Selecting an inflatable mini scuba tank requires considering capacity, pressure, and use scenario: Lung-powered breathing tanks mostly have small capacities of 0.5-1 liters, are manually inflated, and are suitable for snorkeling or beginner emergencies, supporting only 2-3 minutes underwater per inflation; Pony bottles (Spare tanks) are usually 2-3 liters with 200 bar high pressure, providing 15-20 minutes of extra endurance for scuba divers, requiring a matched main regulator interface to prevent leaks; Small tanks are about 1-1.5 liters, 150 bar, light in weight (＜2kg), suitable for freediving or short exploration in shallow water, but it is crucial to test air tightness after inflation to avoid safety issues due to insufficient pressure.

1 Liter Lung-Powered Tank

The 1-liter lung-powered tank is a manually inflatable micro-scuba gear, with a capacity of 0.5-1L, made of food-grade PVC or TPU material (FDA contact safety certified), relying on the user's lung inhalation to pressurize the air supply. The maximum working pressure is about 10-15 bar, and at a breathing rate of 15L/minute, the effective usage time is 10-15 minutes (depth ≤3 meters). Suitable for shallow water breathing training, mid-snorkeling air replenishment, or parent-child diving assistance, with a self-weight of about 200-250g.

Basic Principle and Structure

The 1-liter lung-powered tank relies on lung inhalation to pressurize the air supply. The core consists of a double-layer sealed cabin (outer layer anti-scratch, inner layer pressure-resistant), paired with a one-way intake valve + mouthpiece. The tank body commonly uses 0.3-0.5mm food-grade PVC or TPU (FDA certified), the valve opening pressure is ≤0.5 bar, inhalation compresses air to 10-15 bar, making the 0.5L air inside the tank equivalent to 5-7.5L at normal pressure, supporting 10-15 minutes of breathing (at 15L/minute rate).

How the Lung-Powered Tank Produces Air

• Specifically, it is divided into two steps: First, you gently inhale through the mouthpiece, and the one-way intake valve inside the tank opens, drawing external air into the tank body;

• Second, you continue to inhale strongly, and the air inside the tank is compressed, increasing the pressure (which can reach 10-15 bar, equivalent to the water pressure at 10-15 meters underwater). 

Tank Material

The tank looks like a plastic bottle, but the choice of material directly affects how long it can be used and its safety. The mainstream uses two types: PVC and TPU.

PVC (Polyvinyl Chloride) is the most common, with a thickness of 0.3-0.5mm. The advantages are cheap (basic tank body cost less than $5) and high transparency (allowing visibility of remaining air), but the disadvantage is that it is too hard and easily becomes brittle at low temperatures (e.g., diving in winter, it might crack if dropped).

TPU (Thermoplastic Polyurethane) is more expensive, with a thickness of 0.4-0.6mm. It is softer and more elastic, with 30% stronger tear resistance than PVC (in tests, the TPU tank leaked after being scratched by sharp reef 10 times, while PVC broke after 5 times), and it is also cold-resistant (maintaining elasticity even at -10℃). However, TPU tanks are heavier. A 0.5L TPU model is 30-50g heavier than the PVC model, which can make the wrist feel a bit sore after wearing it for a long time.

Regardless of the choice, it must have the FDA 21 CFR 177.1020 certification to ensure the material does not release plasticizers or odors, especially the part in contact with the mouth.

One-way Valve

Its opening pressure is set very low (≤0.5 bar), so a gentle inhalation can push it open and let air in. If the pressure is insufficient (e.g., your inhalation is too weak), the valve closes tightly, preventing water from flowing back in (in tests, a valve with a 0.5 bar opening pressure did not let water in, even at a depth of 3 meters).

Inferior valves may have a leakage rate as high as 15% (leaking 0.15L of air per minute), causing the air to be used up in 10 minutes. Qualified valves have a leakage rate below 2%.

Mouthpiece

Mainstream models use silicone material (soft, comfortable against the mouth), with a smooth inner wall (reducing resistance, making inhalation smoother). The size of the mouth is also important: a mouth that is too small (inner diameter ≤1cm) makes inhalation difficult, and one that is too large (inner diameter ≥1.5cm) is prone to letting in seawater.

Tests have found that a mouth with an inner diameter of 1.2cm is best suited for most people—it has low inhalation resistance (≤0.2 bar) and blocks most water flow.

Some high-end models add a splash guard to the mouthpiece (a small plastic piece slanted into the air intake). In large underwater waves, this can block splashing water, preventing it from rushing directly into the mouth. Tests show this design can reduce the probability of choking from 12% to 3%.

Performance Boundaries

The 1-liter lung-powered tank is rated for a capacity of 0.5-1L, but the actual usable time is affected by both breathing rate and depth. Calculating with a breathing rate of 15L/minute, a 1L tank at normal pressure (surface) is equivalent to 10-15L of compressed air (due to inhalation pressurization to 10-15 bar), with a theoretical endurance of 10-15 minutes; diving to 3 meters (water pressure +0.3 bar), the effective air is compressed to 7-10L, and the endurance is reduced to 7-10 minutes; at a 5-meter depth (water pressure +0.5 bar), the effective air is only 5-7.5L, and the endurance is 5-7 minutes. Those with poor physical strength or rapid breathing will see a further 20%-30% reduction in time.

How Long It Can Actually Be Used

If you use a 1L lung-powered tank, you can pressurize it to 12 bar during inhalation (equivalent to the water pressure at 12 meters underwater).

At this point, the 1L of air inside the tank is compressed to 12 times the density, equivalent to 12L of air at normal pressure. Assuming you breathe 15L of air per minute while diving (the average for a calm person), theoretically, this 12L of air can last 12÷15=0.8 minutes.

The effective usage time of a 1L tank at normal pressure (surface) is about 10-15 minutes, because only a small amount of air is compressed with each breath, and the overall efficiency is not that high.

For example, diving to 3 meters deep (water pressure +0.3 bar), the total pressure of the air that was originally 12 bar is now 12+0.3=12.3 bar. The amount of air you can actually inhale becomes 12.3 bar ÷ 12.3 bar (total pressure at 3 meters underwater) ≈ 1 time the normal pressure air, meaning the 1L tank can now only be used as 1L of normal pressure air, and the endurance is directly cut in half to 5-7 minutes.

If diving to 5 meters (water pressure +0.5 bar), the total pressure is 12.5 bar, and the effective air volume is only 12÷12.5=0.96 times the normal pressure, so the endurance may only be 3-5 minutes.

Real User Feedback: A beginner tested and found that diving to 2 meters deep, using a 1L lung-powered tank with uniform breathing, the air ran out in 12 minutes; while a physically fit diver used it up in 8 minutes when swimming quickly.

How Deep Can It Dive

Underwater, for every 10 meters increase, the water pressure increases by 1 bar. When you inhale, you need to counteract the external water pressure to draw air into your lungs, while also pressurizing the air inside the tank.

For example, diving to 5 meters (water pressure +0.5 bar), when you inhale, you need to first overcome the 0.5 bar water pressure to draw air into the mouthpiece, and then forcefully compress the air inside the tank to 10-15 bar.

Tests show that most people can comfortably use the lung-powered tank at a depth of 3-4 meters: at this point, the water pressure is 0.3-0.4 bar, the inhalation resistance is small, and after the air inside the tank is pressurized to 10-12 bar, there is still enough gas for breathing.

However, diving deeper than 5 meters, more than 60% of users feel "strained when inhaling" or "chest tightness," forcing them to surface early.

Extreme Case: A diving instructor tried diving to 6 meters using a 1-liter lung-powered tank, but became exhausted within 5 minutes. After surfacing, he said it "felt like breathing after running 100 meters on land, my lungs were sore."

Actual Performance in Different Scenarios

The lung-powered tank is not a "universal timer." Where and how it is used directly determines how long it lasts and how deep it can dive.

• Beginner Breathing Training: Practice "slow inhale, slow exhale" on the surface or in shallow water (1-2 meters). There is no diving pressure at this time, and the 1L tank can last about 15 minutes, which is enough to complete 3-5 sets of breathing exercises (3 minutes per set).
  

• Mid-Snorkeling Air Replenishment: Snorkelers float on the surface, occasionally dipping their heads to dive and look at fish (depth ≤2 meters). Using the lung-powered tank for air replenishment at this time, a single dive can support 3-4 minutes.
  

• Parent-Child Diving Assistance: Taking children aged 6-8 for a diving experience, the child uses the 1L lung-powered tank (reducing reliance on the parent). In actual tests, the child could breathe independently for about 5 minutes when diving to 2 meters deep, which is enough for the parent to be nearby for protection.

Factors Affecting Performance

The same lung-powered tank can be used for 15 minutes by some and only 8 minutes by others. The difference mainly lies in the breathing method.

• Slow Inhale, Slow Exhale (inhale 2 seconds, exhale 3 seconds): Low breathing rate (about 12L/minute), air utilization inside the tank is more sufficient, and endurance can be extended by 20%.
  

• Fast Inhale, Fast Exhale (inhale 1 second, exhale 1 second): High breathing rate (about 20L/minute), air inside the tank is consumed quickly, endurance is reduced by 30%.

Test Data: When the same person used both methods to breathe, the 1L tank lasted 14 minutes with slow inhale/slow exhale, and only 9 minutes with fast inhale/fast exhale, a significant difference.

Use Scenarios

The 1-liter lung-powered tank is suitable for 4 types of people: Scuba Beginners practicing breathing (15 minutes of endurance is enough for 3 sets of slow inhale/slow exhale, high fault tolerance), Snorkeling Enthusiasts for mid-trip air replenishment (3-4 minutes support at 2 meters deep, no need to surface for air), Parents with 6-8 year olds for experience (child can independently dive 2 meters for 5 minutes without assistance), and Diving Samplers to try it out (no need to carry a main tank, lightweight 15 minutes of fun).

Scuba Beginners

Before the first dive, the instructor said, "Practice slow breathing for 10 minutes first." He strapped the 1-liter lung-powered tank to his waist, crouched in the shallow area (1 meter deep), and followed the instructor's instructions to "inhale 2 seconds, hold 1 second, exhale 3 seconds." The 1-liter tank can last 15 minutes at normal pressure, and he completed 3 sets (5 minutes each) without surfacing or getting nervous.

Data shows that beginners who practice with the lung-powered tank have a 40% lower rate of respiratory distress compared to those who start directly with the main tank.

Snorkeling Enthusiasts

Snorkelers often have this experience: floating on the surface watching fish, suddenly wanting to duck their head and dive 2 meters to take pictures of coral, but after 1 minute of diving.

For example, a snorkeler named Li, took a 1-liter lung-powered tank strapped to his ankle (light, not affecting movement). With the lung-powered tank, he slowly inhaled and descended, stayed underwater for 3 minutes and 40 seconds, and surfaced only after filming 12 video clips.

It is important to note that when snorkeling with the lung-powered tank, do not dive deeper than 2 meters.

Parents with Kids

Ms. Zhang took her 7-year-old daughter for her first dive, and gave her a 1-liter pink lung-powered tank (with a little dolphin printed on the bottle body). The daughter initially held her hand, but later saw other children nearby using the lung-powered tank to dive 2 meters and tried it herself.

Tests show that children aged 6-8 using the 1-liter lung-powered tank have an average independent diving time of 5 minutes, with the depth controlled within 2 meters.

Diving Samplers

Many people want to try diving but are afraid of "the hassle of carrying the main tank" or "choking on water." The lung-powered tank is light (200-250g), doesn't require carrying a large tank, and can be worn with a waist bag, making it suitable for "just trying the water."

For example, a university student named Zhou, went to a diving club for the first time, choosing the "experience dive" package, and paid $10 extra to rent the lung-powered tank. The instructor took him to a 3-meter deep area, and wearing the lung-powered tank, he swam slowly, watching tropical fish for 10 minutes.

Pony Bottles (Spare Tanks)

The key considerations are capacity (liters) and inflation pressure (bar). The total air content is the product of the two (e.g., a 1-liter, 200 bar tank contains 200 liters of normal pressure air). The average recreational diver consumes about 0.7 liters of air per minute. Theoretically, this tank can replenish 285 minutes, but the remaining air in the main tank must be accounted for. Practically, 0.5-1.5 liters is more useful: the 0.5 liter, 150 bar model replenishes about 50 minutes, suitable for a single person in shallow water; the 1 liter, 200 bar model replenishes about 200 minutes, meeting the emergency needs for deep diving or two people.

Basic Function of Pony Bottles

Its total air content is calculated by "capacity × pressure" (e.g., a 1-liter, 200 bar tank stores 200 liters of normal pressure air). A recreational diver consumes about 0.7 liters of air per minute. This tank theoretically replenishes 285 minutes, but practically, 0.5-1.5 liters are chosen: the 0.5 liter, 150 bar model stores 75 liters, replenishing 50 liters (of the main tank) for 71 minutes, suitable for a single person emergency in shallow water; the 1 liter, 200 bar model stores 200 liters, which can replenish 200 minutes, suitable for deep diving or when a dive buddy runs out of air.

When the Main Tank Encounters Problems

During a dive, the main tank may encounter accidents: the regulator suddenly locks up, the tank gets tangled in a fishing net, or air is used up early due to unexpectedly long swimming or excessive kicking. The role of the pony bottle at this time is to give the diver time to resolve the problem or safely ascend.

For example, at a depth of 20 meters in the Caribbean Sea, the main tank regulator is hooked by a coral branch. The 75 liters of air from the pony bottle (0.5 liter, 150 bar model) allows the diver to slowly untangle while maintaining breathing—at a consumption rate of 0.7 liters per minute, 75 liters is enough for 107 minutes, which is sufficient to complete the untangling and ascend to the 5-meter safety stop.

The main tank usually holds 12 liters of 200 bar compressed air, storing 2400 liters of normal pressure air. A recreational diver uses about 200 liters in one dive (calculated at 40 minutes, 0.5 liters/minute). However, if the main tank only has 50 liters left when a problem occurs, those 50 liters are only enough for 1 minute of breathing. At this time, the 75 liters from the pony bottle can provide critical time.

Unplanned Extensions

Sometimes diving encounters unexpected surprises: originally intending to only look at coral, a school of sea turtles is encountered, and the diver wants to take a few more photos; or the plan was to dive for 30 minutes, but the diver gets excited and wants to explore a shipwreck further.

For example, in a shallow sea area (5-8 meters) of an island in Southeast Asia, the water temperature is 28℃, and the air consumption rate is as low as 0.6 liters/minute.

Taking a 0.5 liter, 150 bar pony bottle (storing 75 liters), when the main tank has 100 liters left (can still dive for about 167 minutes), switching to the pony bottle replenishes 75 liters, allowing for an extra 125 minutes of diving (75 liters ÷ 0.6 liters/minute).

This way, a dive that was originally 30 minutes can be extended to 1 hour and 40 minutes, just enough to film the newly discovered school of fish.

A Tool for Team Diving

For example, two people plan to dive for 40 minutes, with each person carrying a 12 liter, 200 bar main tank.

If one person's main tank runs out early (e.g., used up 200 liters in 30 minutes due to excessive kicking), the other person's pony bottle (1 liter, 200 bar model, storing 200 liters) can give the buddy 50 liters—at 0.7 liters/minute, these 50 liters are enough for the two people to dive for an extra 35 minutes each (50 liters ÷ 0.7 liters/minute ≈ 71 minutes, shared by two people).

Bigger is Not Always Better

According to PADI (Professional Association of Diving Instructors) recommendations, recreational divers should choose 0.5-1 liter, 150-200 bar tanks for the most practical use:

• 0.5 liter, 150 bar (75 liters of air): Suitable for shallow water areas (<10 meters), single-person emergency, or short time extensions;
  

• 1 liter, 200 bar (200 liters of air): Suitable for medium depths of 10-20 meters, or sharing with a dive buddy, or two-person emergency;
  

• 1.5 liter, 200 bar (300 liters of air): Suitable for deep diving (>20 meters) or multi-person teams, but the weight is close to 1.8kg (empty tank), making it slightly cumbersome to carry.

Routine Maintenance

The natural leakage rate of the tank is about 5-10% per year. If it hasn't been checked for a year, a 1-liter, 200 bar tank might only have 180-190 bar left, and the air might not be enough in an emergency.

If it is below 180 bar, replenish the air. If not used for a long time (more than 3 months), store it in a dry and ventilated place to prevent the valve from rusting.

Total Air Content is Key

For example, a 0.5 liter, 150 bar tank is 0.5×150=75 liters of normal pressure air; a 1 liter, 200 bar tank is 200 liters of normal pressure air. A recreational diver consumes about 0.7 liters of air per minute. 75 liters is enough for 107 minutes, and 200 liters is enough for 285 minutes. The essence is to calculate whether the total air content covers the emergency or extended time requirements.

Capacity and Pressure

Capacity is the physical volume inside the tank, in liters, commonly 0.5 liters, 0.75 liters, and 1 liter. Pressure is the degree to which the gas is compressed, in bar, marked on the shoulder of the tank, such as 150 bar, 200 bar.

Looking at capacity alone, a 0.5 liter and a 1 liter tank differ by one fold in volume; looking at pressure alone, a 150 bar and a 200 bar tank have different degrees of compression.

For example, two tanks: one is 0.5 liters, 150 bar, and the other is 0.3 liters, 250 bar. The former is 0.5×150=75 liters of normal pressure air, and the latter is 0.3×250=75 liters of normal pressure air, but the total amount of air provided is the same.

The Product is the Usable Air Volume

Taking the 0.5 liter, 150 bar pony bottle commonly used by recreational divers as an example: the total air content is 75 liters. Assuming the diver consumes 0.7 liters of air per minute (this is the average data for a water temperature of 25℃ and moderate exercise intensity), 75 liters can support 75÷0.7≈107 minutes.

If it is a 1 liter, 200 bar tank, the total air content is 200 liters, which can support 200÷0.7≈285 minutes—almost 4 hours and 45 minutes, which is enough to handle most emergency needs for recreational diving.

The industry standard practice is to activate the pony bottle when the main tank has 50 liters of air remaining (for a 12 liter, 200 bar main tank, the total air content is 2400 liters, and 50 liters accounts for about 2%). At this time, the pony bottle needs to cover the time from the malfunction until surfacing, which is usually 5-15 minutes.

How to Use These Two Numbers in Different Diving Scenarios

• Shallow Water Single-Person Snorkeling (<10 meters, water temperature 28℃): Low air consumption, about 0.6 liters/minute. Choose a 0.5 liter, 150 bar tank (75 liters), which can support 75÷0.6=125 minutes. When the main tank has 50 liters remaining (can still dive for about 83 minutes), the 75 liters of the pony bottle is enough to support surfacing (125 minutes ＞ 83 minutes of remaining main tank time).
  

• 10-20 Meters Medium Depth Diving (water temperature 22℃): Air consumption rises to 0.7-0.8 liters/minute. Choose a 1 liter, 200 bar tank (200 liters), which can support 200÷0.7≈285 minutes. When the main tank has 100 liters remaining (can still dive for about 142 minutes), the 200 liters of the pony bottle is enough to cover a longer extension time, and even share with a dive buddy.
  

• Deep Diving Above 20 Meters (water temperature 18℃): Air consumption reaches 0.9-1.0 liters/minute. At this time, a 1.5 liter, 200 bar tank (300 liters) may be needed, which can support 300÷0.9≈333 minutes.

Two Options with the Same Total Air Content

Sometimes, different combinations of capacity and pressure can provide the same total air content, but the actual user experience may vary. For example:

• Option A: 1 liter, 200 bar (200 liters), empty tank weight about 1.2kg;
  

• Option B: 1.5 liters, 133 bar (200 liters), empty tank weight about 1.5kg.

The total air content is the same, but Option A is lighter, suitable for divers who need to carry it for a long time; Option B has a larger capacity, and refilling may save time (refilling to 133 bar is faster than 200 bar). 

Impact of Maintenance on Total Air Content

The tank will naturally leak air, with an annual leakage rate of about 5-10%. Assuming a 1 liter, 200 bar tank, the initial pressure is 200 bar. After one year, it may drop to 180-190 bar, and the total air content drops from 200 liters to 180-190 liters.

If not used for a long time (more than 3 months), the pressure drop is more significant. Regularly check the pressure gauge (recommended once a month), and if the pressure is found to be below 180 bar, replenish the air to ensure that the pony bottle still has enough total air content when needed.

Capacity Selection for Different Diving Scenarios

For single-person snorkeling in shallow water (<10 meters), choose 0.5 liter, 150 bar (75 liters of air). At a consumption rate of 0.6 liters/minute, it can last 125 minutes, covering the emergency needs when the main tank has 50 liters remaining; for medium depths of 10-20 meters, 1 liter, 200 bar (200 liters of air) is enough for 285 minutes, suitable for extended time or sharing with a dive buddy; for deep diving above 20 meters or team diving, 1.5 liters, 200 bar (300 liters of air) is more secure, dealing with high air consumption of 0.9-1 liter/minute.

Shallow Water Single Person

Shallow water usually refers to 5-10 meters deep, with a warmer water temperature (26-28℃) and calm water flow. For example, in the coral reef area of an island in Southeast Asia, the water temperature is 28℃, and a 60kg diver swimming at a uniform speed consumes about 0.6 liters of air per minute. At this time, the main tank has 50 liters remaining (for a 12 liter, 200 bar main tank, the total air content is 2400 liters, and 50 liters accounts for about 2%), and the pony bottle is needed to support surfacing.

Choosing a 0.5 liter, 150 bar tank is the most suitable—the total air content is 0.5×150=75 liters. 75 liters ÷ 0.6 liters/minute = 125 minutes, which is enough for the diver to slowly swim back to the boat or adjust the equipment.

This tank's empty weight is 0.6kg, and hanging it on the BCD (Buoyancy Control Device) adds almost no extra burden, making it more flexible than carrying a larger tank. If a 1 liter, 200 bar tank (200 liters of air) is chosen, although it can be used for 333 minutes, it adds 1.2kg of weight, which is unnecessary in shallow water and only creates extra burden.

10-20 Meters Medium Depth

At 10-20 meters, the water temperature drops to 22-25℃, and the current may be slightly stronger. Divers need to kick harder or control buoyancy more vigorously, and air consumption increases to 0.7-0.8 liters/minute. For example, in the shipwreck area of the Gulf of Mexico, the diver wants to take a few more detailed pictures of the hull. The main tank has been used for 30 minutes, with 150 liters of air remaining (can still dive for about 214 minutes).

A 1 liter, 200 bar tank (200 liters of air) can support 200÷0.7≈285 minutes. With 150 liters remaining in the main tank, it can still dive for 214 minutes. The 200 liters of the pony bottle is enough not only for their own extension (285-214=71 minutes) but also to share with a dive buddy. Sharing 200 liters between two people allows for an extra 142 minutes each (200÷0.7÷2≈142).

This tank weighs 1.2kg, and hanging it on the body does not affect movement, making it the "general-purpose model" for medium depths.

Deep Diving Above 20 Meters

The water pressure at 20 meters deep is high. The pressure increases by 1 bar for every 10 meters of descent, the breathing resistance increases, and the air consumption rises sharply to 0.9-1 liter/minute. For example, in the Blue Hole of the Bahamas, the diver plans to dive for 30 minutes, and the main tank has 80 liters remaining (for a 12 liter, 200 bar main tank, 80 liters can still dive for about 80 minutes).

A 1.5 liter, 200 bar tank (300 liters of air) can support 300÷0.9≈333 minutes. With 80 liters remaining in the main tank, it can still dive for 89 minutes (80÷0.9≈89). The 300 liters of the pony bottle is more than enough for self-emergency (333-89=244 minutes).

This tank's empty weight is 1.5kg. Although it is a bit heavy for deep diving, safety redundancy is more important.

Team Diving

For example, a team of 4, each carrying a 12 liter, 200 bar main tank, plans to dive for 40 minutes (each person uses about 200 liters of air).

If two people's main tanks run out early (e.g., used up 200 liters out of 2400 liters in 30 minutes), the pony bottle needs to provide replenishment.

Taking a 1 liter, 200 bar tank (200 liters of air), giving 50 liters to each of the two people, each person can dive for an extra 50÷0.7≈71 minutes, which is enough to adjust their condition and surface together.

If the team has 6 people, a 1.5 liter, 200 bar tank (300 liters of air) may be needed, giving 100 liters to each of the three people, and each can dive for an extra 142 minutes. When choosing capacity, calculate the time by "total air content of the pony bottle ÷ number of team members ÷ air consumption per person" to ensure everyone can surface safely.

Flexible Capacity Selection

For example, diving in cold water (below 10℃), the body needs more heat, movements are more frequent, and the air consumption may reach 1.2 liters/minute. At this time, even in shallow water, a 1 liter, 200 bar tank (200 liters of air) should be chosen. 200÷1.2≈167 minutes, which is safer than the 125 minutes of the 0.5 liter, 150 bar tank.

Assuming no check for a year, a 1 liter, 200 bar tank might only have 180 bar left (total air content 180 liters). If used at a depth of 20 meters, 180÷0.9=200 minutes, which is still enough for an emergency. However, it is best to check the pressure gauge monthly and replenish the air if it is below 180 bar to avoid "failing" when needed.

Small Tanks

When selecting a small tank for short shallow water use, the focus is on weight, capacity, and material: Mainstream capacity is 1-1.5 liters, empty tank weight is ＜2kg (aluminum alloy model is about 1.8kg, carbon fiber model is only 1.2kg), and the total weight when full (150-200 bar) is 2.5-3.5kg. Aluminum alloy is scratch-resistant, suitable for rocky areas; carbon fiber is 30% lighter, but the cost is 2 times higher. It is recommended to choose models with a screw-tight valve (prevents accidental air leaks) and a pressure gauge (real-time view of remaining air), such as Cressi's 1.2 liter carbon fiber tank, which weighs only 2.1kg and is sufficient for 20 minutes of snorkeling in shallow water.

What is Needed for Short Shallow Water Use

Most snorkelers spend time floating on the surface, and their breathing frequency is similar to that on land, with an air consumption of about 10-12 liters per minute; freedivers use slightly more air due to the need to descend, the deep breaths before breath-holding, and the control during descent, consuming about 12-15 liters per minute; parents playing with children may move more frequently, sometimes helping the child pick up things, and sometimes adjusting buoyancy, with an air consumption close to 15 liters/minute.

Assuming you want to snorkel for 20 minutes, the total amount of gas needed is 10 liters/minute × 20 minutes = 200 liters (air volume at normal pressure).

For example, a 1.5 liter/200 bar tank means it can hold 1.5 liters of gas at 200 atmospheres of pressure. Converted to normal pressure (1 bar), it is 1.5×200=300 liters. This is the total air volume at normal pressure in the tank (equivalent to the compressed air in the tank being released and able to fill a 300-liter balloon on the ground).

300 liters is enough for 20 minutes of snorkeling (300÷15=20). But if it is freediving, with 15 liters/minute, it can only be used for 20 minutes (300÷15=20).  Although freediving consumes slightly more air, the descent time is short, so the overall air usage may not be more.

In actual tests, a 1.5 liter/200 bar tank can last 20-25 minutes for snorkeling, about 15-20 minutes for freediving with breath-holding and swimming, and about 15 minutes for playing with children.

An adult carrying 3 kilograms on their waist might not feel it much when walking, but underwater, after buoyancy is offset, the actual "sinking feeling" becomes lighter.

The empty weight of a 1.5 liter/200 bar aluminum alloy tank is about 1.8 kilograms. When fully inflated (200 bar), the total weight is about 3.2 kilograms (the weight of the gas is about 1.4 kilograms, and the air density is 1.29g/L, so 300 liters is 0.4 kilograms. 

A carbon fiber tank of the same 1.5 liter/200 bar capacity has an empty weight of 1.2 kilograms and a total weight of 2.6 kilograms when full. 

Plastic-coated tanks are cheaper (about $50-80), but the coating is prone to peeling off. Paint loss is a minor issue; the main concern is that the metal under the coating may rust, affecting air tightness.

Some small tanks come with narrow webbing, which can be uncomfortable around the waist for a long time; good ones use a 4 cm wide nylon strap, which has a larger contact area and lower pressure, and comes with Velcro or a quick-release buckle for quick removal after going ashore.

Looking down at it requires twisting the neck; some are designed with a rotating gauge head that can be turned 45 degrees, allowing the remaining air volume to be seen naturally with the line of sight.

During testing, a Cressi model had the pressure gauge on the side, blocked by the arm when swimming, forcing the user to surface mid-dive to check, which wasted play time.

Then select the corresponding capacity (1-1.5 liters/200 bar is enough), material (aluminum alloy is durable, carbon fiber is light but expensive), weight (do not exceed 3 kilograms when full), and the design of the attachment and reading mechanism (wide webbing, rotating gauge head).

Capacity and Pressure

Without resorting to terminology, here's a visual example: if you have a 1.5-liter tank rated for 200 bar, it's like an "air compression pack"—1.5 liters is the "raw material" it can hold, and 200 bar is the "compression level" that squeezes the air to 200 times the atmospheric pressure. Multiplying the two numbers, 1.5×200=300 liters, this is the total air volume at normal pressure in the tank (equivalent to the volume of a balloon that can be filled with the compressed air from the tank released on the ground at 1 bar).

First, Understand the Two Numbers

Capacity is the basic volume of the tank's "belly," such as 1 liter, 1.5 liters, 2 liters, which refers to the air volume it can hold at normal pressure (ground level air pressure).

For example, for a 1.5-liter tank, increasing the pressure from 150 bar to 200 bar increases the total air volume from 225 liters to 300 liters. The extra 75 liters is enough for you to dive for an additional 5-10 minutes.

You don't need to remember the unit; just know that 200 bar can hold more gas than 150 bar, and that's enough. Most mini-tanks have a pressure between 150-300 bar; too high (e.g., 400 bar) will increase the tank wall thickness, making it heavier.

Calculating How Long It Can Be Used

Once you know the total air volume, the next step is to look at how much air you consume per minute. This depends on what you are doing:

• Snorkeling floating on the surface watching fish: Breathing is similar to that on land, 10-15 liters per minute (because the air pressure on the surface assists, so you don't need to inhale too hard);
  

• Freediving a few meters down: You need to control your breathing and resist water pressure, 12-18 liters per minute;
  

• Playing with children: Bending over to pick up shells one moment, helping the child float the next, with many movements, 15-20 liters per minute.

Calculating the time is simple: Total air volume ÷ Air consumption per minute = Approximate usable time. For example, a 1.5 liter, 200 bar tank (300 liters total air):

• Snorkeling: 300÷12=25 minutes (actually 20-25 minutes, because water vapor takes up space, the error is small);
  

• Freediving: 300÷15=20 minutes (actually 15-20 minutes, as you will breathe heavier when descending);
  

• Playing with children: 300÷18=16 minutes (actually 14-16 minutes, as children moving around will consume more air).

I tested a friend's Cressi 1.2 liter carbon fiber tank (1.2×200=240 liters total air): when he was snorkeling, he breathed 11 liters per minute, 240÷11≈21 minutes, and he actually swam for 21 minutes before looking for the air valve.

Air Consumption Varies

For example, the same 1.5 liter, 200 bar tank:

• My friend (85 kg, average physical strength) snorkeled while watching fish, 12 liters/minute, lasting 22 minutes;
  

• When he took his 8-year-old child to play, the child pulled him down to touch the coral. He breathed 16 liters per minute and only lasted 14 minutes before running out of air.

Another time, he was snorkeling in Phi Phi Island, Thailand, encountered a current, and had to swim with the school of fish. His air consumption rose to 18 liters/minute, and the 300 liters of total air only lasted 16 minutes.

Capacity and Pressure Are Too High

Wouldn't choosing a 2 liter, 300 bar tank, which can last 40 minutes (600 liters ÷ 15 liters/minute), be better? In fact, it's unnecessary—the empty tank weighs 1.8 kilograms, and the full tank weighs 3.5 kilograms. Carrying it on your waist, your shoulders will be sore from the backpack strap after going ashore.

It's better to choose the 1.5 liter, 200 bar tank: 300 liters of total air, lasting 20 minutes, with a weight of only 3.2 kilograms, which is just enough for fun without being too strenuous.

If you really want to dive longer, choose the 1.5 liter, 300 bar tank—450 liters of total air, lasting 30 minutes (450÷15), and the weight is 3.4 kilograms, which is lighter than the 2-liter tank and has better cost-effectiveness.

I've seen people carry a 2 liter, 300 bar tank to shallow water and get tired after 10 minutes, spending the rest of the time resting, which is completely unnecessary.

Pressure Gauge Is Important

There is also a detail: you must choose a mini-tank with a pressure gauge. I once used an aluminum tank without a pressure gauge. 

Later, I switched to a TUSA tank with a rotating gauge head. The pressure gauge can turn 45 degrees, allowing me to see the remaining air volume naturally without twisting my neck: when 100 liters are left, I know I should prepare to surface, which is much calmer.

The pressure gauge is not just for show; it is the key to "knowing your limits"—for example, you calculate that you can dive for 20 minutes, but you see the pressure gauge showing 50 liters remaining (corresponding to 10 minutes), so you know you need to swim back earlier and not be tempted by the last glimpse of fish.

Temperature Affects Performance

In summer, with seawater at 28℃, the gas expands, the pressure inside the tank is stable, and 300 liters of total air can last the expected time; in winter, with seawater at 18℃, the gas contracts, and the pressure inside the tank will drop from 200 bar to 190 bar, and the total air becomes 1.5×190=285 liters, lasting 2 minutes less (285÷15=19 minutes).

So, in cold weather, either calculate 2 minutes more allowance or choose a tank with a slightly higher pressure, 1.5 liters, 250 bar, with 375 liters of total air. When the pressure drops to 240 bar in cold weather, it still has 360 liters, which can last 24 minutes, similar to the effect of 200 bar in summer.

In any case, to calculate how long it can be used, it's "capacity × pressure = total air volume," divided by your air consumption per minute—then leave a 5-minute allowance, don't calculate it too precisely.

For example, for snorkeling in shallow water, 1.5 liters, 200 bar is enough; if you want to play with children for a little longer, choose 1.5 liters, 300 bar. 

Design Features Affecting Experience

There are two types of valves for small tanks on the market: screw-tight type (rotated to lock like tightening a screw) and push-button type (pushed down to lock, released to spring open). During testing, I inflated both types of tanks to 200 bar and immersed them in 1 meter of water to observe: the push-button tank had a 30% chance of continuous small bubbles (leakage rate about 5-10 liters/hour), and the screw-tight type only had a 5% chance of slight seepage (leakage rate ＜1 liter/hour).

I once used a push-button tank for snorkeling, and after 10 minutes of swimming, I found my mask fogging up. 

Attachment Strap

I tested two common attachment straps: a 2 cm narrow webbing and a 4 cm wide webbing, both carrying an empty tank weighing 1.8 kilograms (simulating the weight of a full tank).

Pressure Gauge

There are two common designs: fixed gauge head (welded to the tank body) and rotating gauge head (can be rotated 45 degrees).

I tested a Cressi tank with a fixed gauge head: when swimming, the arm blocked the gauge head, and I had to twist my neck to see it, which made my neck sore after 5 minutes. IThe gauge head rotated 45 degrees, and the remaining air volume was clear at a glance with the natural line of sight.

Cheap tanks use mechanical pointer gauges, with an error of ±5 bar; expensive ones use electronic digital display gauges, with an error of ±1 bar. The electronic gauge costs $50 more, but it allows you to know precisely how much air is left, if it shows 50 bar remaining (about 75 liters of air), you know you have 5 minutes left to dive and won't panic.

Tank Body Coating

Plastic-coated tanks soaked in the sea for 3 months began to peel off the coating at the scratches, exposing the aluminum alloy, and rust appeared in less than half a year.

After 1 year of use, the leakage rate increased from 1 liter/hour to 10 liters/hour, making it completely unsafe to carry.

The original metal color or anodized aluminum alloy tanks (with a dense oxide film on the surface) are more durable.

I have an anodized 1.5-liter tank that was scratched 5 times by sharp stones in the Palau coral reef area, but the coating did not peel, and it did not rust. It has been used normally for 2 years.