How to Store a Scuba Diving Tank | Long-Term Storage Tips

The cylinder should be stored upright, with pressure maintained at 300-500 PSI to isolate moisture.

Place in a cool and dry place.

Be sure to have a visual inspection every year and a hydrostatic test every five years.

Preparation and Pressure

For long-term storage of cylinders, the internal pressure needs to be maintained at 300-500 PSI (20-35 Bar).

This pressure level is sufficient to resist the external atmospheric pressure of 14.7 PSI, preventing moisture from seeping in through the valve seals.

For 6061-T6 aluminum alloy or Chromoly steel cylinders, residual salt (sodium chloride) on the body must be thoroughly removed before storage, and ensure the valve O-ring is not damaged.

It is recommended that the ambient temperature be controlled between 15°C - 25°C to slow down the metal oxidation reaction.

Pressure Standards

According to the Compressed Gas Association (CGA) P-1 guidance document, cylinders should be maintained in the range of 300 PSI to 500 PSI (20 Bar to 35 Bar) during long-term storage.

Scuba cylinders are filled with multi-stage filtered ultra-dry air, with water content typically below 0.02 mg/L, whereas the humidity of ambient air usually leads to oxidation reactions on the internal metal walls within weeks.

Storage Pressure Status	Pressure Value (PSI/Bar)	Physical Manifestation and Technical Consequences
Recommended Storage Range	300-500 PSI / 20-35 Bar	Maintains a positive pressure differential to resist external moisture while minimizing static stress on the cylinder walls and valve components.
Completely Empty State	0 PSI / 0 Bar	Internal connection to the atmosphere. Due to the breathing effect caused by temperature differences, moisture enters the bottle, causing aluminum bottles to produce aluminum oxide powder or steel bottles to produce iron oxide scale.
Critical Low Pressure	< 100 PSI / 7 Bar	Insufficient pressure gradient. Under extreme environmental temperature differences, local negative pressure may be generated inside the bottle, drawing in trace condensation from the valve threads.
Working Pressure Storage	2250-3442 PSI / 155-237 Bar	Causes the Burst Disc to undergo continuous tensile stress, shortening its metal fatigue life and increasing the probability of O-ring deformation.

For a common AL80 (80 cubic foot aluminum tank), its rated working pressure is typically 3000 PSI, while its burst disc rupture threshold is set between 140% to 166% of the working pressure, which is approximately 4200 PSI to 5000 PSI.

If the cylinder is stored at full pressure for a long time, the internal 6061-T6 aluminum alloy structure will always be in a state of high-stress tension.

In North American visual inspection standards, technicians focus on observing thread cracks in full-pressure stored cylinders because high pressure amplifies the expansion rate of microscopic defects in metal materials.

When gas is rapidly discharged from 3000 PSI, according to the Joule-Thomson effect, the temperature of the gas drops sharply.

If the discharge speed is too fast, the temperature of internal valve parts may drop below 0°C, causing moisture in the air to freeze on the valve surface or generate condensation.

If this condensation remains trapped in the valve passages, it will be pushed into the cylinder by pressure after the valve is closed, triggering pitting corrosion during storage.

The correct operation is to slowly vent at a rate not exceeding 300 PSI per minute until the pressure gauge needle stabilizes at around 400 PSI.

Physical Cleaning

Treating chemical residues on metal surfaces is the first step before long-term storage of scuba cylinders.

Whether it is a 6061-T6 aluminum alloy or Chromoly steel cylinder, after use in seawater, a large amount of sodium chloride crystals will adhere to the surface.

These salts are highly hygroscopic; even in a room with ambient humidity below 50%, salt particles will absorb trace amounts of moisture from the air to form an electrolyte solution.

For aluminum tanks, this can trigger pitting, and pits deeper than 0.015 inches (0.38 mm) may result in the cylinder being scrapped during visual inspection.

It is recommended to completely soak the cylinder in warm fresh water at 30°C to 40°C for at least 15 minutes;

warm water is more effective at dissolving salt scale in the valve thread gaps.

During rinsing, pay special attention to the junction between the cylinder valve and the bottle body, where two different metals (usually a brass valve and an aluminum or steel body) meet, which is prone to electrochemical corrosion.

According to Compressed Gas Association (CGA) standards, if external corrosion of a metal cylinder results in a wall thickness reduction of more than 10% of the original design, the cylinder must be taken out of service. For common S80 aluminum tanks, if external oxidation layer peeling reaches a depth of about 1.2 mm, structural integrity is compromised.

Aluminum alloy cylinders need to air dry naturally after cleaning; do not use heating equipment to speed up drying, as excessive temperatures may affect the metal hardness of the T6 heat-treated state.

The Tank Boot must be removed before storage.

The bottom of the boot usually has drainage holes, but in actual use, fine sand and moisture accumulate between the bottom of the boot and the bottle, forming a closed high-humidity microenvironment.

For steel cylinders, this accumulated water leads to large areas of oxidation layer peeling at the bottom, and can even produce penetrating corrosion invisible to the naked eye.

Although Chromoly steel cylinders (DOT-3AA standard) have higher pressure strength than aluminum tanks, they are extremely sensitive to moisture. In environments with humidity exceeding 60%, the inner walls of steel cylinders without rust prevention treatment will show yellowish flash rust within 48 hours.

The handwheel shaft core and Burst Disc assembly are precision parts; if calcified substances are deposited inside, it will lead to poor valve switching or pressure leakage.

When cleaning the valve interface, use a clean, lint-free cotton swab to wipe the sealing surface of the DIN or Yoke interface.

It is strictly prohibited to use cleaners containing silicone oil or grease to spray internal valve passages, as these chemicals may cause combustion or even explosion under high pressure during future filling processes.

If white powdery substances are found at the valve threads, they are typical aluminum oxides, indicating that moisture has invaded the interior of the threads.

At this time, a professionally certified technician (such as a PSI-PCI certified technician) must disassemble the valve and measure it using a 3/4-14 NPSM standard thread gauge.

Steel cylinders usually adopt hot-dip galvanizing or powder coating processes.

If bubbles or cracks appear in the paint layer, they must be addressed immediately, as moisture will seep under the paint layer along the cracks for hidden corrosion.

For cylinders intended for long-term storage of more than 6 months, it is recommended to install a clean sealing plug (only for DIN valves) at the valve interface after external cleaning is completed.

This provides a physical barrier to prevent dust, mold spores, or insects from entering the interior of the valve.

If using a Yoke valve, ensure the dust cap is dry and tightly fitted.

Ideal Environment

The storage environment needs to be maintained at a constant temperature of 15°C to 22°C (60°F - 72°F), and relative humidity should be below 50%.

For every 1°C increase in temperature, the pressure inside the bottle increases by approximately 5 PSI;

severe thermal expansion and contraction will accelerate metal fatigue.

The ozone concentration in the ambient air must be below 0.05 ppm to prevent the rubber parts of the valve from becoming brittle.

Cylinders must be placed vertically on rubber or wooden pads, strictly prohibited from contacting concrete floors, and ambient air flow should be maintained above 100 FPM to prevent condensation accumulation.

Temperature and Humidity Control

During the static storage of diving cylinders, environmental thermal energy is converted into kinetic energy of the gas inside the bottle.

For an aluminum alloy cylinder with a standard working pressure of 3000 PSI (207 Bar), for every 1°C change in ambient temperature, the internal pressure will fluctuate by approximately 5.5 PSI (0.38 Bar).

If the storage environment reaches above 40°C (104°F) in summer, the internal pressure of a cylinder originally at 500 PSI residual pressure will rise to above 560 PSI.

This temperature-driven pressure cycle is called "thermal fatigue induction."

Although the value of a single fluctuation is small, over a storage cycle of several months, repeated thermal expansion and contraction will cause microscopic displacement of the lattice structure of the 6061-T6 aluminum alloy or Chromoly steel material.

Ideal temperature control standards should be strictly set between 15°C to 21°C (59°F - 70°F), and the temperature fluctuation within 24 hours must not exceed 3°C.

The table below shows the quantified impact of different ambient temperatures on the internal pressure of a standard 80 cubic foot (11.1 liter) aluminum cylinder:

Ambient Temperature (°C/°F)	Internal Pressure (PSI/Bar)	Pressure Change Percentage	Structural Safety Assessment
10°C / 50°F	2891 PSI / 199 Bar	-3.6%	Generates thermal contraction, beneficial for reducing static stress
21°C / 70°F	3000 PSI / 207 Bar	0.0% (Benchmark)	Ideal Storage Calibration Point
35°C / 95°F	3142 PSI / 216 Bar	+4.7%	Long-term excess state, accelerates sealant aging
50°C / 122°F	3294 PSI / 227 Bar	+9.8%	Near burst disc fatigue limit, strictly prohibited for long-term storage

When the relative humidity (RH) exceeds 50%, water molecules in the air begin to form an invisible monomolecular water film on the metal surface.

For DOT-3AA specification steel cylinders, this water film combines with carbon dioxide in the atmosphere to form a micro-acidic carbonic acid environment, rapidly initiating the redox reaction of iron.

In an environment with 60% RH, the pitting rate at the bottom of steel cylinders is approximately 0.05 mm per year;

when humidity rises to above 80%, the corrosion rate grows exponentially to over 0.2 mm per year.

This localized metal loss reduces the pressure-bearing capacity of the cylinder wall, leading to the cylinder being scrapped due to exceeding expansion rates during five-year hydrostatic testing.

For aluminum alloy cylinders, high humidity causes the generation of aluminum hydroxide scale on the surface.

Although this film can to some extent stop deep corrosion, in cases where water accumulates inside the Tank Boot, it creates an oxygen-deficient cell effect, inducing severe ulcer-like corrosion.

Relative Humidity (RH %)	Corrosion Risk Level	Metal Surface State Prediction	Suggested Intervention Measures
< 30%	Extremely Low	Metal surface stays dry, electronic exchange stalls	No special treatment required
30% - 45%	Low	Oxidation rate extremely slow, in passive state	Professional storage warehouse standard index
50% - 65%	Moderate	Steel cylinders begin to show scattered rust spots	Run industrial-grade dehumidifiers
> 75%	Extremely High	Water accumulates in the boot, aluminum cylinders produce white powder	Immediately change storage environment and clean the body

If the surface temperature of the storage area is lower than the air dew point, condensation droplets will form on the outer wall of the cylinder.

Assuming an indoor temperature of 25°C and a relative humidity of 70%, the dew point temperature is approximately 19°C.

If a cylinder is moved from a cool outdoor environment to indoors, or if the room cools down at night, the cylinder surface will "sweat."

Condensation will seep into the tiny gaps of the cylinder valve threads, not only leading to external rust but potentially contaminating the seal area of the bottle mouth through capillary action.

To maintain structural integrity, the storage environment must be equipped with an HVAC system with automatic adjustment functions to ensure the ambient temperature is always more than 5°C above the dew point temperature.

During frequent temperature cycles, the fit gap between the valve stem and handwheel undergoes micron-level changes.

In a stable environment of 15°C to 22°C, this physical gap can be maintained within a nominal tolerance of 0.02 mm.

If the ambient temperature is consistently higher than 30°C, the fluorocarbon O-rings inside the valve will undergo thermal degradation, and their hardness will drop by about 2-5 Shore A every 6 months.

Ozone Defense

When storing scuba cylinders long-term, Ozone (O3) molecules in the environment manifest as continuous oxidative stress damaging valve components.

Normal ozone concentration in the atmosphere is typically maintained between 0.01 ppm to 0.03 ppm, but in closed indoor environments, especially areas with electric motors, transformers, alternators, or high-voltage electronic equipment, ozone concentration will rapidly rise above 0.05 ppm.

The O-rings inside the cylinder valve are usually made of Nitrile (NBR) or Fluorocarbon (Viton); these elastomers will undergo "ozone cracking" when exposed to high concentrations of ozone.

This specifically manifests as microscopic cracks perpendicular to the stress direction on the sealing surface of the O-ring, with a depth of about 0.1 mm to 0.5 mm.

This reduces the airtightness of the valve seal, leading to a slow pressure loss of 1-3 PSI every 24 hours.

To prevent this structural damage, cylinders must be stored at least 3 meters (10 feet) away from any spark generation sources and ensure ambient ozone levels remain below the threshold detectable by human senses.

• Electronic Equipment Isolation Distance: The physical distance between cylinders and large household appliances (e.g., refrigerator compressors, dryers, air purifiers) should be maintained above 5 meters, as the electric arcs generated at startup are the main source of indoor ozone.
• Sealing Material Selection: If it is certain that the storage environment has unavoidable slight ozone exposure, it is recommended to replace all peripheral O-rings with 90 Shore A fluorocarbon material before long-term storage, as its chemical stability is more than 5 times higher than standard nitrile rubber.
• Physical Shielding: Using polyethylene (PE) plastic bags with a thickness of more than 0.1 mm to wrap the valve part can reduce air flow contact by more than 90%, thereby blocking ozone molecules from attacking the rubber polymer chains.

When storing cylinders in a garage or home workshop, the concentration of volatile organic compounds (VOCs) such as gasoline vapors, paint thinners, cleaning solvents, and lubricants is often high.

For cylinders made of 6061-T6 aluminum alloy, certain cleaners containing chlorinated solvents (e.g., trichloroethylene) can induce severe intergranular corrosion.

When the hydrocarbon concentration in the air reaches 50 mg/m³, the Powder Coating or epoxy paint on the cylinder surface will soften or even peel off, and the exposed metal substrate will rapidly undergo electrochemical reactions in moist air.

Once these chemical vapors enter the bottle and dissolve in the high-pressure air, the air the diver breathes next time will contain hydrocarbon pollutants exceeding standards, violating CGA (Compressed Gas Association) Grade E breathing air standards.

• Chemical Storage Red Line: It is strictly forbidden to store cylinders in the same ventilation cycle system as petroleum-based products (gasoline, kerosene, diesel) in open containers. Even if the physical distance is far, the diffusion of gas molecules will still cause rubber parts to swell, increasing their volume by 5% - 15% and leading to increased resistance in valve knob operation.
• Coating Protection Specification: The polyurethane coating thickness of a cylinder shell is typically between 100 microns to 200 microns. Long-term contact with acidic or alkaline volatiles will cause the coating to lose luster and produce bubbles. It is recommended to use activated carbon filtration devices in the storage area to reduce the total concentration of volatile organic compounds in the air.
• Ventilation Flow Control: The air exchange rate of the storage area should reach 4-6 times per hour, or maintain the air flow rate above 100 FPM (feet per minute), using air flow to dilute potentially accumulated chemical vapor clouds.

Batteries release trace amounts of sulfuric acid mist during charging; these acidic particles settle on the Chromoly steel or aluminum alloy surfaces of the cylinders.

For Chromoly Steel cylinders, acidic attachments accelerate the formation of Pitting Corrosion, creating rust pits deeper than 0.2 mm within a mere 6-month storage period, which is usually judged as failing in annual visual inspections (VIP).

In environments with high temperature and humidity, acid mist combines with moisture in the air to form a dilute sulfuric acid solution, causing water accumulation corrosion inside the Tank Boot at the bottom of the cylinder.

• Acid-Base Neutralization Monitoring: If the storage environment is near battery packs, pH test strips or electronic monitoring instruments should be placed near the cylinder to ensure the environment is neutral. If white or yellow powdery precipitates are found on the metal surface, wash immediately with fresh water and a weak alkaline soap solution.
• Physical Barrier Specification: It is recommended to use shelves made of high-density polyethylene (HDPE) and avoid using metal racks to prevent galvanic corrosion generated between different metals mediated by chemical spills.
• Cleaning Process: Before entering long-term storage, the outer walls and valve corners of the cylinder must be thoroughly rinsed with fresh water to remove all salt residues, as salt creates a synergistic effect with environmental chemical volatiles, increasing the corrosion rate by 3 to 10 times.

To ensure that the cylinder maintains structural integrity over a storage period of more than 12 months, the air in the storage room should ideally be multi-stage filtered to exclude particulates, oil mist, and acidic/alkaline gases.

Positioning

When storing scuba cylinders long-term, upright vertical placement is preferred.

According to CGA (Compressed Gas Association) technical standards, the metal thickness at the bottom of a cylinder is typically 12mm-18mm, while the side walls are only 4mm-8mm.

Vertical placement ensures that any condensation that may be generated inside the bottle gathers at the thickest bottom, slowing down the loss of structure due to oxidative corrosion.

Use a dedicated base or fixed rack when storing, and maintain an internal residual pressure of 300-500 psi (20-35 bar) to prevent moisture from entering through the valve.

Vertical vs. Horizontal Placement

According to manufacturing standards, the thickness of the reinforced area at the bottom of a cylinder is usually maintained between 12mm and 18mm, while the thickness of the middle section of the bottle wall is only 4.1mm to 6.4mm.

During long-term storage, gravity exerts a continuous influence on trace amounts of moisture remaining inside the bottle.

Even after strict filtration at a filling station, compressed air may still contain a tiny percentage of humidity, or generate condensation during temperature changes.

When the cylinder is placed vertically upright, this moisture will collect along the bottle walls at the thickest bottom.

Since the metal redundancy at the bottom is extremely high, even if a slight oxidation reaction occurs, it will not cause substantial damage to the overall structural integrity of the cylinder in a short time.

This method effectively coordinates with the annual visual inspection (VIP), as technicians can clearly observe the state of the bottom using an inspection light.

If oxidation scale or water accumulation occurs inside the bottle, vertical storage allows these impurities to concentrate in an area easy to clean, rather than scattering on the fragile side walls.

Evaluation Dimension	Vertical Storage	Horizontal Storage	Data and Physical Performance
Moisture Force Distribution	Concentrated in the thickened bottom area (15mm+ steel/aluminum)	Distributed along the long axis of the side wall (approx. 5mm thickness)	The metal thickness at the corrosion stress point for vertical storage is 2.5-3 times that of horizontal storage
Oxidation Contact Area	Extremely small, limited only to the diameter of the round bottom	Larger, moisture forms a long "wetting line"	The contact area of moisture with metal in horizontal storage is more than 400% higher than in vertical storage
Structural Pressure Risk	Corrosion occurs in non-main stress zones	Corrosion occurs on the side wall, affecting pressure limits	Pitting depth on side walls exceeding 0.6mm may lead to hydrostatic test failure
Valve and O-ring Pressure	Internal pressure acts uniformly on valve components	Valve may bear lateral gravity or pressure from rolling	If lateral bias occurs at valve threads, it may trigger minor leakage
Storage Density and Safety	Occupies small area, but requires physical fixation (racks/ropes)	Good stability, but occupies large floor or rack space	A 12.2-liter cylinder stored vertically needs only about 0.04 square meters

If horizontal placement is chosen, when the cylinder is placed sideways, the internal condensation will be distributed along the side walls of the body, forming a long strip of water.

Since the cylinder side walls are designed to withstand radial tension, the thickness is far lower than that of the bottom.

Long-term contact of moisture with the side walls will trigger electrochemical corrosion, manifesting as white powdery aluminum oxide on aluminum tanks and iron rust on steel tanks.

This corrosion is usually dot-like and is called "pitting."

During five-year hydrostatic testing, the detection equipment is very sensitive to the reduction in wall thickness;

any depth loss on the side walls may result in the cylinder being scrapped for failing the expansion rate test.

If a horizontally stored cylinder rolls, the kinetic energy from its dead weight of about 14 kg will impact the valve.

Valves are usually made of chrome-plated brass, which is softer than the aluminum alloy or steel of the body;

if an impact causes deformation, the cylinder will have poor sealing issues during filling or use.

In the absence of a professional cylinder rack, although horizontal placement has a low center of gravity, it leads to inconvenient inspection, and ground moisture in garages or storage rooms will penetrate the metal surface layer through the longer contact surface.

Physical Fixation

Scuba cylinders typically weigh between 12kg and 18kg when full.

For example, a common S80 aluminum alloy tank weighs about 14.3kg, while a 12-liter high-pressure steel tank weighs closer to 16kg.

Due to the slender cylindrical shape with a diameter usually around 184mm, the center of gravity is high.

Once tilted, the torque generated by gravity quickly exceeds the support force of the bottom, causing the cylinder to collapse.

When designing a fixation system, it is necessary to consider that valve handles are typically made of brass, whose impact resistance is far lower than that of the cylinder body.

If a fully loaded cylinder falls from a vertical state and hits a hard floor, the valve interface may sustain an instantaneous impact force of over 5000 Newtons, which is enough to damage the valve or cause seal failure.

Therefore, setting up a specialized Tank Rack in a garage or storage room is standard operation.

These racks are typically made of high-density polyethylene (HDPE) or preservative-treated wood to avoid metal-to-metal contact triggering corrosion.

The rack height should be installed at 60% to 70% of the total cylinder height. For a standard cylinder 650mm tall, the fixation point should be between 40cm and 45cm off the ground. This height setting effectively balances the center of gravity, maintaining stability even when subjected to a lateral push of about 15 degrees, relying on the reverse tension from the fixation point.

In wall-mounting solutions, using nylon webbing or rubber bungee cords with stainless steel hooks is the most common practice.

Nylon webbing width is recommended to be no less than 25mm.

Webbing of this specification typically has a breaking strength of over 500kg, enough to handle the pressure from multiple cylinders tilting simultaneously.

When installing, maintain a gap of 2cm to 5cm between each cylinder to prevent the bodies from being scratched by collisions with each other.

If the storage area is in an environment with slight vibrations, such as a basement, it is recommended to use webbing with a Cam Buckle to adjust tension, ensuring the cylinder fits tightly against the wall bracket.

For aluminum tanks, the contact surface of the fixation system should avoid unpainted carbon steel to prevent galvanic corrosion between two metals with large potential differences when air humidity exceeds 60%, leading to pits on the aluminum surface.

In commercial dive centers or equipment rooms, a common fixation density is 4 to 5 cylinders per meter of wall space. If using modular plastic mounts, their bases are usually designed with ventilation holes to ensure an air circulation layer of about 1cm between the bottom of the cylinder and the ground, reducing the probability of condensation accumulating inside the base.

The Tank Boot is an auxiliary component in vertical fixation systems. A qualified boot should have multiple drainage grooves and ventilation holes.

A completely sealed boot after six months of storage can accumulate enough moisture to cause a rust layer up to 0.2mm deep at the bottom of a steel tank.

To enhance stability, it is recommended to choose boot models with widened side designs, or square/hexagonal boots; these shapes significantly increase the torsional resistance of the cylinder on the ground.

When placing cylinders, ensure the boot is in full contact with the ground;

do not place on carpets or soft mats, as an unstable support surface will cause the cylinder to wobble slightly, wearing down the protective coating at the bottom over time.

For multi-bottle storage, tiered or honeycomb fixation layouts can improve space utilization. In this layout, the valve orientation of the rear row should stagger with the front row, ensuring that any cylinder's handwheel or dust cap can be operated freely without moving others. The Visual Inspection Protection (VIP) sticker for each cylinder should always face outward for easy monthly status checks.

In fixation solutions, while valve caps are not part of the support structure, threaded protection covers should be installed on valve interfaces during storage to prevent dust or insects from entering.

If there is vehicle access in the storage environment (e.g., a garage), blocks or guardrails at least 10cm high should be set in front of the rack.

This is to prevent accidental vehicle collisions from causing the rack to collapse.

If the rack material is metal, it must be powder-coated to isolate environmental salts, especially in coastal areas where chloride ions will quickly penetrate ordinary spray paint, triggering corrosion of the fixed structure.

If horizontal stacking is required (typically limited to cases with extremely restricted space), recessed anti-roll racks must be used. Rubber gaskets no thinner than 10mm should be placed between each layer of cylinders, and the stack height must not exceed three layers. When stored horizontally, valve handles should point in opposite directions alternately to balance the center of gravity and prevent valve components from colliding with each other.

For individual users, a simple frame can be made using 2x4 inch pressure-treated lumber when building a fixation system.

Applying 3mm thick Neoprene pads to the inside of the frame can further increase friction and reduce cylinder displacement.

Fixing bolts should penetrate the wall at least 50mm to ensure the bracket itself does not fall off the wall if a cylinder accidentally tips over.

Inspection and Maintenance

Before long-term storage, please confirm that the VIP annual inspection sticker and the five-year Hydrostatic Test stamp are within their validity periods.

Check the valve O-ring (typically #011 for Yoke interfaces and #014 for DIN) for hardening or cracks.

A residual pressure of 300-500 psi (20-35 bar) must be kept inside the bottle to maintain internal positive pressure and prevent moisture from the air from seeping in.

Steel cylinders (e.g., Steel 100/120) need special attention for moisture in the boot, while aluminum tanks (e.g., AL80) should be checked for white oxidation powder at the valve threads.

Cylinder Maintenance

The current scuba cylinder market is primarily dominated by two materials:

6061-T6 aluminum alloy and Chromoly Steel 4130, following different Department of Transportation (DOT) specifications, such as DOT-3AL for aluminum and DOT-3AA for steel.

The material characteristics of aluminum tanks mean their wall thickness far exceeds that of steel tanks.

Taking the AL80 specification as an example, its nominal working pressure is 3000 psi (207 bar), and wall thickness is typically between 12 mm and 15 mm.

Aluminum rapidly generates a dense aluminum oxide film upon contact with air; this self-protection mechanism performs well in freshwater but is prone to electrochemical corrosion at the neck threads if the sacrificial brush effect at the valve fails in high-salinity seawater.

Before storage, use a magnifying glass of 20x or more to check the first to third threads of the neck, confirming no fine cracks caused by stress fatigue.

6351 aluminum alloy cylinders manufactured before 1990 must undergo mandatory Eddy Current Testing, whereas the 6061-T6 material popularized after 1990 mainly relies on annual visual inspections (VIP) to confirm whether internal walls have pitting pits deeper than 0.015 inches (0.38 mm).

Due to high material strength, steel cylinders can withstand higher working pressures.

Common ones include 3442 psi (237 bar) High Pressure Steel (HP Steel) and 2400 psi (165 bar) Low Pressure Steel (LP Steel).

Internal walls of steel tanks are usually phosphated or galvanized to enhance corrosion resistance.

Even 5 ml of residual seawater in a 3000 psi high-pressure environment will lead to large areas of iron oxide (red rust) on the inner walls of a steel tank within 3 to 6 months.

This oxidation process consumes wall thickness.

Once the pitting depth exceeds 10% of the original wall thickness or reaches 0.06 inches (approx. 1.5 mm), the cylinder will fail the hydrostatic test.

When storing steel cylinders, the rubber boot must be removed, as the gap between the boot and the body attracts salt water via capillary action.

Even if it looks dry on the outside, moisture inside the boot causes ring-like corrosion at the bottom, potentially leading to metal peeling in severe cases.

Physical and Maintenance Parameter Comparison	Aluminum Alloy Cylinder (DOT-3AL)	Chromoly Steel Cylinder (DOT-3AA)
Standard Working Pressure	3000 psi / 207 bar	2400 psi (LP) - 3500 psi (HP)
Dead Weight (AL80 / ST100)	Approx. 31.3 lbs (14.2 kg)	Approx. 33.0 lbs (15.0 kg)
Empty Bottle Buoyancy	+4.4 lbs (Positive)	-2.0 lbs (Negative)
Typical Wall Thickness	0.48" - 0.60"	0.15" - 0.25"
Corrosion Product Form	White Powder (Aluminum Oxide)	Red/Black Particles (Iron Oxide)
Thread Inspection Requirements	Focus on stress cracks and galvanic corrosion	Focus on thread wear and pitting
Clean Pickling Standards	Strong alkali forbidden, use neutral or weak acid	10% phosphoric or citric acid solution commonly used

In long-term storage maintenance, as the air pressure drops in aluminum tanks, their buoyancy changes from negative to significantly positive.

If the air is empty during storage, the physical center of gravity shifts, affecting stability when placed vertically.

For steel cylinders sprayed with polyurethane coating, check for bubbles in the coating before storage; these bubbles often enclose ongoing chemical corrosion.

If the coating is damaged, sand with fine sandpaper until the raw metal is exposed and apply a specialized zinc-rich primer for local repair.

Aluminum tanks do not require external coating maintenance, but attention is needed for stamped text on the neck; if too much salt accumulates at the stamp, it accelerates stress concentration.

In the Hydrostatic Test process, the cylinder is placed inside a steel water jacket filled with water.

A high-pressure pump increases the internal pressure to 1.67 times the working pressure (5/3 rule).

For example, a 3000 psi aluminum tank needs to be pressurized to 5000 psi.

The equipment records the Total Expansion of the body via pressure sensors and the Permanent Expansion after pressure release.

If the permanent expansion exceeds 10% of the total expansion, it indicates the metal material has undergone plastic deformation, and the cylinder must be scrapped.

For steel cylinders, if stamped with a "+" sign, it indicates the bottle can be used with a 10% overpressure for the next five years (i.e., a 2400 psi bottle can be filled to 2640 psi).

However, in pre-storage checks, if any mechanical scratch deeper than 0.02 inches is found, this overpressure permit automatically expires.

Valves and Sealing Components

The physical structure of cylinder valves is divided into K-valves (single outlet lever type) and DIN valves (screw-in type).

In the field of high-pressure cylinders, the common 3/4"-14 NPSM neck thread fits the vast majority of North American manufactured steel and aluminum tanks.

Before long-term storage, it must be confirmed that the #214 O-ring (for 3/4 inch threads) at the junction of the valve body and the neck shows no signs of extrusion or aging.

If using European standard M25x2 metric threads, a corresponding metric O-ring must be matched.

When disassembling the valve for inspection, observe the smoothness of the Valve Stem and ensure the internal Teflon valve seat gasket has not been excessively compressed and deformed.

For Yoke interfaces (CGA 850 standard), the #011 O-ring at the interface withstands 3000 psi (207 bar) of static pressure.

It should be removed before storage to see if its cross-section has turned from round to flat.

Shore Hardness is typically chosen between 70 and 90 to ensure anti-extrusion capability under high pressure.

• O-ring Specifications and Material Compatibility:
  • #011 O-ring: Used for Yoke regulator interfaces; Nitrile/Buna-N is common in air environments, but must be replaced with Fluorocarbon (Viton/FKM) for Nitrox.
  • #014 O-ring: Used inside the DIN valve port (G5/8" thread); the O-ring at this position bears force uniformly when screwed into the regulator, but is also prone to salt crystal accumulation.
  • #214 O-ring: Located at the valve-to-mouth junction, the final line of defense against neck leaks.
• Lubricant Use Standards:
  • Strictly forbid any petroleum-based grease, as it will trigger violent oxidation or even combustion in high-pressure oxygen environments.
  • Recommend using perfluoropolyether (PFPE) lubricants like Christo-Lube MCG 111 or Molykote 111.
  • Application amount should be controlled so only a slight luster appears on the O-ring surface; excessive grease attracts dust and metal chips, causing wear during valve rotation.

The Burst Disc assembly consists of a plug body, gasket, and metal diaphragm.

For an AL80 aluminum tank rated at 3000 psi, the burst disc typically bursts between 4000 psi and 4500 psi.

During long-term storage, if environmental humidity fluctuates significantly, microscopic cracks (stress corrosion cracking) invisible to the naked eye may occur on the metal diaphragm surface.

When installing the burst disc plug, a torque wrench must be used to precisely control between 45 and 50 inch-pounds;

insufficient torque leads to leakage, while excessive torque crushes the copper gasket, leading to premature diaphragm fatigue.

According to CGA recommendations, even if the burst disc looks intact, it should be mandatorily replaced during every five-year hydrostatic test.

If turning the handwheel requires force to stop the air, the valve seat gasket is worn.

Before storage, soaking the valve in an ultrasonic cleaner mixed with warm water and a citric acid solution (5%-10% concentration) for 15 to 20 minutes effectively removes internal salt stains and calcified substances.

After cleaning, internal water channels must be blown dry with nitrogen or filtered dry compressed air, as residual moisture will cause internal pitting during long-term storage.

For DIN valves, the thread depth of 5 or 7 turns determines the connection strength with the regulator.

Check for wear or deformation at the top of the threads to ensure Pitch Gauge results comply with G5/8" standards.

Component Name	Maintenance Standard/Data	Operation Instruction
Valve Neck Torque	50 - 85 ft-lbs (68 - 115 Nm)	For 3/4"-14 NPSM threads, use a professional torque wrench.
Oxygen Compatibility	Above 40% Oxygen Concentration	Must use Viton O-rings and undergo degreasing.
Valve Handwheel	Rotate clockwise to natural stop	Do not overtighten the valve during storage to prevent permanent seat gasket deformation.
Burst Disc Material	Coated Copper or Stainless Steel	Match according to valve manufacturer standards; different pressure ratings cannot be mixed.

The resistance felt in the cylinder handwheel usually comes from the friction of internal Packing gaskets.

If there is a slight leak under the handwheel, it is usually a loose Packing Nut or aged internal plastic gaskets.

In cases of storage exceeding six months, it is recommended to rotate the handwheel to fully closed and then back off 1/4 turn.

This prevents internal metal parts from seizing due to long-term cold welding effects or subtle oxidation.

At the same time, observe if there are pale green verdigris traces at the valve exhaust port; this is usually residue from seawater backing into the cylinder.

Once found, the valve must be disassembled to check the internal damage to the cylinder wall.

The Yoke slot plane of the cylinder valve must remain absolutely flat.

Use a metal ruler to check this plane.

If a depression or scratch exceeding 0.005 inches is found, the regulator O-ring will not form an effective sealing pressure on this plane, leading to frequent leaks during dives.

For this wear, the only solution is repair via professional grinding tools or replacement of the valve body.