Selecting a scuba cylinder requires focusing on material and capacity.
The most mainstream AL80 aluminum cylinder (11.1 liters, 3000 psi) is suitable for beginners, offering high cost-performance but showing a significant increase in buoyancy when air is low;
The S100 steel cylinder (13.2 liters, 3442 psi) is smaller in volume, provides gas for longer, and has stable buoyancy, making it the top choice for advanced divers.
When purchasing, be sure to confirm DOT certification and the hydrostatic test stamp to ensure safety.
Bottom Time
Bottom time is jointly determined by the cylinder's usable air capacity (Free Air Capacity) and the diver's surface air consumption rate (RMV/SAC Rate).
Taking an 80 ft³ (AL80) cylinder as an example, at a working pressure of 3000 PSI, deducting a 500 PSI reserve, the usable air volume is approximately 66 ft³.
If a diver's RMV is 0.5 ft³/min, at a depth of 66 feet (3 ATA), the theoretical bottom time is 44 minutes.
Using a 100 ft³ (HP100) steel cylinder under the same conditions can extend this to 55 minutes.
Specifications Impact
The Aluminum 80 (AL80) is the most common specification in dive centers worldwide.
Its nominal capacity is 80 cubic feet, but at a rated working pressure of 3000 PSI (207 Bar), its actual internal volume only holds about 77.4 cubic feet of compressed air.
For a medium-sized diver with a surface consumption rate (RMV) of 0.65 ft³/min, at a depth of 60 feet (2.8 ATA), this cylinder theoretically provides a total supply time of approximately 42 minutes.
Strictly following the principle of a 500 PSI (about 13 cubic feet) safety reserve, the actual bottom time available for exploration will be reduced to around 35 minutes.
In contrast, the high-pressure steel HP100 provides a full 100 cubic feet of air at 3442 PSI (237 Bar). At the same depth and consumption rate, the usable time increases to 46 minutes, a duration increase of approximately 31.4% compared to the AL80.
The physical specifications of different cylinder materials intervene in the diver's underwater work. An AL80 has a buoyancy of about -1.5 lbs in seawater when full; as air is consumed, its buoyancy changes to +4.4 lbs at 500 PSI. To counteract this positive buoyancy, the diver must wear extra lead weights. Extra weight increases the body's cross-sectional area and water resistance; the diver consumes more oxygen to overcome this drag while moving, indirectly accelerating the consumption of air in the cylinder.
| Cylinder Model | Rated Pressure (PSI) | Full Air Volume (ft³) | Total Time at 60ft Depth (min) | Usable Time after 500PSI Reserve (min) |
|---|---|---|---|---|
| AL80 | 3000 | 77.4 | 42.5 | 35.3 |
| LP85 | 2400 | 85.0 | 46.7 | 37.1 |
| HP100 | 3442 | 100.0 | 54.9 | 46.9 |
| HP117 | 3442 | 117.0 | 64.3 | 56.3 |
| HP130 | 3442 | 130.0 | 71.4 | 63.4 |
Note:
The above calculations are based on an average consumption rate of 0.65 ft³/min; LP85 includes a 10% overfill calculation.
Low-pressure steel cylinders (LP series) are usually rated at 2400 PSI, but in regions like North America, steel cylinders that pass hydrostatic testing can receive a "+" mark, allowing a 10% overfill to 2640 PSI.
An LP85 steel cylinder provides 93.5 cubic feet of air at 2640 PSI.
Because the internal volume (water volume) of an LP cylinder is much larger than an AL80, even if the filling station's compressor cannot reach the high pressure of 3000 PSI, the LP cylinder still provides more usable gas at 2400 PSI than the AL80.
In a cold water environment of 50 degrees Fahrenheit (10 degrees Celsius), according to Charles's Law, gas pressure decreases with temperature. An AL80 cylinder filled to 3000 PSI on the surface will see its pressure drop rapidly to about 2700 PSI upon entering cold water, causing the diver to lose approximately 10% of their preset stay time the moment they enter. Steel cylinders, due to their thermal conductivity and higher volume-to-pressure ratio, provide higher gas redundancy when dealing with such pressure drops caused by environmental temperature differences.
Cylinder diameter and length specifications affect gas consumption efficiency by influencing the diver's underwater trim.
When a diver over 1.8 meters tall uses a shorter HP80 cylinder, the center of gravity shifts toward the upper body, causing the legs to sink and increasing forward resistance;
Using longer HP120 or LP105 steel cylinders helps elongate the center of gravity distribution, achieving a steadier horizontal trim.
A reduction in drag leads to a lower heart rate and a subsequent decrease in breaths per minute;
an original 40-minute stay might be extended by 3 to 5 minutes due to improved posture.
This biomechanical optimization brought by equipment geometry is a hidden benefit that cannot be seen simply by looking at volume numbers.
| Depth (ft) | AL80 Usable Time (min) | HP100 Usable Time (min) | HP130 Usable Time (min) | Percentage Increase (vs AL80) |
|---|---|---|---|---|
| 33 (2 ATA) | 59.5 | 79.2 | 107.0 | 33% / 80% |
| 66 (3 ATA) | 39.7 | 52.8 | 71.3 | 33% / 80% |
| 99 (4 ATA) | 29.8 | 39.6 | 53.5 | 33% / 80% |
Assuming diver RMV is 0.65 ft³/min, deducting a 500 PSI reserve.
When conducting deep dives beyond 80 feet (2.4 ATA), the choice of cylinder specifications often needs to be paired with Nitrox.
Since nitrogen in the air limits the No-Decompression Limit (NDL), divers using an AL80 often use up most of their air before reaching the NDL.
However, when using EAN32 Nitrox, the NDL is significantly extended;
at this point, the air capacity of the AL80 becomes the bottleneck, preventing the diver from fully utilizing the time benefits of Nitrox.
Switching to HP100 or HP120 cylinders allows the gas supply reserve to match the extended NDL window, enabling long stays at depths of 80-100 feet.
Dive computer algorithms real-time monitor the rate of pressure drop when calculating Air Time Remaining (ATR). For a diver using an HP100 with a 13.3-liter water capacity, the pressure drop per breath (PSI/min) is smaller than for a diver using an 11.1-liter AL80. A slower rate of pressure drop reduces psychological stress on the diver, maintaining a steadier breathing rhythm. Physiological studies show that anxiety and the psychological fluctuations of frequently checking the pressure gauge can increase SAC by 15%, so large-specification cylinders not only provide more air physically but also extend actual bottom time through psychological feedback.
In high-intensity drift diving environments, a diver's RMV may surge from a normal 0.5 to 1.2 ft³/min.
In such extreme cases, an AL80 can support less than 20 minutes of usable time at a depth of 60 feet.
Since the exit point for drift diving is usually restricted by the boat's recovery path, consuming air too quickly forces the diver to ascend early and spend long periods floating and waiting.
Choosing HP130 specification cylinders in this environment provides double the margin for error;
even if consumption increases while fighting strong currents, it ensures the full dive plan can be completed with ample gas remaining during safety stops for emergencies.
Consumption Rate
Surface Air Consumption (SAC) rate represents the volume of air a diver consumes per minute while breathing at the surface, usually measured in cubic feet per minute (ft³/min) or liters per minute (L/min).
An adult male's resting surface consumption rate is typically between 0.4 and 0.6 ft³/min, but in practice, this value changes drastically based on the intensity of underwater activity.
For example, in a steady horizontal glide, an experienced diver might maintain a low level of 0.5 ft³/min, but once fighting a 0.5-knot current, the consumption rate can quickly soar to 1.0 ft³/min or higher, causing a cylinder originally estimated for 50 minutes to be exhausted in 25 minutes.
Biological heat loss is a physical trigger for increased air consumption. Water conducts heat about 25 times faster than air; in 75°F (about 24°C) water, the human body still loses heat much faster than in air of the same temperature. To maintain a body temperature of 98.6°F, the body must generate heat by speeding up metabolism. This enhanced metabolic activity manifests as a passive increase in breathing rate; even if the diver does not feel cold, the autonomic nervous system consumes more oxygen to produce heat. In 50°F cold water, a diver's consumption rate is typically 15% to 30% higher than in 80°F tropical waters.
- Differences in Body Size and Physiological Metabolism: Divers with larger lung capacities or individuals weighing over 200 lbs naturally consume more air for basal metabolism than a 120-lb individual, due to the minimum oxygen flow required to maintain larger muscle mass.
- Respiratory Dead Space and Regulator Resistance: The Work of Breathing (WOB) of a regulator increases at depth. The WOB of a high-quality regulator is typically below 0.9 Joules/liter, whereas poorly maintained or low-end models may reach 1.5 Joules/liter at a depth of 100 feet.
- CO2 Accumulation Effect: Shallow and rapid breathing leads to incomplete discharge of carbon dioxide from the lungs. The brain's respiratory center is extremely sensitive to CO2 concentration; high levels of CO2 will force a breathing reflex, making it impossible for the diver to control a rapid breathing rhythm.
- Neutral Buoyancy and Ineffective Work: Frequent use of BCD inflator/deflator valves consumes 3% to 8% of the cylinder's reserve air. Additionally, being overweighted—causing the body to tilt at a 45-degree angle—increases swimming resistance by over 200%, thereby raising heart rate and gas consumption.
At the surface (1 ATA), one 0.5-liter breath consumes a volume of air at standard atmospheric pressure;
At a depth of 99 feet (4 ATA), the same 0.5-liter breath consumes four times the amount of gas molecules as at the surface.
Therefore, if a diver's surface consumption rate (SAC) is 0.5 ft³/min, their actual respiratory minute volume (RMV) at 99 feet depth becomes 2.0 ft³/min.
Cortisol secretion from psychological stress can cause physiological hyperventilation. When facing murky visibility (less than 10 feet) or strong headcurrents, the human body enters "fight or flight" mode; this change in mental state typically causes the consumption rate to double in a short period. This increase in consumption due to emotional fluctuations is difficult to pre-calculate with physical formulas and usually requires divers to include 1.5 to 2 times the air safety redundancy in their dive plans to cope with such emergencies.
- Choice of Thermal Protection Gear: Wet suits lose insulation performance as depth increases because they are compressed by ambient pressure. While dry suits provide stable insulation, divers need to inject extra gas to prevent squeeze, which is part of the total consumption across multiple depth adjustments.
- Optimization of Underwater Trim: A perfectly horizontal posture minimizes the frontal area. Research shows that when maintaining a speed of 0.8 knots, a diver with poor trim consumes about 40% more oxygen than a diver in a perfect horizontal trim.
- Navigation Efficiency and Search Patterns: Aimless movement generates a lot of wasted work. Efficient dive path planning reduces the number of turns at the bottom, thereby lowering muscle fatigue and high-frequency breathing caused by exhaustion.
In cold water regions like Southern California or the North Atlantic, divers may observe a pressure drop of 300 to 500 PSI in the first 5 minutes after entering the water.
This is not a leak, but rather the internal pressure dropping according to Charles's Law because the water temperature is significantly lower than when the cylinder was filled.
This physical pressure loss is often mistaken by divers for a high consumption rate, but it actually reduces the available bottom time.
When calculating the SAC Rate, if this temperature-induced pressure drop is not deducted, the resulting data will be about 10% higher than actual physiological consumption.
While Nitrox is primarily used to extend NDL, its impact on consumption rates is psychological. Because Nitrox reduces the possibility of nitrogen narcosis, divers can maintain clearer thinking and a steadier breathing rhythm at 100 feet depth. This physiological stability brought by reduced nitrogen narcosis helps maintain a lower SAC value and avoids disordered breathing caused by the deep alcohol effect.
For technical divers or long-duration shipwreck explorers, precise consumption data can be reverse-calculated by recording depth, duration, remaining pressure, and the cylinder's volume coefficient.
After more than 50 recorded dives, a senior diver's SAC Rate fluctuation typically narrows to within 0.05 ft³/min.
Buoyancy
Cylinder buoyancy is determined by the difference between its displaced volume and its own weight.
Taking the commonly used Luxfer AL80 aluminum cylinder as an example, with an outer diameter of 7.25 inches and a volume of 11.1 liters, it exhibits -1.6 lbs of negative buoyancy in seawater when full (3000 psi).
When pressure drops to 500 psi, it becomes +4.4 lbs of positive buoyancy, creating a buoyancy swing of 6 lbs.
In contrast, the Faber HP100 steel cylinder remains negatively buoyant throughout, at -8.4 lbs when full and still -0.8 lbs when empty (500 psi).
The choice of cylinder material alters the diver's weight distribution by 5-8 lbs and their underwater trim layout.
Air Weight
Under standard Earth gravity, every cubic foot of dry air weighs approximately 0.0807 lbs.
Taking the Luxfer AL80 aluminum alloy cylinder common in the North American dive market, although labeled 80, its actual internal volume at a working pressure of 3000 psi (about 207 bar) holds 77.4 cubic feet of gas.
Calculations show the total mass of air in a full cylinder reaches 5.8 lbs (about 2.63 kg).
As the dive progresses, the regulator continuously discharges this mass-bearing gas into the surrounding water as bubbles, and the total weight of the cylinder decreases accordingly.
When the pressure gauge reading drops to the reserve pressure value of 500 psi (about 35 bar), the residual air weight is only about 1 lb, which physically strips away about 4.8 lbs of the diver's initial negative weight feel.
Physical Parameter Reference: In a standard environment of 20°C, gas density increases with pressure. Inside a 3442 psi high-pressure (HP) steel cylinder, the spacing between air molecules shrinks due to van der Waals forces, allowing an HP100 specification cylinder to hold 100 cubic feet of air. The air weight of such a cylinder when full is 8 lbs (about 3.63 kg). At the end of the dive, due to the loss of 6 to 7 lbs of gas mass, the stress state of the cylinder underwater undergoes a physical shift.
The following is a comparison of buoyancy parameters in seawater for several common cylinder models in the North American and European markets:
| Cylinder Model | Material | Working Pressure (psi) | Full Buoyancy (lbs) | Buoyancy at 500 psi (lbs) | Buoyancy Span (lbs) |
|---|---|---|---|---|---|
| AL80 | Aluminum | 3000 | -1.6 | +4.4 | 6.0 |
| HP100 | Steel | 3442 | -8.4 | -0.8 | 7.6 |
| LP85 | Steel | 2400 | -3.8 | +2.6 | 6.4 |
| AL100 | Aluminum | 3000 | -0.7 | +6.8 | 7.5 |
Note:
Negative values represent sinking (negative buoyancy), positive values represent floating (positive buoyancy).
Divers in the Caribbean or Florida waters must pre-calculate lead weight to offset this disappearing mass.
If a diver adjusts weights only based on a full cylinder so they can just submerge at the surface, their overall gravity will decrease by about 6 lbs as the dive ends and air is exhausted.
At a safety stop depth of 15 feet (about 4.5 meters), ambient pressure is 1.45 atmospheres.
At this point, exposure suits (like a 5mm wetsuit) expand in volume because the depth is shallower, making positive buoyancy peak.
If the cylinder becomes lighter due to the disappearance of air, the diver will feel a continuous upward pull.
To stay at that depth without unintentional ascent, the diver needs to add lead weights equal to the air weight to their waist or weight pockets before starting the dive.
Gas Physical State: Air is not an ideal gas. In high-pressure environments, the compression factor (Z-factor) of air deviates. At 3000 psi, the Z-factor is about 1.05. Physical calculations show that as depth increases, each breath exhaled contains more molecules, so the mass of air lost per unit time increases. A diver with a consumption rate (SAC Rate) of 0.75 cu ft/min under high load will discharge about 0.18 lbs of air mass from the cylinder per minute in 66-foot-deep seawater.
For divers using ultra-large capacity steel cylinders like the Faber HP130, which holds nearly 10.4 lbs (about 4.7 kg) of air mass when filled to 3442 psi:
Throughout the dive, as the pressure gauge needle drops from 3500 psi to 500 psi, the total system weight of the diver decreases by about 9 lbs.
The physical structure of a steel cylinder concentrates most of its weight at the bottom, but the internal air is distributed uniformly.
When the air disappears, the balance between the upper and lower parts of the cylinder changes.
For divers using back-mount buoyancy adjustment devices (Backmount BCD), this change will push the body toward a forward-tilting torque during horizontal swimming, affecting the steadiness of the trim.
Dive Environment Comparison: In high-salinity waters like the Red Sea or the Atlantic, water molecules have a higher density than fresh water (Seawater 64 lbs/ft³ vs. Fresh water 62.4 lbs/ft³). While the physical mass of disappearing air remains consistent across different waters, ambient density determines the initial weight base a diver must carry. In seawater, because the displacement buoyancy on the cylinder is greater, the upward tendency caused by the loss of air weight is perceived more acutely.
To withstand 3000 psi of pressure, the wall thickness of an aluminum alloy cylinder far exceeds that of a steel cylinder, which makes the total density of an aluminum tank close to the density of seawater once air is emptied.
When the 5.8 lbs of air in an AL80 cylinder is breathed away, the physical attribute of the cylinder transforms from slight sinking to generating about 4 lbs of upward lift.
This shift requires the diver to still have enough weight to overcome the cylinder's buoyancy after emptying the BCD bladder.
In contrast, the 232 bar steel cylinders common in Europe maintain negative buoyancy even in a completely empty state because the gravity of the material itself is greater than the buoyancy produced by its displaced volume.
Technical Parameter Conversion: 1 kg of air is approximately 2.2 lbs. When a 12-liter steel cylinder is filled to 200 bar, the air mass is 2.88 kg. If the residual pressure at the end of the dive is 50 bar, the consumed air mass is 2.16 kg. This value is defined in dive physics as the dynamic buoyancy increment. A diver must place 2 to 3 kg of lead weights in the weighting system to correspond with this variable, ensuring depth control throughout the dive.
When performing multi-cylinder diving or sidemount diving, the center of gravity of the cylinders mounted on either side of the body shifts outward or upward as gas is consumed.
If aluminum cylinders are used, the bottom of the cylinder will pivot upward late in the dive.
At this point, divers often move the Bolt Snap's fixed position on the cylinder to physically compensate for the torque imbalance caused by the loss of internal gas weight.
This operation is essentially dealing with the physical displacement brought by the loss of about 5 lbs of mass inside the cylinder, ensuring the body's axis does not tilt due to the cylinder getting lighter in narrow spaces or environments requiring precise hovering.
Physical Effort
A 12L steel cylinder weighs about 16kg, while a standard AL80 aluminum cylinder is about 14.3kg.
Since the aluminum cylinder generates about 1.9kg of positive buoyancy at the end of a dive, an additional 2-3kg of lead must be added to maintain neutral buoyancy, increasing total land weight by about 15%-20%.
When moving underwater, an 8-inch diameter cylinder has a 23% larger frontal cross-sectional area than a 7.25-inch diameter one.
Maintaining the same displacement in a 1-knot current requires approximately 12% more breathing gas.
Surface Transport
A standard AL80 aluminum alloy cylinder without a valve weighs about 14.3 kg; once the valve is installed and it is filled to 207 Bar, the total weight climbs to about 17.1 kg.
High-pressure steel cylinders like the HP100, while holding about 25% more gas than an AL80, use Chromoly steel with thinner walls.
Its bare weight is about 15.4 kg, and the total weight after filling to 232 Bar is close to 19 kg.
Since air itself has mass, the weight of every 1000 liters of compressed air is approximately 1.2 kg;
this dynamic weight manifests as a heavy physical burden during the transport stage before the dive begins.
| Cylinder Model | Material | Fill Pressure (Bar) | Bare Weight (kg) | Gas Weight (kg) | Initial Land Total Weight (kg) |
|---|---|---|---|---|---|
| AL80 | Aluminum | 207 | 14.3 | 2.8 | 17.1 |
| HP100 | HP Steel | 232 | 15.4 | 3.6 | 19.0 |
| LP85 | LP Steel | 184 | 14.1 | 3.0 | 17.1 |
| HP120 | HP Steel | 232 | 17.7 | 4.3 | 22.0 |
| AL63 | Aluminum | 207 | 12.2 | 2.2 | 14.4 |
In a typical shore diving environment, a diver often needs to walk 50 to 150 meters carrying heavy gear.
For a 75 kg diver, carrying a 19 kg HP steel cylinder plus regulator, BCD, and weights can mean the load exceeds 40% of their body weight.
The heart rate can surge from resting to over 110 bpm just from transporting heavy items before even entering the water.
This pre-generated physical expenditure causes lactic acid buildup in muscles and is accompanied by a premature increase in breathing rate, resulting in higher-than-normal air consumption in the early stages of the dive.
In research focused on Bonaire's shore diving environment, divers using 12-liter aluminum cylinders had an SAC (Surface Equivalent Consumption Rate) about 18% higher in the first 5 minutes underwater after walking 100 meters on gravel than divers who used carts to transport gear. This indicates that the physical expenditure of land transport extends to the underwater phase through physiological metabolic reactions.
The length of an AL80 is typically 66 cm, while the HP100 is only 60 cm.
Longer cylinders shift the Center of Gravity backward and downward, creating greater torque and requiring the diver to lean excessively forward while walking to counteract this backward pull.
Stubby cylinders keep weight closer to the center of the back, reducing body sway while walking.
For smaller divers, a long cylinder may frequently hit the back of the legs while walking;
this uncoordinated movement pattern significantly increases unnecessary physical expenditure.
Underwater Frontal Drag
Standard aluminum AL80s and high-pressure steel HP100s usually have a diameter of 7.25 inches (about 18.4 cm), while larger capacity cylinders like the LP108 or HP130 increase to 8.0 inches (about 20.3 cm).
From geometric area calculations, this 0.75-inch diameter increase results in a 21.5% increase in the frontal cross-sectional area.
In fluid dynamics, because water is approximately 800 times denser than air, any minor expansion in cross-sectional area produces a significant drag increment when moving.
This resistance may manifest as a slight increase in finning effort in still water, but in currents over 0.5 knots, the drag effect of a large-diameter cylinder forces the diver to expend much more stamina to maintain the same relative position.
Experimental data shows that when a diver propels through water at 0.5 m/s, using an 8-inch diameter cylinder generates about 15 to 18 Newtons more fluid drag than using a 7.25-inch cylinder. For a technical diver performing a 60-minute dive, this constant additional thrust requirement leads to earlier leg muscle fatigue and pushes the heart rate higher.
If a diver doubles their propulsion speed to fight a strong current, the drag they experience increases fourfold.
Large-diameter cylinders produce significant turbulence in high-flow environments; the vortex zone at the end of the cylinder disrupts the smoothness of water flow, increasing pressure-drag.
For smaller divers, an 8-inch diameter cylinder occupies over 60% of the torso width, making it difficult for the body to fully shield the drag brought by the cylinder.
In contrast, thinner steel cylinders (like 7L or 10L specifications) can better hide behind the diver's torso shadow, optimizing overall streamlined performance and reducing energy expenditure during long swims.
In comparative tests between the common Caribbean AL80 and the North American HP120 steel cylinders, divers wearing the HP120 (8-inch diameter) were on average 0.12 knots slower underwater at the same finning frequency than those wearing an AL80. To bridge this speed gap, the diver with the large bottle needs to increase their work output by about 12%.
When muscles increase contraction intensity to overcome drag, carbon dioxide concentration in the blood rises.
Once the brain's respiratory center senses the rising CO2 levels, it automatically accelerates the breathing rate.
This physiological feedback creates a paradox:
The diver chooses a larger cylinder to get more air, but because the large bottle increases swimming resistance, leading to a higher breathing rate, the extra air provided is often canceled out by the disadvantage of the cylinder's volume.
At a depth of 20 meters, if an increase in resistance leads to breathing 5 liters more gas per minute, the extra stay time provided by a large 15L bottle will be shortened by about 8 to 10 minutes.
If the cylinder is mounted too high, the cylinder valve and first stage are exposed to the water flow above the head, creating an extra drag point.
Mounting it too low causes the cylinder bottom to extend beyond the curve of the hips, interfering with the water flow above the legs during finning and creating unstable swaying.
This instability forces the diver to constantly use muscle groups for micro-adjustments to maintain body balance.
This subtle but continuous muscle activity is particularly evident in a dry suit environment, as the suit itself already significantly increases the frontal area; adding cylinder diameter creates a stacking effect, causing the diver's drag performance in the water to grow exponentially.
Statistics show that in typical boat drift environments, divers using compact 7.25-inch diameter cylinders usually rate their subjective fatigue about 25% lower than companions using 8-inch diameter cylinders. This difference mainly stems from the lateral torque produced by the interaction between the back load and water flow; thinner bottles show better directional stability in crosswinds or cross-currents, reducing antagonistic work of the waist muscles.
Cylinder diameter must be compatible with the bladder size of the BCD (Buoyancy Control Device).
If an 8-inch large diameter cylinder is mounted on a small travel BCD designed for standard aluminum tanks, the cylinder will protrude excessively backward.
This structure increases the total thickness of the body, enlarging the vertical frontal area.
This increase in "thickness" creates a drag effect similar to a submarine's conning tower when the diver is swimming in a horizontal trim.
Especially during long-distance penetrations or antagonistic swimming, this physical expenditure brought by mismatched equipment size significantly lowers the overall dive safety margin.
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