×200bar). Assuming a recreational diving consumption of 15 liters/minute, it can only support about 6-7 minutes, suitable for onshore regulator practice; a 1L tank stores 200 liters of gas, usable for 13-14 minutes at the same consumption rate, suitable for short explorations in shallow water; a 2.3L tank (or the common 12L standard tank, storing 2400 liters of gas) can support 15 liters/minute consumption for about 160 minutes, covering the needs of most open water standard dives.

First, Calculate How Much Gas the Tank Can Hold

The "0.5L," "1L," and "2.3L" marked on a scuba tank refer to its water volume. But this is not related to the actual usable volume of compressed air. You must first convert the water volume into the volume of compressed air.

For example, a tank marked "0.5L" has an internal space that can hold exactly 0.5 liters of water. When it is filled with compressed air, this air is "squeezed" into the space that originally only held 0.5 liters of water.

Assuming the tank's working pressure is 200bar (bar is a unit of pressure, 1bar≈1 kg-force/square centimeter, 200bar is 200 times atmospheric pressure), the actual amount of compressed air it holds is the water volume multiplied by the pressure: 0.5L×200bar=100 liters of compressed air.

This is like forcing 100 liters of air into a bottle that can only hold 0.5 liters of water, which is how the tank can "store" so much.

For safety and to protect the tank's inner wall, divers usually stop using the tank when the pressure drops to around 5bar. For a 200bar tank, the usable pressure is 195bar (200-5), so the usable storage capacity of a 0.5L tank is 0.5L×195bar=97.5 liters, which is still about 100 liters, with a small error. In daily calculations, you can directly multiply the water volume by the working pressure.

Next, look at the 1L tank. At the same 200bar pressure, the total stored gas volume is 1L×200bar=200 liters of compressed air, with a usable volume of about 195 liters.

The common 2.3L small tank (not the standard 12L tank) stores 2.3L×200bar=460 liters of compressed air, with a usable volume of about 448 liters. If it is a larger standard recreational tank, such as 12L at 200bar, the stored gas volume is 12×200=2400 liters of compressed air, with a usable volume of 2340 liters.

How are these numbers used? Assuming you are a diver with moderate skills, consuming 15 liters of compressed air per minute (this value varies, as detailed later).

Then the 100 liters of compressed air in the 0.5L tank can only last for 100÷15≈6.7 minutes, used up in about 7 minutes.

The 200 liters of compressed air in the 1L tank can last for 200÷15≈13.3 minutes, about 13 minutes.

The 460 liters of compressed air in the 2.3L tank can last for 460÷15≈30.7 minutes, nearly half an hour.

The 2400 liters of compressed air in the standard 12L tank can last for 2400÷15=160 minutes, 2 hours and 40 minutes.

For example, some high-pressure tanks can be filled to 300bar, so the 0.5L water volume can hold 0.5×300=150 liters of compressed air, with a usable volume of 142.5 liters. Calculated at 15 liters/minute, it can last for 9.5 minutes.

However, these high-pressure tanks are uncommon; recreational diving mostly uses 200bar.

Novices, due to tension, may consume 20 liters or more per minute; cold water environments (e.g., below 15℃) will accelerate body metabolism, increasing gas consumption by 10%-15%; the deeper the descent, the more the air in each breath is compressed, and the volume of gas consumed in the same time will also increase.

For example, at 30 meters depth (4bar pressure), the air you inhale per minute is equivalent to inhaling 4 times the volume at the surface—at this time, the usable time of the 0.5L tank might shorten from 7 minutes to less than 2 minutes.

How Much Gas the Tank Can Hold

A tank marked "0.5L" has an internal space that can hold exactly 0.5 liters of water. When it is filled with compressed air, this air is "squeezed" into the space that originally only held 0.5 liters of water.

This is like forcing 100 liters of air into a bottle that can only hold 0.5 liters of water, which is how the tank can "store" so much.

For safety and to protect the tank's inner wall, divers usually stop using the tank when the pressure drops to around 5bar.

For a 200bar tank, the usable pressure is 195bar (200-5), so the usable storage capacity of a 0.5L tank is 0.5L×195bar=97.5 liters, which is still about 100 liters, with a small error. In daily calculations, you can directly multiply the water volume by the working pressure.

Next, look at the 1L tank. At the same 200bar pressure, the total stored gas volume is 1L×200bar=200 liters of compressed air, with a usable volume of about 195 liters.

The common 2.3L small tank (not the standard 12L tank) stores 2.3L×200bar=460 liters of compressed air, with a usable volume of about 448 liters.

If it is a larger standard recreational tank, such as 12L at 200bar, the stored gas volume is 12×200=2400 liters of compressed air, with a usable volume of 2340 liters.

How are these numbers used? Assuming you are a diver with moderate skills, consuming 15 liters of compressed air per minute (this value varies, as detailed later).

Then the 100 liters of compressed air in the 0.5L tank can only last for 100÷15≈6.7 minutes, used up in about 7 minutes. The 200 liters of compressed air in the 1L tank can last for 200÷15≈13.3 minutes, about 13 minutes.

The 460 liters of compressed air in the 2.3L tank can last for 460÷15≈30.7 minutes, nearly half an hour. The 2400 liters of compressed air in the standard 12L tank can last for 2400÷15=160 minutes, 2 hours and 40 minutes.

However, these high-pressure tanks are uncommon; recreational diving mostly uses 200bar.

For example, at 30 meters depth (4bar pressure), the air you inhale per minute is equivalent to inhaling 4 times the volume at the surface. The usable time of the 0.5L tank might shorten from 7 minutes to less than 2 minutes.

Scenario Matching and Duration

Selecting a tank should not only focus on how much gas it can hold but also on what you plan to do with it. The 0.5L, 1L, and 2.3L tanks correspond to different diving scenarios—short practice, shallow water exploration, or longer dives offshore. Using the wrong one can lead to panic.

Onshore/Pool Practice

Novices just learning to dive always practice fundamentals on the shore or in the pool: adjusting the regulator, mask clearing, and equalizing ear pressure. These actions do not require swimming, so gas consumption is slow, but they require repeated operation. A 0.5L tank at 200bar pressure can hold 100 liters of compressed air (minus the 5bar safety margin, about 97.5 liters). Calculated at a moderate consumption of 15 liters/minute, it can last for 6-7 minutes.

During actual practice, for example, adjusting the regulator might take 2 minutes, mask clearing twice for 1 minute each, practicing ear equalization 3 times for 1 minute each, plus breaks to adjust equipment. 6-7 minutes is just enough to complete one round of basic practice.

If practicing too intently, such as clearing the mask a few more times or repeatedly confirming the breathing rhythm, it might be used up in 5 minutes.

Changing the tank is also convenient ashore; having a small 0.5L tank ready allows for one set of practice after another without the hassle of carrying a large tank.

However, novices should not stick to the 0.5L for too long. 6 minutes might not be enough. In this case, a 1L tank can last for 13 minutes, which is more relaxed.

Shallow Reef Exploration

A friend invites you to snorkel in the shallow sea, 5-10 meters deep, to see coral reefs and chase small fish.

A 1L tank at 200bar has 200 liters of compressed air, usable for about 13 minutes.

In actual use, descending from the surface to 5 meters takes 1 minute, swimming 10 meters to the coral reef and stopping takes 2 minutes, taking 5 photos takes 30 seconds each, swimming back to the shallow area and stopping takes 3 minutes, plus fragmented time for adjusting breathing. 13 minutes is generally enough for a round trip.

If a school of tropical fish comes by and you stay for an extra 5 minutes, you might have 8 minutes left.

However, if the water temperature is low that day (e.g., 18℃), the body needs to consume more oxygen for warmth, and the gas consumption rate might rise to 18 liters/minute.

The 200 liters of compressed air in the 1L tank can only last for 200÷18≈11 minutes. In this case, 1L might be tight. If you bring a 2.3L small tank, 460 liters of compressed air can last for 460÷18≈25 minutes, which is more reassuring.

Offshore Wreck Diving

This scenario involves significant activity: swimming to the wreck (possibly 200 meters away), circling the ship to observe details, and dealing with currents.

A 2.3L tank at 200bar has 460 liters of compressed air, usable for about 30 minutes.

Specific allocation: descending to 20 meters takes 2 minutes (gas consumption slightly faster as depth increases), swimming 100 meters to the wreck takes 3 minutes, circling the wreck to observe the hull and find marine life takes 10 minutes, resting to adjust buoyancy takes 5 minutes, and swimming back to the shore for 100 meters takes 3 minutes, totaling 23 minutes. The remaining 7 minutes are for buffer, or if you want to take a couple more panoramic photos of the wreck, the time is sufficient.

If you switch to 0.5L or 1L, it simply won't last. The 0.5L can only last for 7 minutes; you would have to rush back as soon as you reach the wreck, seeing nothing; the 1L lasts 13 minutes, and after 5 minutes of swimming to the wreck, the remaining 8 minutes only allow for a quick half-circle, a much worse experience.

The standard 12L tank can last for 160 minutes, but it is too heavy to carry. The 2.3L small tank has a suitable weight and can still meet the demand.

Cold Water or Strong Current

If the diving environment changes, such as water temperature below 15℃ or noticeable currents, the gas consumption rate will increase. In 15℃ water, a novice's gas consumption might reach 20 liters/minute. In this case:

0.5L tank: 97.5 liters÷20 liters/minute≈4.8 minutes (less than 5 minutes)
1L tank: 195 liters÷20 liters/minute≈9.7 minutes (less than 10 minutes)
2.3L tank: 448 liters÷20 liters/minute≈22.4 minutes (less than 23 minutes)

Choosing a tank is even more cautious in this situation. In cold water, body metabolism is fast, and oxygen consumption is high. 0.5L and 1L might not even allow for basic activities, so you must choose 2.3L or a larger tank.

Scenario and Duration

Onshore Practice: 0.5L (6-7 minutes) is enough for basic practice; choose 1L (13 minutes) if you want to practice more.
Shallow Reef Exploration: 1L (13 minutes) is enough for normal conditions; choose 2.3L (23 minutes) for cold water or more extensive exploration.
Offshore Wreck Diving: 2.3L (23 minutes) is the minimum; for relaxed exploration or complex environments, go directly for the standard 12L tank (160 minutes).