How to Choose a Small Diving Tank | Capacity & Size Guide

Small cylinders are commonly 3-12L, primarily made of aluminum.

Select 5-7L for recreational diving (e.g., 5L × 200bar = 1000L air; at 20L/min consumption, it lasts 50 minutes).

Select 7-12L for technical diving (e.g., 12L × 232bar = 2784L air).

Working pressure is mostly 200bar for aluminum and 232bar for steel;

prioritize matching the regulator interface.

5L aluminum: ~60cm height, 14cm diameter, 7kg weight;

7L aluminum: 70cm height, 9kg weight;

7L steel: 65cm height, 13cm diameter, 11kg weight (negative buoyancy provides more stability).

When choosing, try it on: if height ＜ 170cm, avoid cylinders ＞ 70cm.

If buoyancy control is weak, choose steel (negative buoyancy).

For transport, prioritize small diameters (＜ 15cm).

Capacity

Currently, the mainstream small cylinders have a water capacity between 0.5L and 2L, with a standard working pressure of 200 bar (3000 psi).

A 1L cylinder at 200 bar contains approximately 200L of breathable air.

Based on an average adult male gas consumption rate of 20L/min at rest, this 1L cylinder can last 10 minutes at the surface;

However, at a depth of 3 meters (10 feet), affected by 1.3 times the atmospheric pressure, the duration will reduce to approximately 7.5 minutes.

Water Capacity vs. Gas Capacity

Essential Differences

Water Capacity (WC)

Definition: The volume of water required to fill the cylinder from a tap up to the brim if the valve were removed.
Units: International standards (ISO) and European standards use Liters; North America (DOT standards) sometimes uses cubic inches.
Data Constancy: This value never changes regardless of how much gas pressure is filled or if it is empty. It is the most reliable data stamped on the cylinder, usually identified by WC or V followed by a number.

Gas Capacity

Definition: The volume occupied by all the compressed air inside the cylinder if it were released entirely to standard atmospheric pressure (1 atm).
Units: Internationally known as Free Liters; North America commonly uses cubic feet (cu ft).
Calculation Logic: At a standard diving industry pressure of 200 bar, the gas is compressed 200 times.
- Formula: 1 Liter Water Capacity × 200 bar pressure ≈ 200 Liters Gas Capacity.

Comparison of Common Small Cylinder Water Capacity and Gas Capacity (Based on 207 bar / 3000 psi)

Model Code (NA)	Water Capacity (Physical Size)	Gas Capacity (Total Air)	Gas Capacity (Cubic Feet)	Cylinder Height (Shell Only)	Full Weight (Approx.)
S6	1.0 L	207 L	6 cu ft	280 mm	1.6 kg
S13	1.9 L	393 L	13 cu ft	340 mm	2.5 kg
S19	2.7 L	558 L	19 cu ft	420 mm	3.4 kg

Imperial vs. Metric

North American System: Named by "Cubic Feet"

North American manufacturers typically name cylinder models based on gas capacity.

For example, model "S13":
- The "13" represents a gas capacity of 13 cubic feet (13 cu ft).
- It does not mean the cylinder is 13 liters in size, nor does it mean it weighs 13 pounds.
- The flaw in this naming convention is that it assumes a fixed working pressure (usually 3000 psi). If it is under-filled or is a low-pressure version, the "13" is no longer accurate.

Euro-Asian/ISO System: Labeled by "Water Capacity"

Europe and most of Asia prefer to label water capacity.

For example, specification "2L, 200 bar":
- This explicitly tells you the physical size of the bottle is 2 liters.
- You need to calculate: 2 × 200 = 400 liters of air.
- This method is more rigorous as it separates physical attributes from the filling state.

Conversion Practice:

If you see a Spare Air source labeled 3 cu ft (3 cubic feet) and want to know its actual size:

1 cubic foot ≈ 28.3 liters.
3 cu ft ≈ 85 liters (total gas).
Assume the working pressure is the standard 200 bar.
Water Capacity = 85 ÷ 200 = 0.425 Liters.
Conclusion: This is a cylinder smaller than a 500ml mineral water bottle.

Interpreting Stamp Markings

Data on advertising brochures might be embellished (e.g., rounded);

the metal stamp markings on the cylinder neck are the legally valid source of data.

You need to learn to identify the following codes:

DOT-3AL 3000 S8675309 07/25

TC-3ALM 207 luxfer 11.1 S13

Service Pressure:
- Look for the numbers immediately following the material code (3AL, 3AA).
- 3000: Represents 3000 psi (approx. 207 bar). This is the standard pressure for aluminum cylinders.
- 2216: Low-pressure steel cylinder, common in older gear. Although the size is large, the pressure is low, meaning less actual gas.
- 4500: Represents 4500 psi (approx. 310 bar). Common in carbon fiber composite cylinders. Note that for the same 1L water capacity, a 4500 psi cylinder holds 1.5 times the gas capacity of a 3000 psi cylinder.
Water Capacity Markings:
- Some cylinders are stamped V1.9 or WC1.9, representing Water Capacity 1.9 Liters.
- Some are stamped with P or V followed by a cubic inch value, which must be divided by 61.02 to convert to liters.
Test Pressure (TP) Trap:
- Some stamps show TP 4500 or PH 310 BAR. This represents the limit for hydrostatic testing and is absolutely not the daily filling pressure. If you accidentally use TP as the working pressure for capacity calculations, you will overestimate gas by 50% and trigger a burst disk failure during filling.

Water Capacity (WC) is the physical internal space of the cylinder, usually in Liters (L) or cubic inches (ci), and is a fixed value.

Gas Capacity is the total amount of compressed air actually stored at the rated working pressure, in cubic feet (cu ft) or Liters (L).

The relationship is governed by working pressure, using the formula:

Gas Capacity ≈ Water Capacity × (Working Pressure bar / 1 bar).

Using the common S13 small cylinder as an example: its water capacity is ~1.9L, rated pressure 207 bar (3000 psi), resulting in a gas capacity of ~393L (or 13 cu ft).

Ignoring this conversion can lead to a 200-fold error in underwater breathing time estimates.

Actual Duration

Calculating Your Real Bottom Line

Usable Gas Volume:
You must never calculate using all the air in the cylinder. You must subtract a 50 bar reserve pressure.
- Formula: (Fill Pressure - 50 bar) × Water Capacity = Usable Gas.
- Example: 1L cylinder filled to 200 bar -> (200 - 50) × 1 = 150L Usable.
Ambient Pressure (ATA):
Pressure increases by 1 atmosphere for every 10 meters of depth.
- Surface (0m) = 1 ATA
- 3m (10ft) = 1.3 ATA
- 10m (33ft) = 2 ATA
- 20m (66ft) = 3 ATA
Gas Consumption Rate (SAC / RMV):
This is the largest variable.
- Rest/Relaxed (15 L/min): Highly experienced diver hovering with no current.
- Normal/Light Activity (20 L/min): Includes slow swimming or light hull inspection. (Baseline data for this table)
- High Load/Work (30 L/min): Swimming against current, scraping hull barnacles, or stress.
- Panic/Emergency Ascent (40-60 L/min): Adrenaline-fueled state during an accident.

Duration Comparison Table

The table below shows the real usable time for the three most common small cylinders after subtracting a 50 bar safety margin.

Theoretical Maximum Bottom Time at Different Depths (Minutes)

Depth (m/ft)	Pressure (ATA)	0.5L Cylinder (3 cu ft) Usable: 75L	1.0L Cylinder (6 cu ft) Usable: 150L	2.0L Cylinder (13 cu ft) Usable: 350L
Surface (0m)	1.0	3.7 min	7.5 min	17.5 min
3m (10ft)	1.3	2.9 min	5.8 min	13.5 min
5m (16ft)	1.5	2.5 min	5.0 min	11.6 min
10m (33ft)	2.0	1.9 min	3.7 min	8.7 min
20m (66ft)	3.0	1.2 min	2.5 min	5.8 min
30m (99ft)	4.0	0.9 min (Unusable)	1.9 min	4.3 min

Data Interpretation:

Limitations of 0.5L: At 10m depth, it provides less than 2 minutes of air. After reaction time and gear adjustment, it is only sufficient for ascent and cannot handle any troubleshooting.
The 1.0L Misconception: Many buy 1L cylinders to "take a look at the seafloor." But at 5-10m, you only have 3-5 minutes. This is not "diving"; it is merely "a slight extension of breath-holding."
The 2.0L Threshold: Only at the 2L spec do you get a working window of nearly 10 minutes at 10m (e.g., clearing a tangled anchor chain), which is the minimum size with practical engineering value.

Dynamic Conditions

Scenario A: Heavy Hull Cleaning (3m depth, 1.3 ATA)

Assume you are vigorously scraping hard growth off a propeller; breathing is heavy, SAC rises to 30 L/min.

1L Cylinder Calculation:
- Consumption per min = 30 L/min × 1.3 ATA = 39 L/min.
- Usable gas = 150L.
- Duration = 150 / 39 ≈ 3.8 minutes.
Result: You thought you could work for 10 minutes, but in reality, the cylinder will begin to breathe "hard" (insufficient supply) in less than 4 minutes.

Scenario B: Emergency Bailout Mode (20m depth, 3 ATA)

Assume primary gas failure, you switch to the bailout bottle;

heart racing, near panic, SAC spikes to 50 L/min.

0.5L Cylinder Calculation:
- Consumption per min = 50 L/min × 3 ATA = 150 L/min.
- Usable gas = 75L.
- Duration = 75 / 150 = 0.5 minutes (30 seconds).
Result: At 20m, a 0.5L cylinder only supports 30 seconds of breathing. Considering the time needed to control buoyancy and ascend, this capacity has almost no margin for error in deep water.

Ascent Profile Verification

The most common use for small cylinders is as a Bailout Bottle.

We need to verify if the capacity supports standard PADI/SSI safety ascent procedures.

Standard Ascent Requirements:

Max Ascent Rate: Not exceeding 18m/min (9m/min recommended).
Safety Stop: 3 minutes at 5m depth.

Calculation Case: Emergency Ascent from 20m

Phase 1: Ascent (20m -> 5m)
- Distance 15m, at 9m/min, takes 1.6 minutes.
- Average depth pressure 2.25 ATA (at 12.5m).
- Gas consumed = 1.6 min × 20 L/min × 2.25 ATA ≈ 72L.
Phase 2: Safety Stop (3 min at 5m)
- Ambient pressure 1.5 ATA.
- Gas consumed = 3 min × 20 L/min × 1.5 ATA = 90L.
Phase 3: Final Ascent (5m -> Surface)
- Duration 0.5 min.
- Gas consumed ≈ 10L.

Total Gas Required = 72 + 90 + 10 = 172L

Conclusion:

0.5L Cylinder (Usable 75L): Fail. Cannot complete a safety stop; must bolt to the surface by dropping weights, which is extremely risky.
1.0L Cylinder (Usable 150L): Marginal/Fail. Even using all gas (including the 50 bar reserve), you only have 200L. Under stress, a standard safety stop cannot be completed.
2.0L Cylinder (Usable 350L): Pass. Sufficient gas for a slow ascent and 3-minute safety stop, with surplus for handling minor issues like a flooded mask.

Charles's Law Cooling Effect

If you fill a tank on a hot deck (35°C) and jump into cold water (15°C).

The temp difference causes pressure to drop by ~7-10%.

Your 1L cylinder loses 15-20L of air before you even take a breath.

For large tanks, this is negligible; for small cylinders, it’s a lost minute of life-saving time.

Small cylinders are usually connected to regulators.

If the O-ring isn't checked before entry, a tiny bubble leak could lose 1-2L of gas per minute.

During a 10-minute dive, that’s another 10-20L loss.

Applicable Scenarios

0.5L (3 cu ft)

Strictly One-way Escape (The "Get Out" Unit)

Technical Limit:
- Total Gas: Approx. 85L.
- Breaths: ~57 at surface; ~30 at 10m; ~15-18 at 20m.
- Deployment: Usually features a Regulator Integrated design—no assembly, just pull and breathe.
Best For:
- Psychological Placebo for Recreational Divers: For OW divers within 18m, it provides just enough gas to reach the surface if the primary fails.
- Anti-drowning for Surfing/Kayaking: Provides 1-2 minutes of "calm down time" to untangle oneself when trapped under waves or a skirt.
- Helicopter/Racing Driver Escape: For cabin egress after ditching in water; DCS isn't the concern here, survival is.
Absolute No-Go Zones:
- Forbidden for "Taking a Look": Don't try to dive down to pick up dropped sunglasses. The gas you consume descending leaves you with none for the return.
- Forbidden Below 20m: At 30m, the volume is insufficient for controlled ascent; you’d be forced to rocket up like a balloon, risking lung overexpansion injury.

1.0L to 1.5L (6 - 9 cu ft)

Boat Maintenance and Ultra-Shallow Work

Technical Limit:
- Total Gas: 200L - 300L.
- Effective Window: ~6-9 minutes at 3m depth.
- Mounting: Must use a chest or back harness. Hand-holding a mouthpiece causes jaw fatigue and prevents two-handed work.
Best For:
- Boat Owner's Kit:
  - Clearing Tangles: Props fouled by nets or ropes are common. This usually happens 0.5-1.5m underwater. A 1L cylinder allows you to cut lines calmly without repeated breath-holding dives.
  - Checking Anodes and Intakes: Quick checks of corrosion protection or clearing seaweed-blocked cooling intakes.
- Pool and Still Water Maintenance: Cleaning drains in deep areas (2m+), repairing tiles, or checking underwater lights.
- Metal Detecting (Shallow): Finding rings/coins in chest-deep or overhead water at the beach.
Operational Risks:
- Buoyancy Trap: Many users don't wear a weight belt. As gas is consumed, the cylinder lightens (1L air weighs ~1.2g; 200L loss = 0.25kg lighter). If you are already positively buoyant, you might drift up and hit the hull/propeller while focused on work.
- "Just a bit longer" Mentality: This is the most dangerous size. It gives enough gas to start a job but not necessarily to finish it. Many ignore the pressure gauge to finish a bolt, leading to an Out of Air situation.

2.0L to 3.0L (13 - 19 cu ft)

Professional Redundancy and Light Engineering

In technical diving, this size is known as a Pony Bottle.

Technical Limit:
- Total Gas: 400L - 600L.
- Effective Window: Up to 10-15 minutes at 10m; also supports safe ascent and deco stops from 40m.
- Config: Usually secured to the primary tank via metal clamps or side-slung (Sidemount style). Requires an independent Submersible Pressure Gauge (SPG).
Best For:
- Solo Diving Standard: With no buddy for gas sharing, you must carry a "mechanical buddy." 2-3L is enough to end a dive methodically during total system failure.
- Deep Dive Safety Margin: On dives below 30m, use Pony gas for the 3-minute safety stop to keep the primary reserve for contingencies.
- Light Engineering and Photography:
  - Underwater Photographers: Use small tank gas while waiting for macro critters to save primary gas or as a reserve for delays.
  - Scientific Diving: Transect surveys or deploying small sensors.
Tactical Advantage:
- Gas Sharing: This size can be handed off entirely to an out-of-air diver, letting them ascend independently without the entanglement and panic risks of sharing a single Octopus.

Small cylinders are not universal tools. 0.5L (3 cu ft) is only for emergency bailout within 20m, possessing no work or sightseeing capability;

it acts as an "underwater fire extinguisher."

1.0L - 1.5L (6-9 cu ft) is the workhorse for boat maintenance, suited for 5-8 minutes of shallow work within 3m depth.

2.0L - 3.0L (13-19 cu ft) is the standard redundancy for tech/solo diving, providing 15+ minutes of reserve for deep safety stops or light engineering.

Size

Common small aluminum cylinder diameters fall into three tiers:

3.2 inches (~81mm), 4.4 inches (~111mm), and 5.25 inches (~133mm).

6 cu ft and 13 cu ft cylinders are shorter, ideal for mounting next to the primary tank;

While 19 cu ft cylinders offer better volume, their length often exceeds 17 inches, which may bump the legs of smaller divers.

40 cu ft cylinders are nearly 25 inches long and must be side-slung, otherwise they severely disrupt trim.

Specs and Suitability

Standard Diameters

3.2 Inch (81 mm) Series:
- Typical Capacity: 6 cu ft.
- Grip: Similar to a standard 1L sports bottle. Adults can wrap their hand fully around it. In emergencies, the receiver can easily control the bottle with one hand while operating gear with the other.
- Mounting Limits: Due to the tight radius, standard Cam Bands struggle to grip it, leading to sliding. Specialized Velcro kits or adapters are usually required.
4.4 Inch (111 mm) Series:
- Typical Capacity: 13 cu ft, 19 cu ft.
- Profile: The mainstream choice for recreational redundancy. The cross-section is ~88% larger than the 3.2-inch series, noticeably increasing drag while swimming.
- Compatibility: Fits most standard "Pony Bottle" bracket systems. Thick enough for dual-point mounts to provide anti-torque stability against wobbling.
5.25 Inch (133 mm) Series:
- Typical Capacity: 30 cu ft, 40 cu ft.
- Handling: Cannot be wrapped by one hand; requires a handle or neck leash.
- Profile Impact: Increases lateral profile by ~14cm. When passing through wrecks/caves, the diver must tilt to avoid snagging.

Torso Match

Effective Operating Length Formula:

Cylinder Length + Valve Height (~3-4") + 1st Stage/Hose Bend Radius (~2-3")

Back Mount Length Limits:
- When inverted on the primary tank, the valve sits near the primary valve.
- 13 cu ft (11.2"): Total length ~15". For most adults, the bottom sits mid-back, away from the waist belt.
- 19 cu ft (17.5"): Total length exceeds 21". For divers under 170cm, the bottom extends below the glutes. It may hit boat benches when floating or bump thighs while finning.
Side Sling Geometry:
- For 40 cu ft tanks, side-slinging is the only logical choice.
- Measurement: From armpit (front clip) to hip bone (rear clip/D-ring).
- Conflict: A 40 cu ft aluminum tank is ~24.6". With valve/reg, it's nearly 75cm. On short torsos, the 1st stage hits the armpit, or the bottom drags at the knee, ruining Frog kicks. Adjusting mounts often ruins the trim angle.

Density and Buoyancy

Aluminum Buoyancy Curve:
- Thicker walls mean larger external volume per capacity.
- 6-19 cu ft: Usually slightly negative (full) to neutral/slightly positive (empty).
- 40 cu ft: A "buoyancy chameleon." -1.5 lbs full to +3.3 lbs empty. At the end of a dive, the empty tank will float up, requiring a short leash (Bolt snap leash) to keep it from floating behind like a balloon.
Steel Compactness:
- Huge difference between LP (2400 psi) and HP (3440 psi).
- Faber 23 cu ft (HP Steel): Diameter 3.94", Length 12.8".
- Comparison: Compared to a 19 cu ft aluminum, the 23 cu ft steel holds 20% more gas but is 5" shorter and thinner.
- Suitability: Best "gas-to-volume ratio" for short-torso divers or tight-space ops. However, its constant negative buoyancy (~ -3 to -5 lbs) requires trimming weight off the belt.

Valve Interfaces

DIN vs Yoke (INT):
- Yoke: Bulky, higher height, significant lateral protrusion. A common snag point in kelp or tight spaces.
- DIN: Screws into the valve; lower profile and more streamlined. Reduces protrusion by ~1-2", lessening jaw and neck interference.
Burst Disk Layout:
- On ultra-short 6 cu ft tanks, the valve is compact. Ensure the HP hose doesn't block the burst disk or the handwheel. Large 1st stages can sometimes prevent fully opening the valve on small diameter tanks.

6 cu ft to 13 cu ft aluminum tanks use 3.2" or 4.4" diameters, keeping total length under 14", fitting back-mounts without interfering with waist gear for divers over 165cm.

19 cu ft tanks stay at 4.4" but stretch to 17.5";

low mounts will cause the tank to hit the buttocks while swimming.

40 cu ft tanks widen to 5.25" and weigh over 15 lbs full.

They require side-slinging and an armpit-to-hip distance of at least 45cm to prevent a "seesaw" effect in the water.

Impact of Diameter

Back-Mounting

Torque Calculation:
A full 19 cu ft aluminum tank weighs ~8 lbs (3.6kg).
- If 4.4" diameter, the center of mass is ~15cm from the spine.
- If 5.25", it moves to over 18cm.
- Though only 3cm, this lateral pull is magnified underwater, requiring constant oblique muscle contraction to maintain level Trim.
BCD Bladder Interference:
Wings or Jackets expand when inflated.
- Under 4.4": Diameter is less than the inflated bladder thickness; the tank "nests" into the side with little interference.
- Over 5.25": Acts like a rigid board, preventing full inflation on the Pony side or forcing the bracket to loosen under pressure.

Side-Sling Configuration

Diameters over 5" make side-slinging the only hydrodynamic choice, where diameter dictates the Horizontal Profile.

Armpit Crowding:
- 5.25" (133mm) Tank: Including the DIN protrusion, total width is ~20cm.
- For smaller divers, this fills the armpit, forcing a "Chicken Wing" posture that ruins streamlining.
- Data: Compared to 4.4", a 5.25" tank increases single-side cross-section by ~30%, requiring constant course correction in currents.
Rigging Adaptation:
Requires Jubilee bands or nylon straps for Bolt Snaps.
- Curvature Stability: Standard bands struggle on 3.2" thin tanks, leading to rotation. Rubber strips are needed for friction.
- D-ring Reach: Thicker tanks sit further from the body, making blind clipping harder in low viz or thick gloves.

Pocket Stowage

For micro gas (e.g., 6 cu ft), many prefer soft stowage.

Cargo Pockets:
- Standard pocket openings are ~15-18cm.
- 3.2" tanks slide in easily with room for a spare mask.
- 4.4" tanks might fit but are hard to extract one-handed and act as "speed brakes."
Tank Strap Mounting:
Mounting on the primary's Cam Band.
- Small 3.2" diameters have minimal contact area with the primary tank curve, leading to instability.
- Matching Principle: The closer the Pony diameter is to the primary (~7.25"), the more stable the "tank-to-tank" soft connection becomes.

Gauge Reading

Port Orientation:
- On small diameters (3.2-4.4"), the neck curve is sharp. HP ports might point directly at the tank surface.
- Large 1st stages or short SPG hoses can cause physical interference, making the gauge unreadable or impossible to install.
- Large 5.25" tanks have flatter surfaces, offering more room for hoses.
Valve Knob Tactility:
- Standard knobs look disproportionately large on tiny tanks.
- In emergency "feathering," fingers can hit the tank shell. In 5mm or dry gloves, the 3.2" neck space is cramped. Larger tanks provide a "platform-like" surface for easier fine-motor operation.

3.2" (81mm) micro tanks are too thin for standard cam bands and must use Velcro or pockets, offering near-zero drag;

4.4" (111mm) is the mechanical limit for back-mounting; any wider and the shifting center of mass causes a Roll Moment that leads to muscle fatigue.

5.25" (133mm) and wider must be side-slung, occupying ~13-14cm of armpit space.

For shorter arm spans, this forces a high-drag "wing" posture.

Besides aluminum, steel cylinders are available.

For the same volume, steel is usually smaller.

HP Steel: Operates at 3442 psi or higher, packing more gas into less space.
Size Comparison: A 23 cu ft steel tank can be physically more compact than a 19 cu ft aluminum one.

If gas volume is critical but luggage space or mounting room is tight, steel is superior.

However, it is heavier (more negatively buoyant) in the water, which must be factored into your weighting system.

Would you like me to create a comparison table specifically for the weight and buoyancy of these different cylinder materials?

```