100cf vs. 80cf Diving Air Tank

100cf (≈2830L) has a larger capacity but is heavier (steel cylinder approx. 16kg vs 13kg for 80cf), placing more pressure on the waist during carry (tests show a 40% increase in fatigue for beginners carrying it continuously for 2 hours);

80cf is 3kg lighter, suitable for beginners or short dives.

Based on a beginner's air consumption of 15L/min, the 100cf duration is ≈188 minutes (2830÷15), while 80cf is ≈151 minutes;

For experienced divers at 10L/min, the durations are 283/226 minutes respectively, narrowing the gap.

100cf has a diameter of 18cm and length of 66cm, while 80cf has a diameter of 17cm and length of 61cm;

the latter fits the back better and requires 2-3kg less weight (PADI case study).

Choose 100cf for long boat dives; choose 80cf for shore dives or flexible needs.

Data shows 70% of beginners choose 80cf for their first purchase (Scuba Diving 2024).

Comfort

The essence of comfort lies in the distribution of weight.

On land, a filled HP100 (100cf steel cylinder) is about 8-10 lbs heavier than an AL80 (80cf aluminum cylinder), making it more strenuous to carry.

However, underwater, an HP100 maintains approximately -2 lbs of negative buoyancy even when the cylinder pressure drops to 500 psi, while an AL80 generates +4 lbs of positive buoyancy at that point.

Using an HP100 allows you to reduce 5-6 lbs of lead weight from your weight belt.

Reducing waist weight significantly lowers spinal pressure, and the HP100 body is typically 2-3 inches shorter than the AL80, providing more head mobility and superior overall underwater balance (Trim).

Weight Positioning

Gas itself has weight; at surface atmospheric pressure, every 13 cubic feet of air weighs approximately 1 lb.

When a cylinder is used from a full state (3000 psi or 3442 psi) down to 500 psi, the weight of the gas inside the cylinder decreases by about 5 to 7 lbs.

A Luxfer S80 creates about -1.8 lbs of negative buoyancy when full, but shifts to +4.4 lbs of positive buoyancy at 500 psi.

To counteract this 4.4 lbs of positive buoyancy, the diver must carry an equivalent extra amount of lead on the waist belt or in BCD weight pockets.

If the diver does not carry this extra 4-5 lbs of lead, at safety stop depth, the combination of air expansion and the cylinder becoming lighter will cause the diver to float uncontrollably toward the surface.

A standard Aluminum 80 diver wearing a 3mm wetsuit in tropical waters usually needs 12 lbs of weight, of which about 4 to 5 lbs are solely to keep down the cylinder that wants to float at the end of the dive. 30% to 40% of waist weight is "dead weight," used purely to offset the poor buoyancy characteristics of aluminum cylinders.

An HP100 manufactured by Worthington or Faber has a massive negative buoyancy of about -8.5 lbs when full at 3442 psi; even when gas is depleted to 500 psi, it maintains -2.5 lbs of negative buoyancy.

From a physical calculation standpoint, switching from an AL80 to an HP100 gives the diver a "double relief":

First, there is no need to carry 4.4 lbs of lead to suppress the aluminum cylinder's floatation; second, the steel cylinder itself provides -2.5 lbs of negative buoyancy even when empty, replacing an equivalent weight of lead.

This shift in buoyancy characteristics allows the diver to remove 6 to 7 lbs of lead from the waist.

In cold water or drysuit diving, this difference is even more pronounced; a diver who might have needed 24 lbs of weight often only needs to carry 16-17 lbs when using a steel cylinder.

When a diver walks on land, climbs boat ladders, or treks at shore dive sites, the lead on the weight belt creates downward shear force acting on the L4 and L5 lumbar segments.

Heavy weight belts tend to slide down, compressing hip nerves; long-term wear can lead to persistent lower back muscle tension or even spasms.

Removing 6-8 lbs from the waist and converting it into the material density of the cylinder itself effectively transfers the load from the soft-tissue-heavy waist to the more structurally sound thoracic spine and back.

The weight of the HP100 is distributed evenly across the back surface area via the BCD backplate or hard pack, with shoulder straps sharing most of the downward pull, rather than relying solely on lumbar support.

Physical therapists and diving medicine experts frequently point out that long-term carrying of heavy weight belts is a primary trigger for chronic lower back pain in recreational divers. Redistributing a load of over 6 lbs by changing cylinder material fundamentally alters the spinal stress model.

When using an AL80, because the cylinder tail generates positive buoyancy in the latter half of the dive while the waist lead generates downward gravity, these two opposing forces create a torque on the diver's body.

The cylinder pulls the back up, while the weight belt pulls the abdomen down, forcing the diver to constantly arch their back or use back muscles to fight this "folding" tendency, which accelerates muscle fatigue during long dives.

The center of negative buoyancy for an HP100 is closer to the diver's geometric center of lung volume, and the cylinder itself acts like a long V-weight hugging the spine line.

For drysuit divers, the extra negative buoyancy of the HP100 also effectively counteracts the buoyancy of air bubbles in the back of the suit, preventing instability caused by air pocketing.

Buoyancy and Specifications

Below is a comparison of physical parameters for common specifications from major manufacturers (such as Faber and Catalina), which determine the difficulty of underwater trim:

Cylinder Spec	Material	Full Weight (Land)	Buoyancy (Full)	Buoyancy (Empty 500psi)	Length (No Valve)
AL80	Aluminum Alloy	~31.6 lbs	-1.8 lbs	+4.4 lbs	~26.1 inches
HP100	Steel	~34.0 lbs	-8.5 lbs	-2.5 lbs	~24.0 inches

Note:

Data taken from general standards; minor errors may exist between brands. HP100 usually refers to models with a working pressure of 3442 psi.

Underwater Trim

Underwater trim control is essentially a game between the Center of Gravity and the Center of Buoyancy.

For AL80 users, the process of dropping from 3000 psi to 500 psi is actually a process where back weight gradually disappears and transforms into upward lift.

A full AL80 has about -1.8 lbs of negative buoyancy, with a relatively balanced center of gravity;

But as approximately 6 lbs of air is consumed, the buoyancy characteristics of the cylinder bottom flip.

The specific gravity of aluminum alloy (2.7 g/cm³) is much lower than steel (7.8 g/cm³), and the bottom of an AL80 is typically thicker to withstand impact, but not enough to offset the displacement after air is evacuated.

In the final stages of a dive, the bottom of an AL80 generates +4.4 lbs of positive buoyancy.

Since the cylinder is secured to the BCD via a cam band, this positive buoyancy point acts on the lower-middle back, creating a lever with the cylinder valve as the fulcrum.

The regulator (first stage) at the valve is usually heavy chrome-plated brass, while the cylinder bottom is floating aluminum.

This "top-heavy" mass distribution generates a strong Rotational Torque, attempting to pull the diver's hips up and forcing the head down, destroying horizontal trim.

To counter this "tail-up" effect of the AL80, the most common compensatory action for divers is Hyperextension of the lumbar spine and forceful contraction of the glutes to push the legs down, or adding lead to lower BCD weight pockets or even ankles.

This constant muscle tension not only increases CO2 buildup and the Surface Air Consumption (SAC) rate but also makes it difficult to remain still during hover photography or precision buoyancy control.

Once the diver stops kicking, the AL80's buoyancy takes over, pulling the body into a head-low, feet-high inverted posture.

Torque Lever Differences:
- AL80: Length is approximately 26.1 inches. The longer lever arm amplifies the impact of tail buoyancy on body posture.
- HP100: Length is typically 23.5 to 24 inches. The shorter lever arm reduces the cylinder tail's interference with the body's center of gravity.

HP100 steel cylinders from Faber or Worthington are designed to leverage the high density of steel; even in an empty state at 500 psi, they maintain -2.5 lbs of negative buoyancy.

When the cylinder is snug against the backplate or BCD, it provides a stable downward vector, helping the diver naturally maintain horizontal Trim without leg muscle effort.

During the glide phase after a frog kick, divers using an HP100 can fully relax their bodies, as the negative mass of the cylinder prevents the feet from floating up.

For drysuit divers, this point is especially evident.

Air inside a drysuit tends to migrate toward the legs, increasing foot buoyancy; the negative buoyancy of the HP100 effectively balances this trend, eliminating reliance on ankle weights.

Body Compatibility

A standard Luxfer S80 (AL80) cylinder has a total length of about 26.1 inches (66.3 cm), excluding the valve and first stage regulator mounted on top, which usually add another 3 to 4 inches of vertical height.

For divers with shorter Torso Length (the distance from the C7 cervical vertebra to the iliac crest), especially those under 5 feet 6 inches (approx. 167 cm), this nearly 30-inch total assembly length constitutes a significant mechanical obstacle.

When a diver attempts to look up at surface boats, buddies, or marine life while in a horizontal position, the back of the head (Occipital Bone) hits the DIN connector or Yoke bolt of the first stage.

This persistent "head-bumping" is not only annoying but forces the diver to arch their back or twist their body to gain vision, breaking stable Trim and potentially causing trapezius pain due to neck restriction.

Diving instructors frequently observe shorter students constantly adjusting the height of cylinder straps to solve the head-bumping issue. But with an AL80, if you mount the cylinder too low to avoid the back of the head, the bottom of the cylinder moves down to the thigh area.

When an AL80 is mounted too low, another biomechanical conflict arises:

The collision between the hamstrings and the cylinder bottom.

During the recovery phase of a standard Flutter Kick or Frog Kick, the muscles at the back of the diver's thighs bulge toward the back.

If the cylinder is too long, the tank boot restricts the upward range of the thigh, preventing the kick from being fully executed.

For female divers, this issue is particularly prominent as women's torso proportions are generally shorter than men's.

The length of an AL80 covers almost the entire area from neck to below the buttocks, making the waist completely rigid and losing the flexibility to fine-tune posture via the spine.

HP100 Steel Cylinders offer a superior solution in spatial geometry.

Despite a 20-25% increase in volume, the body length of mainstream HP100s (like the Faber FX series) is usually kept around 23.8 to 24 inches (approx. 61 cm).

This 2-inch surplus provides exactly the buffer needed for neck extension, allowing divers to look up 180 degrees freely while maintaining horizontal trim, without feeling the obstruction of hard metal.

Simultaneously, the shorter bottom typically hovers above the sacrum, completely avoiding the movement path of the hips and thighs.

This "short and stout" configuration (HP100 diameter is typically 7.25 or 8.0 inches, similar to or slightly thicker than AL80) keeps the cylinder's center of mass more compact against the back, reducing the Moment of Inertia during turns.

When a diver turns in narrow wreck cabins or caves, the shorter swing radius of the HP100 means less risk of collision.

Air Consumption

Your Surface Air Consumption (SAC Rate) determines your cylinder choice.

Despite the name Aluminum 80 (AL80), it actually only holds 77.4 cu ft of gas at its 3000 psi rated pressure.

In contrast, the HP100 provides a full 100 cu ft at 3442 psi.

The HP100 has 29% more actual usable gas than a standard AL80 (approx. 22.6 cu ft).

For divers with a SAC rate higher than 0.65 cu ft/min, or those planning dives deeper than 60 feet (18m), the AL80 often runs out of gas before reaching the No-Decompression Limit (NDL), forcing an early end to the dive.

Choosing an HP100 can fill this 15-20 minute dive time gap.

Actual Gas Reserve

There is a significant numerical discrepancy between industrial manufacturing standards and marketing nomenclature.

Specifications from Luxfer and Catalina, the world's major aluminum cylinder manufacturers, show that the internal Water Volume of a standard S80 cylinder is typically 11.1 liters (approx. 0.399 cubic feet).

At the rated working pressure of 3000 PSI (207 Bar), the actual volume of gas held by an S80 is 77.4 cubic feet.

The industry rounds 77.4 up to 80, creating an initial data error of about 3.2%.

By contrast, HP100 (High Pressure Steel) manufacturing standards require it to hold a full 100 cubic feet or even more (depending on manufacturers like Faber or Worthington; some models actually reach 102 cubic feet) at its working pressure of 3442 PSI (237 Bar).

From factory specs, the difference in gas volume between the two is not 20 cubic feet, but 22.6 cubic feet.

"At 60 feet deep, a difference of 22.6 cubic feet of air equates to an increase of about 9-12 minutes of extra breathing time (based on a normal consumption rate of 0.6-0.8 cfm)."

Gas in high-pressure environments must be corrected using a compressibility factor (Z-Factor).

As pressure exceeds 200 Bar, Van der Waals forces between air molecules lead to higher resistance to compression.

Aluminum cylinders, limited by the 3000 PSI DOT-3AL standard, sit right at the critical point where gas compression efficiency begins to drop.

The Chromium-Molybdenum steel used in HP100 allows for higher working pressure (3442 PSI), using physical pressure to overcome gas resistance to compression by brute force.

To force 100 cubic feet of gas into a standard AL80, pressure would need to be raised to a dangerous 3800 PSI or more, which is near the yield strength limit of aluminum alloy and constitutes a violation of operation.

Conversely, to carry only 77.4 cubic feet in an HP100, the gauge would only need to read 2650 PSI.

The Tank Factor is a precise tool for calculating actual gas reserves, defining the specific gas volume represented by every 100 PSI on the gauge.

For an AL80, the tank factor is approximately 2.5 (meaning 100 PSI equals 2.5 cubic feet of gas).

For an HP100, this factor is approximately 2.9-3.0. This mathematical difference alters the physical definition of a "500 PSI safety reserve."

500 PSI in AL80 $\approx$ 12.5 cubic feet of gas.
500 PSI in HP100 $\approx$ 15.0 cubic feet of gas.

In "Minimum Gas / Rock Bottom" calculations, the extra 2.5 cubic feet provided by the HP100 offers an additional 30-45 seconds of buffer for two highly stressed divers during an emergency shared-air ascent from 100 feet (30m).

As depth increases and ambient pressure rises, your air consumption multiplies.

The value of an HP100 is amplified during deep dives.

Assume an adult male diver with a SAC rate of 0.75 cu ft/min (a common slightly high rate) diving at 66 feet (20m/3 ATA), keeping 500 psi as a safety reserve.

Cylinder Type	Actual Usable Gas (minus reserve)	Actual Underwater Consumption (3 ATA)	Theoretical Max Dive Time	Analysis
Aluminum 80	~63 cu ft	2.25 cu ft/min	28 minutes	Must ascend long before NDL is reached.
HP Steel 100	~85 cu ft	2.25 cu ft/min	38 minutes	Increases bottom time by 35%, closer to NDL.

Thus, simply changing the cylinder without changing breathing habits can grant 10 minutes of extra underwater sightseeing.

Psychological Impact

Diving is heavily regulated by the Autonomic Nervous System.

Cylinder capacity is not just a physical reserve; it serves as a powerful psychological cue, intervening in the diver's Respiratory Minute Volume (RMV) via visual feedback loops.

When a diver checks the Submersible Pressure Gauge (SPG), seeing a low capacity that is rapidly depleting (common with AL80) triggers a "resource shortage" signal in the brain's amygdala.

This subconscious anxiety triggers trace cortisol release, leading to shallower diaphragm contractions and increased frequency, creating a Shallow Rapid Breathing pattern.

This pattern reduces alveolar gas exchange efficiency, increases Dead Air Space ventilation, and paradoxically accelerates gas consumption, forming an "anxiety-consumption" positive feedback loop.

Conversely, the high pressure and large volume of an HP100 establish a psychological safety net of "resource abundance."

If a diver checks their SPG 20 minutes into a dive, the HP100 might still show a reassuring 2500 PSI, while an AL80 might have dropped to 1800 PSI.

This numerical difference acts on the vagus nerve, inducing a lower heart rate and extended breathing rhythm.

Research from DAN and observations from instructors show that divers switching to larger cylinders typically see a 5% to 10% non-technical reduction in their SAC rate.

Visual Pressure Threshold: Most recreational divers experience stress when the gauge falls below 1000 PSI. With an AL80, this happens too early. With an HP100, starting at 3442 PSI, consuming 2000 PSI still leaves 1442 PSI, keeping the diver in the comfort zone.
CO2 Tolerance: Anxiety-induced shallow breathing hinders CO2 removal. High CO2 levels (Hypercapnia) are the strongest signal to the respiratory center to increase breathing. The calm provided by a high-volume cylinder helps maintain deep, slow Tidal Breathing, ensuring efficient CO2 wash-out and suppressing the urge to breathe.

"Fear is the number one enemy of air consumption. Eliminate the fear of 'running out,' and your consumption naturally drops."

This effect is especially evident in beginners or "Advanced Beginners."

These divers haven't yet built stable underwater confidence and are extremely sensitive to remaining gas.

The low tolerance of an AL80 forces them to allocate mental bandwidth to monitoring gas rather than environment or buoyancy.

HP100's "ghost gas" (the extra 20 cu ft they might never use) acts as a buffer, freeing up mental bandwidth.

When the brain isn't alerted to "gas out" threats, muscles relax, drag decreases, and efficiency improves.

Physical Size & Trim

HP100 High Pressure Steel cylinders are typically about 2 inches (5-6 cm) shorter than standard AL80 aluminum cylinders, with a more concentrated center of gravity.

This effectively prevents the cylinder bottom from hitting thighs or the first stage from bumping the back of the head.

The biggest physical difference is buoyancy:

AL80 generates about +4 lbs (1.8 kg) of positive buoyancy when empty (500 psi), pulling the diver's back upward;

While an HP100 maintains about -2.5 lbs (1.1 kg) of negative buoyancy even when empty.

With an HP100, you can remove 6-8 lbs of lead from your weight belt, using the cylinder's material density to maintain a stable horizontal Trim without fighting the cylinder's upward pull during safety stops.

Buoyancy Evolution

Cylinder buoyancy is not a static value.

At standard sea level pressure and 70°F, every cubic foot of air weighs about 0.0807 lbs.

Total weight consists of "net metal weight" plus "compressed gas weight."

As you breathe, mass decreases, reducing the gravity term in the Archimedes buoyancy formula while displacement remains constant, resulting in a continuous Positive Shift in buoyancy.

An AL80 filled with 77.4 cu ft of air holds about 6.3 lbs (2.8 kg) of gas.

An HP100 holds about 8.1 lbs (3.7 kg).

AL80 buoyancy crosses the "zero point."

A common Luxfer S80 shows -1.4 lbs of negative buoyancy when full at 3000 psi.

At 1500-1800 psi, it hits the neutral point (0 lbs). By 500 psi, it generates +3.5 to +4.4 lbs of positive buoyancy.

This "polarity reversal" interferes with stability at safety stops: the cylinder tail lifts, creating torque around the tank band that tries to push the hips higher than the head.

The HP100 buoyancy curve stays within the negative range.

An HP100 is -8.5 to -9.0 lbs when full.

Even at 500 psi, after losing 8.1 lbs of gas, it remains -2.0 to -2.5 lbs.

It acts as a constant anchor, preventing the cylinder from trying to float up.

Start of Dive (Full):
- AL80: -1.4 lbs. Trim is easy to control.
- HP100: -8.5 lbs. Strong downward pull; may require BCD inflation to compensate.
End of Dive (500 psi):
- AL80: +4.4 lbs. Tail-up tendency; diver must use muscles to maintain trim.
- HP100: -2.0 lbs. Naturally maintains horizontal Trim.

Weight Adjustment

A "buoyancy budget" consists of Gas Weight and Tank Material Buoyancy.

To ensure you can stay neutral at 15ft (5m) for a safety stop with 500 psi, your weight system must offset the sum of "empty cylinder buoyancy" and "remaining gas weight."

Switching from AL80 to HP100 requires a mathematical subtraction.

If a diver in a 7mm wetsuit needs 18 lbs of lead with an AL80 (where 4 lbs is just to hold down the empty aluminum cylinder), switching to HP100 creates a total shift of 6.5 lbs in buoyancy.

Physically, this diver can remove 6 to 8 lbs of lead.

Physical Parameter	AL80 Configuration	HP100 Configuration	Difference
Full Gas Weight	~6.4 lbs	~8.0 lbs	+1.6 lbs gas
Empty Buoyancy	+4.0 lbs (Positive)	-2.5 lbs (Negative)	6.5 lbs shift
Total Lead Required	18 lbs (example)	11-12 lbs	~35% less lead

For drysuit divers, HP100 acts as a built-in "V-Weight," reducing waist belt load.

However, caution is needed in tropical waters with thin suits.

Lean divers might become overweighted with an HP100 even with zero lead, requiring constant BCD inflation which increases drag and hurts Trim.

Ditchable weight must also be considered.

With 18 lbs on an AL80, you can ditch all 18 lbs for massive lift.

With 10 lbs on an HP100, your ditchable weight is less.

You must ensure that even at full cylinder weight, dropping your remaining lead provides enough lift to swim to the surface if the BCD fails.

100cf vs. 80cf Diving Air Tank | How to Choose