Large vs. Small Scuba Oxygen Tank | Beginner's Buying Guide

A large tank (e.g., 12L/200bar) stores 2400L of air.

Based on a beginner's consumption rate of 15L/min, it has a duration of 160 minutes (PADI standard);

A small tank (3L/200bar) stores 600L of air, with a duration of 40 minutes, making it suitable for short-range shore dives.

Large tanks require professional filling stations (5-10 minutes to reach 200bar), costing $10-20 per fill, with 80% coverage in major cities;

Small tanks can use portable electric pumps (30 minutes to reach 200bar), costing $5-10 per fill, offering more flexibility in remote areas.

A large tank weighs 15kg and has a diameter of 18cm, suitable for long-duration boat diving operations;

A small tank weighs 1.5kg and has a diameter of 12cm, fitting into a backpack (storage space reduced by 70%).

70% of beginners choose small tanks for their first purchase to practice (Scuba Diving Magazine 2024 survey).

Duration

At a standard working pressure of 3000 PSI, an Aluminum 80 (80 cu ft, approx. 11.1L) cylinder stores about 22% more air than an Aluminum 63 (63 cu ft, approx. 9L) cylinder.

For an adult diver with an average Surface Air Consumption (SAC) rate of 0.5 cu ft/min, at a typical recreational depth of 60 feet (approx. 18m), an AL80 provides about 45-50 minutes of dive time, while an AL63 can only sustain 35-40 minutes under the same conditions.

When accounting for the mandatory 500 PSI safety reserve, the actual usable "bottom time" for a small cylinder at 60 feet may be less than 30 minutes.

For every 33 feet (1 ATM) of depth, air consumption doubles, significantly reducing the practicality of small cylinders in deep diving scenarios.

Depth Impact

At sea level standard atmospheric pressure (1 ATA), assuming an adult's lung capacity is 6L and the air exchange per breath is 0.5L, they inhale 0.5L of air at one atmosphere of density.

When descending to 33 feet (approx. 10m), the ambient pressure increases to 2 ATA (1 atmosphere of air plus 1 of water).

Although the gas is compressed and the diver feels the same lung expansion as at the surface, the regulator must deliver air at twice the surface density to fill the same alveolar space.

Each breath taken at this depth actually consumes twice the number of gas molecules compared to the surface volume.

At 33 feet, a cylinder that lasts 60 minutes at the surface theoretically has its duration halved to 30 minutes;

Descending further to 66 feet (approx. 20m, 3 ATA), the consumption rate becomes 3 times that of the surface;

Near the recreational diving limit of 99 feet (approx. 30m, 4 ATA), the consumption rate is 4 times that of the surface.

For small cylinders like the Aluminum 63, their already limited volume (63 cu ft) will visibly deplete as seen by the rapid drop of the Submersible Pressure Gauge (SPG) needle under a 4x consumption rate.

Suppose a diver has a Surface Air Consumption (SAC) rate of 0.75 cu ft/min (typical for an adult male beginner). At the surface, an AL63 tank would last about 84 minutes. However, at a wreck dive depth of 66 feet (3 ATA), his actual consumption rate (RMV) becomes 0.75 × 3 = 2.25 cu ft/min. At this point, the theoretical total duration of the AL63 is reduced to only about 28 minutes.

These 28 minutes are not entirely for exploration.

According to recreational diving safety standards, divers must reserve 500 PSI to 750 PSI of air as a reserve gas for sharing air in emergencies (such as a buddy running out of air) and completing a safe ascent.

In an Aluminum 80 cylinder, 500 PSI represents about 13 cu ft of gas;

In the smaller Aluminum 63 cylinder, due to its smaller internal volume, the same 500 PSI reading actually contains only about 10.5 cu ft of gas.

At a depth of 66 feet, after deducting the mandatory 500 PSI reserve (equivalent to 4-5 minutes of air at that depth) and the approx.

300-400 PSI needed for the ascent and a 3-minute safety stop, a diver using an AL63 might only have 10 to 12 minutes of actual "Bottom Time" upon reaching the floor.

In contrast, the extra 14.4 cu ft of gas in an Aluminum 80 provides an additional 6 to 8 minutes of bottom time under the same depth and reserve strategy.

In a dive profile lasting only a dozen minutes, this represents a 50%+ increase in effective exploration time.

As depth increases, the density of air flowing through the regulator's first and second stages increases, becoming "thicker," which increases frictional resistance through hoses and valves.

For standard rental-grade regulators, delivering air at 99 feet (4 ATA) requires more negative pressure for inhalation than in shallow water.

This physical exertion triggers an instinctive bodily response—faster or deeper breathing—leading to an increase in respiratory minute volume.

For divers using small cylinders, this "depth-resistance-consumption increase" feedback loop is extremely disadvantageous.

Once in this loop, a calculated 20-minute duration can quickly shrink to 15 minutes.

In practice, using an AL63 for dives below 80 feet (approx. 24m) often leads to frequent gauge checking throughout the dive.

This psychological stress itself increases metabolic rate and oxygen consumption, as anxiety leads to shortness of breath.

Data modeling shows that at 100 feet (30m), air density is about 4 times that of the surface (approx. 5g/L). If a diver engages in heavy exertion due to a current, their instantaneous consumption rate (RMV) could soar to 3.0 or even 4.0 cu ft/min. Under such extreme high-load conditions, a full AL63 (63 cu ft), minus the necessary ascent reserve, would sustain breathing for less than 10 minutes.

No-Decompression Limits (NDL) calculated by dive tables or dive computers typically allow a stay of 50-55 minutes at 60 feet (depending on the model and previous dive history).

Experienced divers using an Aluminum 80 usually have enough air to cover most of those 50 minutes, maximizing the NDL.

With an Aluminum 63, the gas often runs out long before the NDL countdown ends, forcing the diver to surface early while the computer still shows plenty of remaining no-decompression time.

To illustrate the difference more clearly, we set the following calculation conditions:

• Surface Air Consumption (SAC): 0.6 cu ft/min (average recreational diver).
• Reserve Gas: 500 PSI (minimum reserve for safety stop and ascent).
• Actual Usable Gas: Gas remaining after deducting the reserve.

Dive Depth (ft/m)	Ambient Pressure (ATA)	AL80 Est. Duration (min)	AL63 Est. Duration (min)	Time Difference (min)
33 ft (10m)	2.0	54 min	42 min	12 min
60 ft (18m)	2.8	38 min	29 min	9 min
80 ft (24m)	3.4	31 min	24 min	7 min
100 ft (30m)	4.0	27 min	20 min	7 min

Note:

The data above is estimated based on mathematical models; actual conditions are affected by currents, water temperature, and the diver's psychological state.

As the table shows, at 60 feet (a very common reef depth), a diver using an AL63 needs to start the ascent procedure at around 29 minutes, while a buddy using an AL80 can continue for nearly 10 minutes more.

Consumption Rate Differences

In recreational diving databases, a petite, experienced female diver might have an SAC value as low as 0.35 cu ft/min (approx. 10L/min), while a large male beginner just earning his Open Water certification may have an SAC hovering between 0.8 and 1.0 cu ft/min (approx. 23-28L/min).

This nearly 3-fold difference means the same AL63 tank could support over an hour of shallow reef roaming for the former, while lasting less than 20 minutes of panicked struggle for the latter.

Divers with higher body weight and muscle mass naturally have higher gas requirements, which are met by the extra 20% to 40% volume provided by Aluminum 80 or larger Steel 100 cylinders.

Specific variables affecting SAC rates are often underestimated; the following factors can multiply gas consumption:

• Thermal Stress: Water conducts heat over 20 times faster than air. When a diver feels a slight chill in warm Caribbean waters, the body produces heat through involuntary muscle contractions (shivering), a process that causes metabolic rates to skyrocket, leading to an instantaneous 50% to 75% increase in air consumption. In cold water, an AL63 can easily be depleted before the diver even realizes they are cold.
• Hydrodynamic Drag: A diver's cross-sectional area and streamlined posture determine swimming resistance. A diver with poor buoyancy control swimming in a 45-degree "seahorse" position must overcome several times more water resistance than a diver in a proper horizontal trim. This extra physical work translates into higher oxygen uptake, making the limited volume of a small cylinder insufficient.
• Anxiety: The so-called "beginner tax" stems mainly from psychological tension. Adrenaline secretion leads to an increased heart rate and shallow breathing—an inefficient pattern that fails to effectively exchange dead-space gas in the lungs, leading to CO2 buildup and a subsequent "air hunger" sensation, forming a vicious cycle of consumption.

When planning a dive, assume a wreck dive at a depth of 60 feet (approx. 18m, 2.8 ATA) with a target stay of 40 minutes.

For an average male diver with an SAC of 0.6 cu ft/min, his actual underwater consumption (RMV) is 0.6 × 2.8 = 1.68 cu ft/min.

A 40-minute dive will consume 1.68 × 40 = 67.2 cu ft of gas.

In this scenario, an AL63 with a physical volume of only 63 cu ft is clearly incapable of the task, as it cannot even meet the total gas requirement, let alone the 500 PSI safety reserve.

By comparison, an AL80 with an actual capacity of approx. 77.4 cu ft, after deducting 67.2 cu ft of consumption, leaves about 10 cu ft (approx. 400 PSI), putting it on the edge of safety.

When considering duration factors, please refer to these standards:

1. Most Adults: Stick with AL80 or Steel 100 (larger steel cylinder, approx. 100 cu ft).
2. Teenagers or Smaller Divers: If you are under 5'3" (approx. 160cm) and find the bottom of an AL80 hits the back of your thighs, or if you consistently surface with over half your air left, then AL63 is a reasonable choice.
3. Technical or Deep Diving: Do not use small cylinders as a primary air source. Gas is consumed extremely fast at depth; the AL63 is only suitable as a Pony Bottle (independent backup source) slung at your side for emergencies.

Refilling Logistics

Filling a standard aluminum tank (AL80) is highly dependent on dive shops, costing only $5 - $8 per fill, but you must present a PADI/SSI certification card (C-Card) and ensure the tank has passed its annual Visual Inspection (VIP).

While 0.5L to 1L mini cylinders claim to be "self-sufficient," the actual operation is extremely difficult:

Using a manual high-pressure pump to fill a 1L cylinder to 3000 PSI requires 20-30 minutes of continuous high-intensity exercise in exchange for only 10-15 minutes of underwater time.

Buying a dedicated electric compressor typically costs over $300, and most legitimate dive shops will refuse to fill mini tanks purchased online that lack DOT (Department of Transportation) or TC (Transport Canada) stamps.

Standard Large Tanks

Unlike tires that can be filled at any gas station, scuba cylinders hold life-sustaining high-pressure gas, with pressure standards typically at 3000 PSI (207 Bar) or 3442 PSI (237 Bar).

In major diving markets like North America, Europe, and Australia, the only legal and safe way to obtain this high-pressure air is to visit a licensed dive center or specific high-pressure gas filling station.

Most reputable dive shops strictly enforce insurance liability clauses and refuse filling services to anyone who cannot produce a valid diver certification card (C-Card).

“In the US, there is no law requiring dive shops to fill air for the public. It is a private commercial service based on trust and qualification certification. If a tank owner cannot prove they have been trained by agencies like PADI, SSI, or NAUI on high-pressure gas handling and diving physics, operators will typically invoke the 'right to refuse service' to avoid potential personal injury lawsuits.”

When you connect an empty AL80 cylinder to a compressor fill whip, the tank temperature will rise rapidly to 50°C or higher within minutes.

Operators typically submerge the cylinders in cold water tanks for "wet filling" to control temperature, but this does not entirely eliminate thermal effects.

When the gauge shows 3000 PSI while the tank is still warm, the internal pressure will naturally drop back to 2600 PSI or 2700 PSI once it cools to room temperature over the next hour.

To achieve a "full tank," experienced users will request a "top-off" after the cylinder has cooled.

You cannot expect to finish in 2 minutes like at a gas station; getting a properly filled cylinder usually requires a wait of 30 to 60 minutes.

Every cylinder used in the US must comply with DOT regulations.

Every 12 months, the cylinder must undergo an internal visual inspection by a certified technician.

They remove the valve and use fiber-optic lights and mirrors to check for moisture, aluminum oxide (white powdery corrosion), or steel rust, and inspect the neck threads for cracks.

For early aluminum cylinders (such as 6351 alloy), an Eddy Current Test is also required to find microscopic cracks invisible to the naked eye.

Cylinders that pass inspection are labeled with a VIP sticker showing the month and year of testing.

“A cylinder without a valid VIP sticker is effectively scrap metal in the diving industry. If you are caught at a fill station with an expired VIP, the operator will require an on-site inspection for a fee, usually between $15 and $25. If it fails, the tank is immediately marked as 'Condemned,' and technicians usually drill through the body to prevent it from being pressurized again.”

The Hydrostatic Test every five years is the ultimate test of metal strength.

The cylinder is sent to a specialized facility, placed in a water jacket filled with water, and pressurized to 5/3 times its working pressure (e.g., a 3000 PSI tank is pressurized to 5000 PSI).

Technicians measure the amount of water displaced from the jacket to calculate the expansion rate under pressure and the contraction rate after release.

If metal fatigue exceeds the standard, the cylinder is mandatory retired.

Passed cylinders receive a new stamp on the shoulder with the month/year of the test and the station's code.

This service typically costs between $45 and $60 and usually has a 3 to 7 business day turnaround.

The air inside a scuba cylinder must meet CGA (Compressed Gas Association) Grade E or equivalent breathing air standards.

This requires dive shop compressor systems to be equipped with expensive filter sets (such as molecular sieves and activated carbon) to keep oil mist below 5mg/m³, carbon monoxide below 10ppm, and to remove odors and moisture.

If filling Enriched Air Nitrox (usually a mix of 32% or 36% oxygen), the cost rises from $5-$8 for regular air to $12-$20.

Users must also sign a fill log and personally confirm the oxygen concentration using an oxygen analyzer.

A full Luxfer AL80 cylinder weighs about 14.2 kg on land, while a 15L steel cylinder can weigh over 18 kg.

During vehicle transport, cylinders must be placed horizontally and secured, or placed in specialized tank racks.

If a 3000 PSI cylinder's valve is sheared off due to rolling during a sudden stop, it instantly becomes a jet-propelled missile with the kinetic energy to penetrate concrete walls.

Consequently, many divers in the West buy rubber "Tank Boots" or specialized foam cradles to ensure safety in the car, which adds extra cost and takes up more space.

Small Cylinders

A. Physical Limits and Moisture Risks of Manual High-Pressure Pumps

Most entry-level mini cylinder kits include a manual high-pressure pump similar to a bicycle pump, using a three- or four-stage compression structure and leverage to convert human power into high pressure.

• Quantifying Work: To fill a 0.5L cylinder from empty to 3000 PSI, an average adult male needs 15 to 20 minutes of high-intensity continuous exercise, totaling 400 to 600 strokes (depending on pump efficiency and residual air). As pressure rises, the required force increases exponentially; beyond 2000 PSI, users often need to use their full body weight (60kg+) to push the handle down.
• Thermal Degradation and Seals: High-pressure friction generates intense heat, causing rubber O-rings to age quickly or even melt. To protect the equipment, users usually must stop to cool the pump for 5 minutes for every 5 minutes of work, potentially extending the full process to over 40 minutes.
• Air Quality Hazards: This is the most critical technical flaw of manual pumps. These simple devices often lack the molecular sieve and activated carbon filtration towers found in professional compressors. In high-humidity environments like Florida or Southeast Asia, manual pumps force significant moisture into the tank. Condensed water accumulates at the bottom, not only corroding the aluminum interior but also potentially causing "green rust" on precision regulator parts or even internal freezing (free-flow) during cold water dives, leading to catastrophic supply failure.

B. Portable 12V/110V Electric Compressors (PCP Compressors)

To avoid the physical toll of manual filling, users turn to micro high-pressure compressors originally intended for PCP airguns.

• Equipment Cost and Specs: A reliable entry-level compressor (like the Yong Heng or GX series) typically costs $300 to $600, often exceeding the value of the cylinder itself. These machines usually support 12V car batteries or 110V/220V household power.
• Cooling System Logic: Most budget models use water cooling, requiring a bucket of water and ice with an external pump. This limits portability—you cannot easily set it up on a dry beach. Air-cooled models exist but are typically very loud, reaching 90dB, equivalent to a lawnmower at close range.
• Maintenance Cycles: Piston rings and check valves in micro compressors are high-wear items. Users must regularly replace tampon-style filters to absorb oil and moisture and monitor oil levels. In oil-lubricated models, improper operation leading to oil vapor in the breathing air can cause serious lipoid pneumonia.

C. The Pressure Equalization Trap of Trans-filling Adapters

Using an adapter with dual Yoke or DIN interfaces, you connect a full source tank to an empty target mini tank and let gas flow naturally by opening the valves.

• The Reality of Incomplete Fills: According to Boyle's Law and the ideal gas law, pressure will equalize between the two tanks but will never reach the source tank's initial pressure. If you have a 3000 PSI AL80 and an empty 1L mini tank, the resulting balance is around 2700 - 2800 PSI. As the large tank is depleted, each subsequent fill yields less pressure. When the large tank drops to 1500 PSI, your mini tank can only be filled to 1500 PSI, halving your underwater time.
• Operational Risks: Trans-filling must be extremely slow (Slow Bleed). If the valve is opened too quickly, the heat from adiabatic compression can instantly heat valve components or even cause burst disks on low-quality mini tanks to pop due to thermal shock.

Mini cylinder users often face rejection when seeking professional fills.

Many cheap mini cylinders sold on Amazon or eBay only carry CE certification or lack any certified stamps entirely.

In the US, under federal regulation 49 CFR, it is illegal to transport or fill high-pressure containers not approved by the DOT.

Reputable dive shop fill stations are bound by insurance terms; technicians will first check for the DOT-3AL mark on the neck.

If it's missing, or if the tank uses non-standard M18x1.5 threads rather than the diving industry's standard 3/4 NPSM, the shop will refuse the fill.

Portability vs. Capability

A standard AL80 (11.1L) aluminum tank weighs approx. 14.2 kg (31.3 lbs) empty and provides 2191L of gas at 207 bar (3000 psi).

This typically supports a diver with a consumption rate of 20L/min for about 45 minutes at a depth of 18m (60 feet).

The smaller AL50 (7.2L) cylinder reduces land weight to 9.6 kg (21 lbs), but the total gas is only 1400L.

At the same depth, an AL50 compresses dive time to under 25 minutes.

For beginners primarily diving in open water, the 4.6 kg reduction in carrying weight does not compensate for the 45% loss of underwater time.

Land Handling

The most widely used standard aluminum cylinder is the Aluminum 80 (AL80).

A standard S80 cylinder made by Luxfer or Catalina has a net metal weight of between 14.2 kg (31.3 lbs) and 14.7 kg (32.4 lbs).

However, this is just the weight of the aluminum tube itself.

The actual weight a diver lifts at a parking lot or pier includes the valve, tank boot, and the mass of the compressed air inside.

A standard K or DIN valve adds about 0.75 kg (1.6 lbs), and a hard plastic boot adds another 0.2 kg to 0.4 kg.

“Air has weight, a fact often overlooked in land load calculations. At 207 bar (3000 psi), an 11.1L AL80 cylinder contains about 2.2 cubic meters of air, which has a physical mass of approximately 2.6 kg (5.7 lbs). Thus, when you pull a full AL80 from your trunk, you are actually lifting nearly 18 kg (40 lbs), not the 14 kg listed in the manual.”

Steel cylinders offer a different volume-to-density ratio.

For example, the high-pressure HP100 has a working pressure of 237 bar (3442 psi).

Although its capacity is 25% larger than an AL80, its body length is typically 61 cm (24 in), which is 5 cm shorter than a standard AL80 (66 cm).

When walking toward an entry point, the shorter HP100 keeps the center of gravity lower on the back, reducing the lever arm effect on the spine.

In contrast, the longer AL80 is more likely to hit the back of the head when a diver looks down or leans over to put on fins—a physical redundancy that translates into inconvenience in tight boat cabins or crowded kitting-up areas.

Mainstream cylinder diameters in the North American market are mainly 184 mm (7.25 in) and 203 mm (8.0 in).

Most AL80 and HP80/HP100 models use a 7.25-inch diameter, perfectly fitting the curvature of standard BCD tank bands.

However, high-capacity low-pressure steel tanks (like LP85 or LP95) and some high-pressure models (HP117/HP133) use an 8.0-inch diameter.

Lifting an 8-inch diameter LP95 weighing over 20 kg with one hand requires a wider grip and places the center of gravity further from the body's midline, worsening shoulder fatigue.

If you use a lightweight travel BCD, its backplate may be designed specifically for 7.25-inch cylinders; forcing an 8-inch cylinder can cause it to wobble during transit, making it highly unstable.

Duration Time

When evaluating actual cylinder performance, one must factor in Surface Air Consumption (SAC) rate/RMV for dynamic calculations.

For an adult male beginner fresh out of Open Water (OW), the resting SAC typically fluctuates between 20 L/min (0.71 cu ft/min) and 25 L/min (0.88 cu ft/min).

When carrying a standard AL80 aluminum tank (11.1L water volume) to a typical recreational depth of 18m (60 feet), ambient pressure increases to 2.8 ATA.

According to Boyle's Law, gas volume is inversely proportional to pressure; the diver's actual consumption at that depth will surge to 56 L/min to 70 L/min.

With a standard fill of 207 bar (3000 psi), the total air in an AL80 is approx. 2297L (11.1L × 207 bar).

Safety procedures require a reserve gas for emergencies or assisting a buddy, usually a minimum of 50 bar (725 psi).

For an AL80, 50 bar represents about 555L of air, which must be deducted from the total.

Thus, at 18m depth, the actual "maximum bottom time" for an AL80 is calculated as follows:

Total 2297L minus 555L reserve leaves 1742L of usable air.

Divided by the depth consumption of 56 L/min, the result is approx. 31 minutes.

If the diver encounters a slight current or poor buoyancy control and SAC rises to 25 L/min, this time drops to 24 minutes.

Looking at "convenient" small tanks like the AL63 (9.0L water volume), it saves weight on land, but at the same 207 bar, total gas is only 1863L.

Deducting the same 50 bar reserve (450L), usable gas is only 1413L.

Under the same conditions (18m depth, 20 L/min SAC), an AL63 sustains only 25 minutes of dive time.

At 25m (3.5 ATA), usable time falls below 20 minutes.

High Pressure Steel cylinders are prized by advanced divers because they change the "working pressure" variable through higher pressure ratings.

An HP100 steel cylinder has a water volume of about 12.9L and is rated for 237 bar (3442 psi).

Its total gas reaches an impressive 3057L, 33% more than a standard AL80.

In the same model (18m depth, 20 L/min SAC, 50 bar reserve), the HP100 extends usable time to 43 minutes.

These 12 extra minutes often correspond to the most exciting parts of wreck or reef exploration.

Steel cylinders also have a flatter pressure decay curve, providing far more air than a full AL63 even with a higher reserve (e.g., 70 bar).

The following table lists No-Stop Gas Time for different cylinder specs at various depths based on a typical beginner's SAC (20 L/min):

Cylinder Model (Material)	Water Volume (L)	Working Pressure (Bar/PSI)	Total Gas (Liters)	Reserve (50 Bar)	15m Depth Time (2.5 ATA)	20m Depth Time (3.0 ATA)	30m Depth Time (4.0 ATA)
AL 50 (Alu)	7.2 L	207 / 3000	1490 L	360 L	22 min	18 min	14 min
AL 63 (Alu)	9.0 L	207 / 3000	1863 L	450 L	28 min	23 min	17 min
AL 80 (Alu)	11.1 L	207 / 3000	2297 L	555 L	35 min	29 min	21 min
HP 80 (Steel)	10.0 L	237 / 3442	2370 L	500 L	37 min	31 min	23 min
HP 100 (Steel)	12.9 L	237 / 3442	3057 L	645 L	48 min	40 min	30 min
LP 85 (Steel)	13.0 L	182 / 2640	2366 L	650 L	34 min	28 min	21 min

Note: LP (Low Pressure) steel tanks are often "overfilled" to higher pressures, but the standard calculations are shown above.

The same 50 bar reading represents entirely different air volumes in different-sized tanks.

In a 7.2L AL50, 50 bar is only 360L of air.

If an emergency occurs at 20m (3 ATA), two divers with high consumption (sharing air might exceed 100 L/min) will deplete that 50 bar in 3 minutes.

In a 12.9L HP100, 50 bar is 645L, providing nearly double the window to manage the crisis.

Weight System

A cylinder that is lighter on land (usually an aluminum tank) often requires more lead on the waist to counter its underwater positive buoyancy, meaning the total load when wearing the total rig on land is not reduced and may even increase.

Air has mass; every 1000L (approx. 35 cu ft) has a mass of approx. 1.2 kg (2.6 lbs) at sea level.

As gas is consumed, the cylinder's total mass drops while its displaced volume remains constant, making it "lighter" (shifting toward positive buoyancy).

The Luxfer S80 (Standard AL80) aluminum cylinder is a classic example of unstable buoyancy characteristics.

At a full 207 bar, it usually exhibits a slight negative buoyancy of about -0.7 kg (-1.5 lbs), sinking slowly at the start of a dive.

However, once depleted to a 35 bar (500 psi) safety reserve, it loses about 2.5 kg of gas mass.

Due to the low density of aluminum, the tank's buoyancy state reverses to about +1.9 kg (+4.2 lbs).

To counter this 2 kg upward pull and ensure neutral buoyancy for a safety stop at 5m at the end of the dive, a diver must add at least 2 kg of lead to their belt or BCD weight pockets.

Including weight needed for exposure suit compression, an adult male in a 5mm wetsuit using an AL80 typically needs 8-10 kg of lead.

Steel cylinders provide a different weighting solution due to their high density.

For example, a Faber HP100 has a negative buoyancy of -3.8 kg (-8.4 lbs) when full, and even when empty, it remains negative at -1.1 kg (-2.5 lbs).

Since the tank remains negative at the end of the dive, the diver can remove corresponding weight from the belt.

In the same 5mm wetsuit scenario, switching from an AL80 to an HP100 allows the diver to remove the 2 kg used to compensate for the aluminum tank, plus an additional 1 kg since the steel tank itself helps them stay down.

The result is a reduction of 3-4 kg on the weight belt.

Detailed physical comparison of three cylinder types on the weighting system (data based on freshwater):

• Standard Aluminum (Luxfer S80):

  • Land dry weight: 14.2 kg

  • Empty buoyancy: +1.9 kg (Positive)

  • Weight adjustment: Must add 2 kg of lead to offset empty buoyancy.

  • Total system effect: Tank is light, but the waist load is heavy; center of gravity shifts to the belt, often causing a drag-heavy "seahorse" position.

• High Pressure Steel (Faber HP100):

  • Land dry weight: 15.4 kg

  • Empty buoyancy: -1.1 kg (Negative)

  • Weight adjustment: Can reduce 3-4 kg of lead.

  • Total system effect: While the tank is 1.2 kg heavier than an AL80, the weight belt is over 3 kg lighter, reducing total rig weight by about 2 kg. Negative buoyancy is evenly distributed on the back, helping maintain horizontal trim.

• Small Aluminum (Luxfer S63):

  • Land dry weight: 12.1 kg

  • Empty buoyancy: +1.1 kg (Positive)

  • Weight adjustment: Must add 1-1.5 kg of lead.

  • Total system effect: Easiest to handle on land, but its short length and positive buoyancy can cause the hips to float and the head to sink, making it hard for tall divers to adjust trim.

Recommended Selection Criteria:

1. Shore Diving: If you must walk far with gear, the AL63 is a compromise. You sacrifice about 10 minutes of dive time, but for less physically strong divers, it reduces the risk of falling on land.

2. Boat Diving: You don't need to carry tanks long distances. Always prioritize Capability. Choose AL80 or HP100 to ensure you don't end a dive early because you ran out of air, which is a better value for expensive boat fees.

3. Depth Considerations: If your planned depth exceeds 12m (40 feet), do not consider small tanks under 50 cu ft as your primary source. Consumption accelerates with depth, and small tanks lack sufficient safety redundancy in deep water.

For most beginners, the standard AL80 or HP100 is the starting point.