Steel (chrome-molybdenum steel) has a high density; the 7L/230bar model has an empty weight ≈4kg, and a total weight ≈6.1kg when full (7L×230bar=1610L of air, weight ≈2.08kg);

Carbon fiber (carbon fiber + aluminum liner) features outstanding lightweight properties; the 9L/300bar model has an empty weight of only ≈1.8kg, and a total weight ≈5.3kg when full (9L×300bar=2700L of air, weight ≈3.48kg), which is about 13% lighter than a steel cylinder of the same volume.

Steel has negative buoyancy (the 7L model sinks by ≈0.5kg), saving weight for deep diving, with a tensile strength ≥800MPa, but requires an annual inspection for internal wall corrosion prevention;

Carbon fiber withstands pressures of 300bar (4350psi) and has near-neutral buoyancy (9L model ≈+0.1kg).

Its high-pressure resistance makes it suitable for technical diving, but it should avoid scratches from sharp objects. Regularly measure the pressure drop of the liner (≤5bar/month).

Weight

A 12L steel cylinder has an empty weight of 15-18kg and a full weight of 27-30kg;

A carbon fiber cylinder of the same capacity has an empty weight of 8-10kg and a full weight of 20-23kg, nearly 1/3 lighter.

According to measurements by Norwegian ice divers, steel cylinders require 2kg of added weight, and after 20 meters, they become lighter due to gas consumption, leading to frequent unintentional ascents;

According to statistics from the Florida Coastal Association in the United States, hiking 100 meters with a steel cylinder on one's back increases the heart rate by 15%, with a physical exhaustion equivalent to 5 minutes less activity underwater.

Empty Weight vs. Full Weight

Different Capacities

Common cylinder capacities in the international diving market are 10L, 12L, 15L, and 18L.

Below are the measured data for four types of capacities by DiveGear Lab in 2023 (Unit: kg):

Capacity (L)	Cylinder Type	Empty Weight Range	Full Weight Range	Weight Difference (vs. Steel of same capacity)	Material Structure Details
10	Steel	13-16	23-26	-	High-carbon steel (4.5mm wall thickness)
	Carbon Fiber	7-9	17-19	6-7kg lighter (approx. 32%)	Carbon fiber + 6061 aluminum alloy liner (2.8mm wall thickness)
12	Steel	15-18	27-30	-	Chrome-molybdenum steel (impact-reinforced)
	Carbon Fiber	8-10	20-23	7-8kg lighter (approx. 30%)	T700 carbon fiber + 7075 aluminum alloy liner
15	Steel	19-22	34-37	-	Stainless steel (seawater corrosion resistant)
	Carbon Fiber	10-12	25-28	9-10kg lighter (approx. 31%)	Carbon fiber + titanium alloy interface
18	Steel	24-27	42-45	-	High-strength steel (deep-sea compression type)
	Carbon Fiber	13-15	31-34	11-12kg lighter (approx. 32%)	Multi-layer carbon fiber winding (6 layers)

Note:

The 18L carbon fiber cylinder is exclusively for technical diving; its liner thickness is increased to 3.2mm, yet it remains 12kg lighter than a steel cylinder (2022 report from the Deep Sea Lab at the University of Bergen, Norway).

Influencing Factors

1. Material Density Difference

The main body of steel cylinders is made of high-carbon steel (density 7.85g/cm³) or stainless steel (7.93g/cm³), while the density of carbon fiber composite materials is only 1.6-1.8g/cm³ (carbon fiber filament density 1.8g/cm³, resin matrix 1.2g/cm³).

Taking a 12L cylinder as an example, the volume of a steel cylinder is about 12L (including the curvature of the body); the mass of the steel material = (volume × density) - the volume of the internal cavity.

Measurements show that the mass of the steel in a steel cylinder body is approximately 12-15kg, while the mass of the carbon fiber layer in a carbon fiber cylinder is only 2-3kg, and the aluminum alloy liner (12L volume) mass is approximately 1.5kg, naturally resulting in a lower combined empty weight.

2. Structural Design Optimization

Steel cylinders have a uniform wall thickness design (e.g., 4.2mm wall thickness for a 12L steel cylinder), while carbon fiber cylinders use "variable thickness winding" (3mm thick in the middle of the body, 4mm thick at the shoulder), reducing material usage while ensuring a 200bar pressure resistance (ISO 11439 standard).

The carbon fiber 12L cylinder launched by the Italian company Cressi in 2023, through topology optimization design, reduced the empty weight to 7.8kg (an industry record), which is 8.2kg lighter than traditional steel cylinders.

Surface Preparation

Shore Diving

Shore diving requires divers to carry the cylinders themselves from their vehicles to the water entry point, a distance usually between 50-200 meters, across terrains including sandy beaches, reefs, or jungle trails.

1. Measured Carrying Distance and Time Consumption

The Florida Peninsula Diving Association in the United States tracked 100 shore dives in 2023 (12L cylinders):

• 50m sandy beach: Steel cylinders took an average of 2 minutes 10 seconds, with a step frequency of 110 steps per minute (normal walking is 130 steps); carbon fiber cylinders took 1 minute 30 seconds, with a step frequency of 125 steps.

• 100m mixed terrain (beach + gravel): Steel cylinders took 4 minutes 50 seconds, with a peak heart rate of 105 bpm; carbon fiber cylinders took 3 minutes 20 seconds, with a heart rate of 98 bpm.

• 200m jungle trail: Steel cylinders required one 30-second rest break midway, while carbon fiber cylinders required no rest.

2. Detailed Differences in the Donning Process

Donning a cylinder involves three steps: strap adjustment, valve alignment, and weight belt pre-hanging.

A diving center in Phuket, Thailand, recorded beginner operations (10L cylinders):

• Steel cylinder (empty weight 13kg): The strap buckle error rate was 12% (due to the center of gravity being too far back, making it difficult to position), and it took an average of 45 seconds to align the valve with the regulator interface.

• Carbon fiber cylinder (empty weight 7kg): The buckle error rate was 3%, and the alignment time was 28 seconds.

• Twin-tank system (main tank + deco tank): The steel combination (e.g., 12L+6L) had a total empty weight of 23kg, with a 40% probability of requiring assistance from others while donning; the carbon fiber combination (12L+6L) had a total empty weight of 15kg, with the assistance probability dropping to 10%.

3. Environmental Factors Magnifying Weight Impact

• High temperature (＞35℃): At shore diving sites for Red Sea liveaboards, the surface temperature of steel cylinders reached 50℃. The burning sensation on the palms when carrying them barehanded caused a 15% drop in grip strength, increasing the risk of dropping (measured dropping probability was 0.8 per hundred for steel and 0.2 for carbon fiber).

• Low temperature (＜5℃): During shore diving in Tromsø, Norway, condensation and ice formed on the metal surface of steel cylinders, resulting in a 25% probability of glove slipping during handling for steel, while carbon fiber cylinders (with composite anti-slip texture) had only a 5% probability.

Boat Diving

1. Collaboration Needs for Boarding and Disembarking

Dive boat logs from Phuket, Thailand (500 boat dives in 2023):

Cylinder Type	Independent Boarding Success Rate	Probability of Needing 1 Person's Help	Probability of Needing 2 People's Help	Average Time (seconds)
12L Steel	65%	25%	10%	40
12L Carbon Fiber	92%	7%	1%	28
18L Steel	30%	50%	20%	65
18L Carbon Fiber	78%	20%	2%	38

Note:

18L cylinders are mostly used for technical diving.

During boat diving, additional fixation for the decompression valve is required.

Steel cylinders are more prone to shifting during boat rocking due to their weight (fixation strap loosening probability: 8% for steel, 2% for carbon fiber).

2. Deck Space and Weight Distribution

Diving boat decks often pile up cylinders, weight belts, masks, and other gear.

Measurements from an Italian Blue Grotto liveaboard (20 passengers):

• A 12L steel cylinder occupies 0.12㎡ (including strap expansion), while a carbon fiber cylinder occupies 0.09㎡. When carrying 8 cylinders on the same boat, carbon fiber cylinders save 0.24㎡ of space (allowing for 2 more sets of spare regulators).

• When stacked 3 layers high, the probability of the bottom steel cylinder deforming due to pressure from the upper layers is 5% (long-term compression), while it is 0% for carbon fiber cylinders (high compression strength of composite materials).

Different Divers

1. Beginner Divers

Diver magazine tracked 50 beginners in 2022 (first shore dive):

• When carrying a 12L steel cylinder, the error rate of the bending to pick up the cylinder movement (such as lumbar hyperextension) was 35%, leading to brief back pain; the error rate for the carbon fiber cylinder group was 10%.

• During the weight belt pre-hanging phase, the weight of the steel cylinder caused the body to lean forward, with an 18% probability of the weights falling off; for the carbon fiber cylinder group, it was 5%.

• Psychological aspect: Beginners self-rated their "carrying pressure" (on a scale of 1-10), with steel cylinders averaging 7.5 points and carbon fiber cylinders 4.2 points. Those with higher pressure values saw a 12% drop in focus during the subsequent dive.

2. Technical Divers

Technical diving commonly uses twin-tanks (main tank + deco tank), and the total weight difference expands with capacity:

• 12L main tank + 6L deco tank: The total empty weight of the steel combination is 15+10=25kg, while the carbon fiber combination is 8+5=13kg, a difference of 12kg.

• 18L main tank + 12L deco tank: The steel combination is 27+18=45kg, while the carbon fiber combination is 15+10=25kg, a difference of 20kg.

A team of Norwegian technical divers measured donning twin-tanks:

• The steel combination requires step-by-step donning (main tank first, then deco tank), with a total time of 3 minutes 10 seconds; the carbon fiber combination can be donned simultaneously, taking 2 minutes 05 seconds.

• When securing twin-tanks on a deck, steel cylinders require 2 fixation straps (each with a 150kg load capacity) due to their weight, whereas 1 strap (100kg load capacity) is sufficient for carbon fiber cylinders.

Special Environments

1. Reef Area Shore Diving

Reef area on Panglao Island, Philippines (entry point requires climbing slippery reefs):

• Steel cylinders have a smooth surface (paint coating), with a 22% probability of hand slipping during transport; carbon fiber cylinders (with diamond anti-slip texture on the surface) have a hand slip probability of 5%.

• The probability of a steel cylinder denting after colliding with a reef is 15% (stress concentration at the dent increases the failure rate in subsequent inspections by 8%); for carbon fiber cylinders (high impact resistance), the dent probability is 2%.

2. Cold Region Boat Diving

Winter boat diving in Newfoundland, Canada (-5℃):

• Metal parts of steel cylinder valves freeze, with an average switching time of 12 seconds (carbon fiber cylinder valves are plastic-wrapped and do not freeze at -2℃, taking 3 seconds).

• Divers wearing thick gloves operate steel cylinder strap buckles with a 30% error rate; carbon fiber cylinder straps (with enlarged buckle rings) have a 10% error rate.

Experience from Norwegian ice divers shows that leaning the body forward by 15° when carrying a carbon fiber cylinder can reduce lumbar stress by 10% (EMG monitoring).

Ascent and Return Journey

Hiking Back

Feedback from divers at the reef area of Panglao Island, Southeast Asia (30m hike):

• Steel cylinder group (12L): 60% experienced shoulder pain, with an average return time of 5 minutes 10 seconds, of which 20% slowed down due to pain.

• Carbon fiber group (12L): The proportion of shoulder pain was 20%, the time was 4 minutes 20 seconds, and there was no deceleration.

Shore diving route at the Great Barrier Reef, Australia (100m sandy beach):

The return heart rate of the steel group was 12 bpm higher than at entry, while the carbon fiber group was only 5 bpm higher.

Boat Diving Cleanup

Measured deck cleanup for an Italian Blue Grotto liveaboard (20 passengers):

• A 12L steel cylinder occupies 0.12㎡ (including strap expansion), with a 5% probability of the bottom layer deforming when stacked 3 layers high (long-term compression); a carbon fiber cylinder occupies 0.09㎡ and shows no deformation when stacked 5 layers high.

• There is a 15% probability of steel cylinder valves denting from hitting the gunwale (failure rate in subsequent inspections increases by 8%), while the dent probability for carbon fiber cylinders (high impact resistance) is 2%.

Special Scenarios

1. Deep Diving (＞40m)

Data from a Norwegian technical diving team (dive depth 55m, 120 minutes):

The steel group frequently adjusted their breathing due to buoyancy fluctuations, resulting in an 8% increase in gas consumption and an additional 5 minutes of decompression stop time (requiring 1.2L more deco gas);

The carbon fiber group had no such issues, with a 10% increase in decompression efficiency.

2. High Temperature Environment (＞35℃)

Measured on a Red Sea summer liveaboard (40℃):

The surface temperature of steel cylinders reached 55℃, and their empty weight increased by 0.2kg due to thermal expansion (weight error);

The surface of carbon fiber cylinders was 42℃, and the weight remained stable.

90% of Egyptian divers choose carbon fiber cylinders to cope with high temperatures and avoid repeated weight adjustments.

3. Low Temperature Environment (＜5℃)

Winter shore diving in Newfoundland, Canada (-5℃):

The empty weight of steel cylinders increased by 0.3-0.5kg (metal contraction), requiring temporary weight adjustments;

The weight change of carbon fiber cylinders was ＜0.1kg, with no operational delays.

Diving Performance

Steel cylinders (such as the 300bar Faber FX) have a gas storage capacity 50% higher than 200bar carbon fiber cylinders, making them suitable for 60m tech diving, but they weigh 7kg more when empty (16kg vs. 9kg);

Carbon fiber cylinders have a +0.5kg positive buoyancy when full (measured in the Norwegian Arctic), reducing weights by 1.5kg, but their brittleness increases at <4℃.

United States NAUI Instructor Certification:

With the same gas consumption, steel cylinders provide an extra 15 minutes of safety stops.

Tests at the Great Barrier Reef, Australia, show that the usable gas volume of carbon fiber actually decreases below 50 meters.

Endurance

Depth

According to Boyle's Law, for every 10 meters of depth increase, the pressure increases by 1bar, and the gas volume is compressed to 1/(depth+10) times the original.

• Expansion Control of Steel Cylinders

  Steel cylinders have a wall thickness of 6-8mm (300bar class), with strong compression resistance and a stable gas expansion rate. During a 60m tech dive in the Egyptian Red Sea (7bar pressure), a 12L steel cylinder at 300bar stores 3600L of gas. Upon release, it expands to 3600×(7+1)=28800L (theoretical value). In practice, due to regulator decompression, the usable gas volume converted to depth is 3600÷(7+1)=450L (equivalent surface air volume), supporting 450÷30 (consumption) = 15 minutes of pure descent, plus a 15-minute safety stop for a total of 30 minutes (US NAUI Instructor Log).

• Depth Penalty of Carbon Fiber

  Carbon fiber cylinders have a wall thickness of 3-4mm (200bar class). At a depth of 50 meters (6bar pressure), the gas expansion rate is 12% higher than that of steel cylinders (2022 test at the Great Barrier Reef, Australia). A 12L carbon fiber cylinder at 200bar stores 2400L of gas. At 50 meters, the equivalent surface gas volume = 2400÷(6+1)≈342L. In reality, because the expansion rate exceeds standards, the usable gas volume is only 342×0.82=280L (18% less). With a gas consumption of 30L/minute, the endurance is shortened to 9 minutes of pure descent + 9 minutes of safety stop.

Depth Comparison Table (12L cylinder, consumption 30L/minute):

Depth (m)	Pressure (bar)	Steel 300bar Equiv. Gas (L)	Carbon Fiber 200bar Equiv. Gas (L)	Steel Endurance (min)	Carbon Fiber Endurance (min)	Case Source
30	4	3600÷5=720	2400÷5=480	24	16	Cancun Recreational Dive
50	6	3600÷7≈514	2400÷7≈342 (Actual 280)	17	9	GBR Test
60	7	3600÷8=450	2400÷8=300	15	10	Red Sea Tech Dive Record

Environment

• Steel Cylinder Advantage in Cold Water (<4℃)

  During ice diving in Svalbard, Norway (-Arctic-) (water temp 2℃), steel cylinders (such as the British Rigid Dive 12L 300bar) have stable gas density, with a consumption of 28L/minute (close to surface); carbon fiber cylinders have a 15% increase in second-stage exhaust resistance due to resin hardening in the cold, causing consumption to rise to 32L/minute. For the same stored gas volume, endurance is shortened by 12% (2021 data from the Norwegian Arctic Expedition Team).

• Pressure Stabilization of Steel in Strong Currents (>2 knots)

  During a strong current dive in Cancun, Mexico (flow rate 2.5 knots), divers consume 35L/minute fighting the current. A steel cylinder at 300bar stores 3600L, with an endurance of 3600÷35≈102 minutes; a carbon fiber cylinder at 200bar stores 2400L, with an endurance of 2400÷35≈68 minutes. US PADI instructor logs show that 90% of divers in strong current zones prioritize steel cylinders to avoid the risk of having to surface mid-way to swap gas.

• Micro-crack Leaks from Impact

  2022 data from a Phuket, Thailand repair station: 30% of carbon fiber cylinder repairs are due to micro-cracks from impacts with coral or wrecks. Although they don't burst, they suffer from slow leaks (0.5L/minute). For a 12L carbon fiber cylinder at 200bar storing 2400L, if the leak rate is 0.5L/minute, the effective gas volume after 45 minutes of diving is only 2400-45×0.5=2377.5L, a 1.8% reduction in endurance (seems small, but could mean a 5-minute difference in safety stop during deep dives).

Steel cylinders have high impact resistance.

In a wreck diving case in Florida, steel cylinders only had paint layers peel off after impact, with no records of leaks.

Buoyancy

Weighting

Weighting = drysuit negative buoyancy + cylinder net negative buoyancy (or minus positive buoyancy).

Measured weighting changes by international divers:

• Tropical Shallow Diving (within 30m)

  At Tubbataha Reef in the Philippines, a diver wearing a 3mm wetsuit (2kg negative buoyancy) using a 12L steel cylinder (-4kg net negative buoyancy) requires weighting = 2+4=6kg; switching to a carbon fiber cylinder (+3kg net positive buoyancy), weighting = 2-3=-1kg (meaning 1kg should be removed, but in practice, due to the weight of regulators and accessories, the final weighting is 0kg). A British BSAC survey shows that steel cylinder users carry 1.5-2kg more weight on average than carbon fiber users, and air consumption increases by 8% due to the extra load.

• Cold Water Drysuit Diving (<4℃)

  During ice diving in Svalbard, Norway, the drysuit negative buoyancy is 8kg. A steel cylinder (-4kg) requires weighting of 8+4=12kg; a carbon fiber cylinder (+0.5kg positive buoyancy) requires weighting of 8-0.5=7.5kg, carrying 4.5kg less lead, which reduces energy consumption during descent by 15% (2021 data from the Norwegian Arctic Expedition Team).

• Strong Current Zones (>2 knots)

  During a strong current dive in Cancun, Mexico (flow rate 2.5 knots), steel cylinders are stable due to their weight; with 6kg of weighting, body deviation is <0.5 meters. With a carbon fiber cylinder and 0kg of weighting, the current easily pushes the diver off route, requiring an additional 2kg of weights or fixation straps (US PADI Instructor Log).

Scenario	Steel Net Buoyancy (12L/300bar)	Carbon Fiber Net Buoyancy (12L/200bar)	Weighting Diff (same -5kg drysuit)	Air Consumption Increase	Case Source
Tropical Shallow (30m)	-4kg	+3kg	Steel 7kg more	+12%	Philippines Diving Club
Arctic Ice Dive (2℃)	-4kg	-1.2kg (Cold water correction)	Steel 2.8kg more	+15%	Norwegian Arctic Team
Strong Current (3 knots)	-4kg (Stable)	+3kg (Drifts easily)	Carbon fiber needs 2kg fix	+10%	US PADI Instructor Log
Cave Dive (20m)	-4kg (Impact resistant)	+3kg (30% collision leak)	Steel: no buoyancy fluctuation	-	Mexico Cave Diving Assoc

In a 2019 upwelling accident at Phi Phi Island, Thailand, a steel cylinder diver with high weighting (8kg) was easily pulled down by negative buoyancy after being washed away by the current, and was pulled back by a companion using spare lead blocks;

A carbon fiber cylinder diver was washed to the surface due to positive buoyancy (0kg weighting) and swam back independently.

Durability

Damage Resistance

1. Impact

• Steel Cylinder Impact Case: Wreck dive record in Florida (water depth 40m): After a 12L steel cylinder (empty weight 16kg) hit a coral reef, only the paint layer peeled off, and the hydro-test showed no leakage (British BSAC diver log). British HSE statistics: In cave diving (Mexico cenotes), the structural damage rate due to rock wall collisions for steel cylinders is 0.1 per thousand dives.

• Carbon Fiber Cylinder Impact Risk: 2022 data from a Phuket, Thailand repair station: Among the carbon fiber cylinder repair cases received, 30% had micro-cracks (0.5-2mm long) due to impacts with coral or wrecks, of which 5% of the cracks extended to the fiber layer, requiring disposal. Mexico Cave Diving Association report: During a cave dive in 2021, a carbon fiber cylinder (9kg empty weight) hit a rock wall, causing a micro-crack leak rate of 0.3L/minute. The diver made an emergency ascent after 60 minutes due to a sudden buoyancy drop (from +3kg to +1.5kg).

2. Chemical Corrosion

• Steel Corrosion Patterns: The British Columbia Coast Guard in Canada tracked coastal diving steel cylinders: every 5 years, corrosion adds 0.2-0.5kg in weight (e.g., a 12L cylinder increased from 16kg to 16.3kg). Most corrosion occurs at the valve interface and can be restored for sealing by grinding away the rust (without affecting gas storage). British HSE allows steel cylinders with a corrosion depth ≤0.1mm to continue to be used; those exceeding the limit must have the shell replaced.

• Carbon Fiber Chemical Corrosion: Feedback from Red Sea divers: Oxybenzone in sunscreen will penetrate the carbon fiber resin coating. After 5 years, the coating peeling area reaches 10%, and the resin absorbs water, increasing weight by 0.1kg (empty buoyancy drops from +3kg to +2.7kg). The EU requires carbon fiber cylinders to be coated with an anti-corrosion layer every 2 years (cost $50); otherwise, gas storage efficiency drops by 10% after 10 years (detection data from a Phuket, Thailand repair station).

Maintenance

1. Steel Cylinder Maintenance

• Inspection Cycle and Items: British HSE mandates that steel cylinders undergo a hydrostatic test every 5 years (test pressure = 1.5 × working pressure). After the test, the shell must be visually inspected for dents and rust, and valve sealing is checked using helium leak detection (leak rate <1×10⁻⁶ mbar·L/s). Italian Cressi HP steel cylinder maintenance manual: Salt must be rinsed off after every dive, and valve O-rings should be lubricated every 2 years (cost $5).

• Maintenance Cost: US NAUI instructor data: The total 5-year maintenance cost for a 12L steel cylinder is about $30 (including testing and lubrication), accumulating to $150 over 30 years, which is 50% of the purchase cost (approx. $300).

2. Carbon Fiber Cylinder Maintenance

• Inspection Cycle and Items: EU EN144-3 requires carbon fiber cylinders to undergo ultrasonic testing every 2 years (scanning wall thickness and micro-cracks) and resin aging tests every 5 years (using a durometer). German Catalina CX maintenance guide: Check the surface for scratches after every dive (depth >0.5mm requires recoating) and perform an eddy current test every 2 years (checking for fiber breakage).

• Hidden Costs: Case from a Red Sea diver: Because the anti-corrosion layer was not regularly applied, the gas storage efficiency of the carbon fiber cylinder dropped by 5% after 5 years, equivalent to storing 120L less air in a 12L cylinder (at 200bar), requiring dives to end early. Phuket, Thailand repair station data: A single ultrasonic test for a carbon fiber cylinder costs $40, and the coating maintenance fee is $50. The cumulative maintenance cost over 10 years exceeds $500 (close to the price of a new cylinder).

Failure Rate

Failure Type	Steel Rate (per 1000 dives)	Carbon Fiber Rate (per 1000 dives)	Case Source
Impact Micro-crack	0.1	3.2	Mexico Cave Diving Assoc
Chemical Corrosion Leak	0.05 (Rust)	1.5 (Coating aging)	Phuket Repair Station
Valve Seal Failure	0.2	0.8	British HSE Report
Extreme Env (<4℃) Brittleness	0	0.5 (Cracking)	Norwegian Arctic Team

The British Columbia Coast Guard in Canada strictly enforces 5-year hydro-tests.

Steel cylinders from the 1980s have had a 0 failure rate during a 30-year service life, proving that "regular detection = extended life."

The EU requires ultrasonic testing every 2 years, which can reduce the scrap rate caused by micro-cracks from 60% to 30% (German TÜV certification data), but high-frequency testing still pushes up maintenance costs.