Steel scuba tanks are divided into HP High-Pressure Tanks (working pressure 200-300bar, e.g., a 300bar/12L tank stores 3600L of air) and LP Low-Pressure Tanks (150-200bar, suitable for beginners). An empty tank weighs 18-25kg (for the 12L model), and after being fully filled with air, its buoyancy is nearly 0 due to the light weight of air, requiring ballast to offset; rust on old tanks may increase their weight by 2-5%, disrupting the buoyancy balance. Key maintenance points: conduct a thickness test every 5 years, and mandatory scrapping if the wall thickness is ＜6mm.

Mainstream Types of Steel Tanks

Steel scuba tanks are mainly divided into two categories: general-purpose high-pressure tanks (e.g., Type 3442 complying with the EN 12245 standard) and scuba-specific tanks. The former has a diameter of 7.6cm, a height of 66cm, a water capacity of 6.8L, a working pressure of 200bar, and an empty weight of about 15kg; the latter is mostly a 300bar ultra-high-pressure tank, which stores 30% more air (about 8.8L) at the same size, suitable for deep diving or long-duration diving.

Two Common Steel Tanks Classified by Pressure

Currently, the most common types on the market are 200bar general-purpose high-pressure tanks and 300bar ultra-high-pressure tanks.

200bar General-Purpose High-Pressure Tank

It has a height of about 66cm and a diameter of 7.6cm, looking similar to an ordinary large cola bottle but heavier.

It weighs about 15kg, and the total weight after being fully filled with air is close to 29kg—don't underestimate this extra 14kg, as this weight becomes a "burden" underwater, but the advantage is that the air storage capacity is sufficient for daily use.

This means that when compressed air is pressurized to 200bar, it can hold 6.8L × 200 = 1360L of air (volume at 1bar pressure).

An average diver consumes about 20L of air per minute underwater (at moderate exercise intensity), and 1360L is enough for 68 minutes. As the depth increases, the pressure becomes higher and the consumption becomes faster, but it is completely sufficient for recreational diving within 30 meters and 1 hour.

This type of tank complies with the European EN 12245 standard or US DOT-3AL certification, and must pass two key tests during production:

• First is the "hydrostatic test": pressurize the tank to 330bar (130bar higher than the working pressure), maintain the pressure for 30 seconds, and the tank must not deform or leak;

• Second is the "leak tightness test": use a small-molecule gas like helium to detect leaks, and the leakage rate must be lower than 1×10⁻⁶ mbar·L/s, which is harder to achieve than a small hole pierced by a needle tip.

It is made of 316L stainless steel, which contains more molybdenum than ordinary 304 stainless steel, making the inner wall less prone to rust in coastal salt spray environments or when stored in humid conditions.

300bar Ultra-High-Pressure Tank

If you often dive below 40 meters or want to stay underwater for a longer time in a single dive, you need this type of tank.

It looks almost the same as the 200bar tank, with dimensions of 66cm in height and 7.6cm in diameter, but the interior can withstand a pressure of 300bar.

With the same water capacity of 6.8L, it can store 6.8×300 = 2040L of air when fully filled (at 1bar pressure).

Calculated at a consumption rate of 20L per minute, it can theoretically be used for 102 minutes, but it stores twice as much air as the 200bar tank, meaning fewer ascents for refilling, making it more suitable for long-distance exploration or technical diving.

Its weight is only about 1kg more than that of the 200bar tank (empty weight about 16kg), but the total weight after being fully filled with air is 32.6kg, which is more difficult to carry.

However, technical divers usually use more professional equipment, so this extra weight is negligible.

This type of tank must also pass the EN 12245 or DOT-3AL certification, and the testing standards are as strict as those for the 200bar tank. However, the strength requirements for the steel are higher, and the inner wall polishing is more precise, with a roughness Ra ≤ 0.8 micrometers (equivalent to 1/70 of a human hair), reducing gas residue and corrosion.

Which One to Choose Based on the Situation

If you are diving for the first time, or only diving near the coast at 18-30 meters, the 200bar tank is sufficient.

90% of recreational diving sites around the world are equipped with inflation equipment for this type of tank, making refilling convenient, and the price is also cheaper (about 500 US dollars in the second-hand market).

However, for technical diving, such as exploring the interior of shipwrecks, cave diving, or if you want to stay below 40 meters for more than 40 minutes, the 300bar tank can reduce the number of ascents for refilling halfway, making it safer.

Consistency of Materials and Manufacturing Standards

The materials and manufacturing standards of steel scuba tanks are highly unified, centered on 316L stainless steel and EN 12245/DOT-3AL certification. 316L contains 2-3% molybdenum, which is 3 times more resistant to salt spray corrosion than 304 stainless steel; during manufacturing, it must pass the hydrostatic test (330bar pressure maintained for 30 seconds, deformation ＜1%) and helium mass spectrometry leak detection (leakage rate ＜1×10⁻⁶ mbar·L/s). For tanks of the same brand and model, the deviation of material composition is ＜0.05%.

What Materials Are Used

The material selected for steel scuba tanks is not ordinary steel, but 316L stainless steel. This type of steel adds 2-3% molybdenum to the basic 304 stainless steel, which is equivalent to putting a corrosion-resistant "armor" on the steel.

For example, in a coastal high-salt spray environment, 304 stainless steel may develop slight rust spots on the inner wall after 1 year, while 316L can last for more than 3 years.

Not only in coastal areas, but 316L can also reduce inner wall corrosion when stored in humid diving equipment boxes or tropical regions.

Its carbon content is less than 0.03% (about 0.08% for 304), resulting in less intergranular corrosion during welding and a stronger tank structure.

During production, the steel composition of the same batch must be strictly consistent: chromium content 16-18%, nickel 10-14%, molybdenum 2-3%. If the deviation exceeds 0.05%, the entire batch will be scrapped.

What Tests Must Be Passed During Manufacturing

After the materials meet the standards, the tank must pass two key tests for forming, as required by mainstream global standards (European EN 12245, US DOT-3AL):

Put the empty tank into a pressurizing device, slowly inject water, and increase the pressure to 330bar (higher than the working pressure of 200bar or 300bar), maintaining the pressure for 30 seconds.

During this period, the tank body must not have any permanent deformation (such as bulging or denting); if the deformation exceeds 1%, it will be scrapped immediately. This is equivalent to a "pressurized physical examination" for the tank, ensuring that it can withstand the maximum pressure of daily use.

The leakage rate must be lower than 1×10⁻⁶ mbar·L/s. For a tank the size of an egg, the amount of gas leaked in 24 hours is less than a bubble the size of a needle tip.

Why Consistency Is Mandatory

If you buy two 200bar tanks of the same model, both made of 316L stainless steel and both passing the 330bar hydrostatic test, their corrosion resistance and pressure-bearing capacity will be almost identical during a 60-minute dive at 40 meters underwater.

In global scuba tank recall records, 90% of the problems are due to substandard materials or cutting corners during testing—for example, using 304 stainless steel to pass off as 316L, or only pressurizing to 300bar in the hydrostatic test.

Therefore, checking for the EN 12245 or DOT-3AL certification mark when buying a tank is essentially confirming that its materials and testing are "in step" with other tanks of the same type, ensuring safety.

Differences Between Various Standards

Some people may notice that Europe uses EN 12245 and the US uses DOT-3AL. The hydrostatic test pressures (330bar for EN and 345bar for DOT) and test details of these two standards are slightly different, but they do not affect material consistency.

Selection Based on Diving Needs

When choosing a steel tank, first consider the diving depth and duration: for recreational diving within 30 meters and a single dive within 1 hour, a 200bar general-purpose tank (empty weight 15kg, air storage capacity 1360L) is sufficient, with convenient global supply; for technical diving exceeding 40 meters or a single dive exceeding 60 minutes, choose a 300bar ultra-high-pressure tank (air storage capacity 2040L at the same size) to reduce the number of refills. Choose 200bar for frequent diving (lower maintenance costs) and 300bar for diving in remote areas (carry more air).

How Deep Do You Dive

Underwater, the pressure increases by 1bar every 10 meters, so the deeper you dive, the more air you consume per minute.

• Recreational diving within 30 meters (e.g., snorkeling, coral reef photography, watching tropical fish): a 200bar general-purpose tank is sufficient. Such dives usually do not exceed 30 meters in depth and last about 1 hour per dive. A 200bar tank stores 1360L of air (at 1bar pressure), which can theoretically be used for 68 minutes at a consumption rate of 20L per minute.

• Technical diving at 40-60 meters (e.g., exploring old shipwrecks, cave diving): a 300bar ultra-high-pressure tank is recommended. At a depth of 40 meters, the pressure is 5bar, and the air consumption per minute becomes 20L × 5 = 100L. The 1360L of air in a 200bar tank can only be used for 13.6 minutes, which is obviously insufficient; the 2040L of air in a 300bar tank can be used for 20.4 minutes.

How Long Do You Dive in a Single Session

• Short-duration diving (＜40 minutes): a 200bar tank is more flexible. Such dives are common in snorkeling or introductory diving, where users may only dive for 20 minutes before surfacing. A 200bar tank is light (empty weight 15kg), easy to carry onto the boat, and suitable for occasional divers.

• Long-duration diving (＞60 minutes): a 300bar tank is more worry-free. The extra 680L of air stored in a 300bar tank allows them to ascend once less—each ascent takes 10 minutes and consumes physical strength, thus reducing risks.

Is Refilling Convenient

• Visiting popular diving sites (Southeast Asia, Caribbean): choose a 200bar tank. 80% of recreational diving centers around the world are equipped with 200bar air compressors, with fast inflation speed (can be inflated to 200bar in 5 minutes), and convenient tank rental or purchase. 

• Visiting remote areas (small islands in the South Pacific, Arctic Circle diving): consider a 300bar tank. A 200bar tank may only last for half an hour after refilling, while a 300bar tank stores more air and can support a longer dive.

Cost and Maintenance Considerations

If you dive frequently, maintenance and replacement costs will accumulate, and choosing the right tank can save money.

• Occasional diving (5-10 times a year): a 200bar tank is more economical. Such users do not need to use air frequently. A 200bar tank has a low price (about 800 US dollars for a new one), and low testing cycle (replacing valve O-rings every 5 years) and storage costs.

• Daily diving (professional divers, diving instructors): a 300bar tank is more cost-effective. Professional divers use air every day. A 300bar tank stores more air, reducing the number of refills (saving 300 minutes per year by refilling once less per day), and can also be paired with a smaller backup tank. Although a new tank is 100-200 US dollars more expensive, it saves time in the long run.

Buoyancy of Empty and Full Tanks

An empty steel scuba tank weighs 18-25kg (taking the common 12L/300bar model as an example).  After being fully filled with air, the volume of air inside the tank is about 12L, and the weight of the displaced water is 12kg (density of water is 1kg/L), generating 12kg of buoyancy. However, the empty tank itself weighs 18-25kg, and the total weight after being filled with air increases to 22-29kg, which is still greater than the buoyancy, so it will still sink overall, requiring additional ballast for balance. Rust on old tanks increases the weight by 2-5%, which may require increasing the ballast.

Basic Data of Empty Tanks and Reasons for Sinking

There are differences in the manufacturing processes of different brands, but the empty weight of 12L/300bar steel tanks from mainstream manufacturers is basically between 18.5 and 24.8kg. This is equivalent to carrying a 10kg bag of rice plus 8-15kg of iron blocks. Some people may wonder: "The tank is not very big, so why is it so heavy?" The secret lies in the material—the density of steel is about 7.8g/cm³, which is 7.8 times the density of water (1g/cm³).

The "12L" here refers to the water capacity of the tank, meaning it can hold 12L of water if filled with water.

In other words, the "displaced volume" of the steel tank in water is 12L.

The weight of 12L of water is 12kg (1L of water = 1kg), so an empty steel tank will receive an upward buoyancy of 12kg in water.

But wait, the empty tank itself weighs 18.5-24.8kg. After offsetting, it will sink into the water with a downward force of 6.5-12.8kg.

To verify from another perspective: find an electronic scale, weigh the empty tank in the air as 20kg, and then completely immerse it in water (excluding air bubbles). The reading will become 7.2kg (20kg - 12kg of buoyancy).

For example, a 15L/200bar tank has an empty weight of about 22-28kg and a water capacity of 15L, displacing 15kg of water.

Its downward force is 22-28kg - 15kg = 7-13kg, which is slightly less than that of the 12L/300bar tank, but the difference is small.

For a small-capacity high-pressure tank of 8L/300bar, the empty weight may be 15-19kg, displacing 8kg of water, resulting in a downward force of 7-11kg.

For tanks of the same brand and specification, a "standard model" with a wall thickness of 0.8mm weighs 19kg empty, while a "reinforced model" with a wall thickness of 1.0mm may weigh 21kg.

The extra 2kg all comes from the steel plate, which will directly increase the downward force. Rust in some areas can slightly increase the wall thickness (about 0.05-0.1mm per year), accumulating a few extra kilograms after several years.

In actual diving, a 70kg diver, fully equipped, has a total weight (including the empty tank) of maybe 85kg.

Usually, 4-8kg of ballast is required for the empty tank to neutralize its sinking tendency.

Changes in Weight and Buoyancy When Full

We use the most common 12L/300bar steel tank as an example to analyze the specific data when the tank is full.

How to Calculate the Total Weight When Full

First, the water capacity of the steel tank is 12L, which is also the volume of air after inflation (because the gas fills the entire space). However, the density of air is related to pressure: the mass of air in a 12L space at a high pressure of 300bar is completely different from the air density at normal pressure (1bar).

According to the physical formula (Boyle's Law: Pressure × Volume = Constant), the volume of 300bar/12L air converted to normal pressure (1bar) is 300×12=3600L (i.e., 3.6m³). The density of air at normal pressure is about 1.29kg/m³, so the mass of air in the tank is ≈3.6m³×1.29kg/m³≈4.6kg.

Adding the weight of the empty tank itself (18.5-24.8kg), the total weight of the fully filled steel tank is ≈18.5+4.6=23.1kg (light model) to 24.8+4.6=29.4kg (heavy model). This is equivalent to holding a 25kg bag of rice plus a 4-5kg schoolbag.

Buoyancy Remains Unchanged

Buoyancy is only related to the volume of displaced water, and the volume of the steel tank (12L) has not changed, so the weight of the displaced water is still 12kg (1L of water = 1kg). The buoyancy of the fully filled steel tank is still 12kg, the same as when it is empty.

However, the total weight has increased by 4.6kg, causing the "downward force" (total weight - buoyancy) to change from 6.5-12.8kg when empty to 11.1-17.4kg when full (23.1-12=11.1; 29.4-12=17.4).

Simply put: when empty, you need to counteract a downward force of 6-13kg; when full, this force increases to 11-17kg.

In actual diving, this change is very intuitive: before inflation, you may need 4kg of ballast to balance the sinking of the empty tank; after inflation, you may need 6-8kg of ballast.

Different Tank Specifications

Tanks with different capacities and pressures have different weight changes when full. We compare two common specifications:

Steel Tank Specification	Empty Weight (kg)	Air Mass (kg)	Total Weight (kg)	Buoyancy (kg)	Downward Force (kg)
12L/300bar	18.5-24.8	4.6	23.1-29.4	12	11.1-17.4
15L/200bar	22-28	3.9 (200×15=3000L=3m³, 3×1.29≈3.87kg≈3.9kg. Total weight 22+3.9=25.9, 28+3.9=31.9. Buoyancy 15kg, downward force 25.9-15=10.9, 31.9-15=16.9)	25.9-31.9	15	10.9-16.9
8L/300bar	15-19	3.1 (300×8=2400L=2.4m³, 2.4×1.29≈3.1kg≈3.1)	18.1-22.1	8	10.1-14.1

Looking at the table, you will find that:

• The large-capacity tank (15L/200bar) has a higher total weight (25.9-31.9kg), but because it displaces more water (15L, 15kg of buoyancy), the downward force (10.9-16.9kg) is slightly less than that of the 12L/300bar tank.

• The small-capacity high-pressure tank (8L/300bar) has a low total weight (18.1-22.1kg) but displaces less water (8kg), and the downward force (10.1-14.1kg) is close to that of the 12L tank, so it is not used as the main cylinder for recreational diving.

Impact of Old Tanks

There is an annual rust increase of about 0.05-0.1mm. After 5 years, a 12L/300bar steel tank may increase in weight by 1-2kg (more accurate calculation: the inner surface area of the tank is about 12L=0.012m³=12000cm³, the increase in wall thickness is 0.1mm=0.01cm, the increase in volume is 12000×0.01=120cm³, and the increase in weight is 120×7.8=936g≈0.94kg/year, 5 years≈4.7kg).

This adds 4-5kg to the total weight when full, increasing the downward force by 4-5kg. 

For example, a 75kg diver, fully equipped, has a total weight (including the empty tank) of 85kg, and may need 6kg of ballast to balance the downward force of the empty tank;

After being filled with air, the total weight becomes 85+4.6=89.6kg, and may require 8-10kg of ballast—1kg less may lead to too fast descent, and 1kg more may cause uncontrollable ascent when surfacing.

Practical Impact on Divers

Taking the most common 12L/300bar steel tank as an example: when empty, its downward force is 6.5-12.8kg (total weight 18.5-24.8kg - buoyancy 12kg).

Assuming a 70kg diver, fully equipped (BCD, wetsuit, fins, etc.), the total buoyancy of the body and equipment is 5kg (meaning the body will slightly float). The 6.5-12.8kg downward force from the empty tank will result in an overall state of "sinking by 5-11kg".

Usually, the empty tank requires 4-8kg of ballast to neutralize the downward force. For example, if the downward force of the empty tank is 10kg and the net buoyancy of the diver's body and equipment is 5kg, then 5kg of ballast is needed (10-5=5) to make the overall gravity equal to buoyancy, achieving a neutral state.

Ballast Adjustment When Full

After inflation, the total weight of the steel tank increases by 4.6kg, and the downward force increases from 6.5-12.8kg to 11.1-17.4kg. At this time, the original ballast may be insufficient.

For example: Diver A, after equipping, has a net buoyancy of 5kg for the body and equipment. When the tank is empty, the downward force is 10kg, and he adds 5kg of ballast to achieve perfect balance.

After inflation, the downward force becomes 15kg (hypothetically), so the total downward force = 15kg (tank) + 0 (own equipment) - 5kg (own buoyancy) = 10kg? This is obviously incorrect; a simpler method is needed.

A simpler example: 4kg of lead may be needed when the tank is empty; 6kg may be needed when full, requiring more lead to counteract the extra downward force.

If 4kg is still used, the descent will be too fast; if too much is added, such as 8kg, forceful air release will be required during ascent due to the heavy lead, otherwise, the diver will "shoot" out of the water.

Ballast Changes in Different Environments

The density of seawater is higher than that of freshwater (about 1.025kg/L vs 1kg/L), so the same equipment gets more buoyancy in seawater.

For example, the same diver using the same set of equipment in freshwater and seawater:

• Freshwater: body buoyancy 5kg, equipment buoyancy 13kg, tank buoyancy 12kg, total buoyancy 30kg. Total weight 98.1kg, net force 68.1kg (will sink).

• Seawater: body buoyancy ≈5kg×1.025≈5.1kg, equipment buoyancy≈13×1.025≈13.3kg, tank buoyancy≈12×1.025≈12.3kg, total buoyancy≈5.1+13.3+12.3=30.7kg. The total weight is still 98.1kg, and the net force≈98.1-30.7=67.4kg. However, in actual diving, it is easier to float in seawater, so less ballast may be needed.

A more direct example: 4kg of ballast is needed for the empty tank in freshwater; only 3kg may be needed in seawater.

Ballast Adjustment for Old Tanks

As calculated earlier, 5 years of use may increase the weight by 4-5kg, increasing the downward force when full by 4-5kg. 

For example, Diver B's steel tank has been used for 5 years, and the downward force when full has increased from 15kg to 19kg. He previously added 6kg of ballast when full, which was just right. Now he may need to add 10kg.

Every time the tank or environment is changed (freshwater/seawater), or even the physical condition changes (e.g., gaining 2kg), the ballast needs to be readjusted. It is like the "balance ruler" of diving equipment: 1kg more may lead to too fast descent, and 1kg less may cause uncontrollable ascent.

Key Maintenance Steps for Steel Tanks

The maintenance of steel scuba tanks requires a hydrostatic test at 1.5 times the working pressure once a year (e.g., a 232bar tank is tested at 348bar, which must be performed by a third-party certified organization), and internal ultrasonic/magnetic particle inspection every 5 years (to check for cracks and corrosion, allowing a corrosion depth ＜0.1mm). Daily inspections include tank dents (discontinue use if the diameter ＞5mm), valve tightness (soapy water leak test with no bubbles), avoiding collision with sharp objects (to prevent coating damage leading to corrosion), storing in a dry and ventilated place (humidity ＜60%), and keeping away from heat sources (temperature ＜60℃ to prevent valve rubber aging).

Daily Basic Inspection

Spending 5 minutes doing this before and after each dive can proactively find 80% of potential problems.

If the protrusion height exceeds 1mm or the dent diameter is greater than 5mm, special attention must be paid. 

Last year, a diver's tank was scratched by a rope on the ship's railing, causing a 6mm-diameter dent. It was ignored, but the tank deformed during the hydrostatic test 3 months later and was immediately scrapped.

If the peeled area exceeds the size of a coin (diameter over 2cm), it will accelerate internal rust. Oxygen in the humid air will penetrate the damaged area and slowly corrode the steel wall.

Then, use soapy water to test the tightness (easier to observe bubbles than soapy water alone) by applying it to the valve connection, O-ring, and regulator connection, and observing for 30 seconds.

There are several key pieces of information on the tank body: manufacturing date (stamped on the tank bottom, format YYYYMM, e.g., 202305 means production in May 2023), working pressure (stamped on the tank shoulder, unit bar or psi, common are 232bar or 3300psi), and test date (third-party hydrostatic test label, affixed to the tank body). If the test date is more than 1 year old, or the manufacturing date is more than 15 years (according to CGA standards, the design life of a steel tank is generally 20 years, but more frequent inspections are required after 15 years).

For example: a 2022 statistic from a diving club showed that out of 37 steel tank anomalies, 32 were due to inadequate daily inspections, leading to slow gas leakage during descent and near pressure loss underwater; some ignored small tank dents, which led to the tank bursting during the hydrostatic test.

Valve and Accessory Maintenance

The maintenance of steel scuba tank valves requires cleaning surface salt spray and dust monthly, disassembling and lubricating the O-ring and valve stem with silicone-based grease every 12 months, and replacing the rubber gasket every 2 years. Daily leak detection at the connection with soapy water is required; continuous bubbling requires immediate discontinuation of use and repair—these details can avoid 80% of valve failures.

Valve Structure

The handwheel is the plastic or metal ring you turn, the valve stem connects the handwheel to the internal valve seat, the O-ring is set on the valve stem for sealing, and the gasket separates the gas at the valve outlet. Divers do not need to remember all the names, but should be able to point out "this rotating ring" and "the rubber piece at the outlet".

Daily Cleaning

A surface cleaning must be done once a month: wipe the outer surface of the valve with a dry cloth dipped in clean water, focusing on the bottom of the handwheel, the valve stem, and the outlet threads. Do not cover it directly with a wet towel, as residual moisture will accelerate the aging of rubber parts.

For valves used near the sea, it is recommended to rinse them with fresh water immediately after diving, as moisture retention can cause the O-ring to harden within 24 hours.

Lubrication Maintenance

The internal moving parts of the valve (where the valve stem contacts the O-ring) need lubrication, otherwise, they may seize due to friction. However, not all oils can be used:

• Do not use Vaseline: It will swell the rubber O-rings, causing the gasket to expand and deform within 3 months, leading to leaks.

• Silicone-based grease is recommended: It is water-resistant and temperature-resistant (-40℃ to 200℃) and does not corrode rubber. Use a small wooden stick to scoop a rice-grain-sized amount of grease and apply it evenly to the valve stem threads and O-rings.

The lubrication frequency is once every 12 months. If you frequently dive in the tropics (high water temperature, fast rubber aging), it can be shortened to 10 months. 

Gasket Replacement

Their service life depends not only on time but also on the frequency of use and environmental impact:

• Rubber gasket: It is compressed every time the regulator is assembled/disassembled. It must be replaced if the surface is sticky, shows no rebound when pressed (does not quickly swell back when pinched), or has cracked edges (length exceeding 1mm). Even if only used for 1 year, frequent divers may need to replace it every 6 months.

• O-ring: If the cross-section has become a flattened oval (it should be a perfect circle) and a new ring of the same size must be replaced (the size is marked on the tank body or manual, with common codes like "AS568-016").

The tools for replacing the gasket are simple: a small flat-head screwdriver (for picking out the old gasket) and a bottle of silicone-based grease (for lubricating the new gasket). After replacement, test for leaks with soapy water; it is only qualified if no bubbles appear for 5 minutes.

Handling Common Problems

• Slight leakage: If the soapy water test shows small bubbles at the valve connection or regulator connection, first wipe it dry and re-tighten (turn the handwheel clockwise to the end, then back half a turn). If it still leaks, disassemble the valve to check if the O-ring is displaced.

• Stuck handwheel: Unable to turn or difficult to turn. 90% of the time, it is due to internal dust accumulation. Disassemble the valve (using the wrench provided with the tank), clean the valve stem threads and valve seat gaps with a small brush, and then lubricate with silicone grease. 

• Regulator cannot be connected: Stripped valve outlet threads, often due to violent disassembly. Slight stripping can be remedied with thread sealing tape (wrap 5-8 times).

Spending 3 minutes cleaning monthly, 10 minutes lubricating annually, and replacing the gasket every 2 years can keep the valve working reliably for more than 15 years.

Storage and Environment

Steel scuba tanks should be stored upright and fixed with a dedicated bracket to avoid tipping; the environmental humidity should be ＜60%, and the temperature ＜60℃, away from heaters or windows (to prevent condensation). Retain at least 0.5bar of residual pressure inside the tank to prevent moist air from entering. When not in use for a long time, check the weight and valve status every 3 months—these measures can reduce 80% of internal rust and valve aging.

How to Control Humidity and Temperature

The humidity of the storage environment should be controlled below 60%. In coastal areas or during the southern rainy season, the humidity often exceeds 80%. In such cases, the tank must be wrapped in a moisture-proof bag (choose one with a desiccant) or stored in a room with a dehumidifier.

The temperature should be below 60℃. The rubber O-rings and gaskets in the valve will accelerate aging above 60℃: experiments show that after 3 months of storage at 70℃, the rubber hardness increases by 30% and the elasticity decreases by 25%, making them prone to cracking and leaking. Do not place the tank on the balcony, near heating pipes in the garage; the temperature inside the car can exceed 70℃ in summer, so temporary storage there is strictly forbidden.

Why Residual Pressure Must Be Maintained

The tank must retain at least 0.5bar of residual pressure (about 0.05MPa), which means there is still some air "holding up" the internal environment.

A diver's tank was used up without residual pressure, and after 3 months, it weighed 1.2kg more. After disassembly, the inner wall was covered with reddish-brown rust, and the corrosion depth reached 0.2mm (exceeding the 0.1mm safety limit).

How to Protect the Valve

Valves of tanks intended for idle storage for more than 3 months require extra protection:

• Wipe the surface clean: Wipe the outer surface of the valve with a dry cloth dipped in fresh water, especially the outlet threads, as residual moisture can corrode the threads.

• Apply a thin oil coat for rust prevention: Apply a tiny bit of watch oil (thinner than silicone-based grease) to the handwheel axle area to prevent the axle from rusting and seizing. 

• Seal the outlet: Use a dedicated tank dust cap (the one with a sealing rubber ring) to tightly screw onto the valve outlet. The dust cap blocks 90% of dust and moisture, making it more reliable than sealing with a plastic bag.

Storage Solutions for Different Scenarios

• Home storage: Store in a corner of the living room rather than the balcony. Choose a dry place without direct sunlight, fix it with a bracket, and place a hygrometer nearby (regularly check if it exceeds 60%).

• Car storage: Use a dedicated tank bag (with fixing straps) for short trips. For long journeys, it must be fixed to the bottom of the car trunk (tied tightly with bungee cords) to avoid collision during sudden braking. Do not place it in the front passenger seat; a deployed airbag might damage the tank.

• Club warehouse: A large number of tanks should be stacked upright, leaving a 30cm aisle between rows (for easy inspection).