How Long Does a Small Diving Tank Last | Air Duration by Size and Depth

When choosing a mini scuba tank, focus on three key numbers: a 0.5L tank typically lasts about 6–10 minutes at a depth of 3–5 meters, while a 1L tank lasts about 10–20 minutes; for every additional 10 meters of depth, air consumption roughly doubles as pressure rises from 1 ATA to 2 ATA. For an average person, breathing rate is about 15–20 L/min, and you should always keep at least 50 Bar in reserve to avoid low-pressure risk.

Factors Affecting Duration

A 0.5L tank filled to 200 Bar holds 100 liters of air. An adult diver’s average surface air consumption (RMV) is typically 15–25 L/min, giving a still, surface-level duration of around 4–6 minutes. At 10 meters (2 ATA), air density doubles and breathing time drops to about 2–3 minutes. At 20 meters (3 ATA), only 1.5–2 minutes remain.

Estimated Air Consumption Under Different Conditions

A 0.5L mini tank filled to 200 Bar stores 100 liters of compressed air. An adult sitting still on land breathes about 15 liters per minute, so at the surface this tank only provides about 6.5 minutes of breathing time. At 10 meters, where pressure doubles, each breath becomes twice as dense, and those 100 liters are gone in just a little over 3 minutes.

If a person remains motionless on the bottom, with heart rate steady at 60 to 70 beats per minute, breathing is at its most efficient. In that state, a large diver with bigger lungs may consume about 18 liters per minute, while a smaller diver may need only 12 liters per minute. If you are using a 1L tank with 200 liters of total air, it can last 11 to 16 minutes at the surface under calm conditions.

The moment you start kicking and swimming, your muscles demand more oxygen. In a light current of 0.5 knots, even a slow swim can push air consumption above 25 L/min. At 10 meters and 2 ATA, that means the tank loses 50 liters of air per minute. At that point, a 0.5L bottle can keep you alive for only about 2 minutes, or roughly 20 to 30 breaths.

Water resistance is about 800 times greater than air resistance. If you try to swim hard against a current, your RMV can surge to 50 L/min or more. At 20 meters (3 ATA), that is equivalent to consuming 150 liters of surface air every minute. A 0.5L mini tank will not even last 40 seconds in that situation.

• Still at the surface: consumption 12–15 L/min; a 0.5L tank lasts about 7 minutes.

• Slow swim at the surface: consumption 20–28 L/min; a 0.5L tank lasts about 4 minutes.

• Fast swim at the surface: consumption 40–60 L/min; a 0.5L tank lasts about 1.8 minutes.

Water temperature is another major factor. In 15°C cold water, the body loses heat about 25 times faster than it does on land. To maintain body temperature, people begin shivering involuntarily, and shivering consumes a great deal of air. Air consumption in cold water is usually about 30% higher than in 28°C warm water. A tank that lasts 5 minutes in warm water may last only 3.5 minutes in cold water.

Larger bodies also consume more air. A 1.9-meter tall man and a 1.6-meter tall woman may differ by nearly 1 liter in tidal volume per breath. The higher the basal metabolic rate, the more oxygen the body burns. At the same depth of 15 meters, a large man may consume 60 liters per minute, while a woman may need only 40 liters.

First-time divers often feel nervous in the water, and that anxiety causes breathing to become fast and shallow. This kind of “inefficient breathing” prevents carbon dioxide from being properly expelled, which makes the brain urge the body to breathe even more. In this state, RMV often spikes to 80 L/min. At 30 meters (4 ATA), that means pulling 320 liters of air from the tank every minute.

1. At 5 meters (1.5 ATA): with consumption at 25 L/min, a 0.5L tank (100 liters) lasts 2.6 minutes.

2. At 15 meters (2.5 ATA): with consumption at 25 L/min, a 0.5L tank lasts 1.6 minutes.

3. At 25 meters (3.5 ATA): with consumption at 25 L/min, a 0.5L tank lasts 1.1 minutes.

If you are wearing a bulky jacket-style BCD, drag is about 15% higher than with a back-inflate wing setup. Cameras and dive lights also add to the workload. Every extra bit of effort makes the needle on the pressure gauge fall faster. At depth, the regulator itself also becomes harder to breathe from, so inhalation starts to require more work.

Put simply, a mini tank is really just emergency insurance. At 20 meters, whether you cramp up or panic, usable air disappears almost instantly. Even a 1L tank with 200 liters of air gives a diver only a little over 1 minute to reach the surface in an emergency. If you factor in the last few meters of ascent, the time window gets even tighter.

Activity Level	RMV (L/min)	0.5L Tank (at 10m)	1L Tank (at 10m)
Completely still	15	200 sec	400 sec
Normal swimming	25	120 sec	240 sec
Hard work	50	60 sec	120 sec
Extreme panic	90	33 sec	66 sec

Body fat also affects air consumption. Divers with lower body fat lose heat faster, and in seawater below 20°C, their breathing rate rises quickly. Experienced divers, on the other hand, control neutral buoyancy well and do not constantly add or dump air to move up and down. Every time you inflate the BCD, you are effectively stealing 5 to 8 liters of air from your tank.

Do the math for a 0.5L tank at 10 meters. If you stay composed, you may get 30 to 40 breaths. If you panic, 15 breaths may be enough to empty the tank. That is exactly why mini tanks can only serve as a last-resort lifeline in deeper water and should never be relied on for extended underwater time.

Air Duration by Size & Depth

At 3000 PSI (200 Bar), 0.5L, 1L, and 2L tanks provide total air volumes of 100L, 200L, and 400L respectively. Based on an average adult surface breathing rate of 20 L/min, a 0.5L tank provides 5 minutes of breathing at the surface. For every additional 10 meters of depth, ambient pressure rises by 1 Bar. At 10 meters (2 ATA), duration drops to 2.5 minutes; at 20 meters (3 ATA), it falls to about 1.6 minutes.

Depth and Pressure

Atmospheric pressure at sea level is 1.013 Bar (14.7 PSI). In seawater, every 10 meters (33 feet) of descent adds 1 Bar of ambient pressure. At 10 meters, a diver is under 2 absolute atmospheres (2 ATA), meaning water pressure plus the atmospheric pressure above the surface.

At 20 meters (66 feet), the reading rises to 3 ATA. If you are carrying a 1-liter tank at 200 Bar, it still contains 200 liters of air, but in a 3 ATA environment, each breath delivers three times as many air molecules into the lungs as it would at the surface.

Depth (Meters/Feet)	Ambient Pressure (ATA)	Air Inhaled per Minute (based on 20L/min)	0.5L Tank Duration
0m / 0ft	1.0 ATA	20 L	5.0 min
10m / 33ft	2.0 ATA	40 L	2.5 min
20m / 66ft	3.0 ATA	60 L	1.6 min
30m / 100ft	4.0 ATA	80 L	1.2 min

Seawater and freshwater do not have the same density. Seawater is about 1025 kg/m³, while freshwater is about 1000 kg/m³. In Florida’s freshwater springs, you need to descend 10.3 meters to gain 1 Bar of pressure, while in Caribbean seawater, only 10.06 meters is required for the same increase.

Take a 0.5L mini tank. At the surface, it may feel like it lasts quite a while. But at 15 meters (49 feet), pressure is 2.5 ATA. Even if your breathing rate stays the same, the pressure gauge will drop 2.5 times faster than it does on land.

Now consider the number of breaths. An adult’s total lung capacity is about 3 to 5 liters, but underwater breathing typically uses only part of that, around 1.5 liters per breath. At 20 meters, one such breath consumes 4.5 liters of compressed air from the cylinder. For a 0.5L tank holding just 100 liters, the tank is empty after only about 22 breaths.

• At 5 meters (1.5 ATA): ambient pressure rises by 50%, and a 0.5L tank supports about 3.3 minutes of breathing.

• At 12 meters (2.2 ATA): a 1.0L tank (200L total air) lasts about 4.5 minutes.

• At 25 meters (3.5 ATA): a 2.0L tank (400L total air) lasts only about 5.7 minutes.

Changes in air density with depth also affect regulator performance. At 30 meters (4 ATA), air is four times denser than at the surface. As this denser gas passes through the regulator valve, friction increases, making the regulator harder to breathe from. Divers instinctively breathe harder, which drives air consumption even higher.

When using a small backup cylinder (pony bottle), you must also account for gas used during ascent. If you begin an emergency ascent from 20 meters, ascent speed should not exceed 18 meters (60 feet) per minute. That alone takes more than 1 minute, and once you add a safety stop at 5 meters, any tank under 1.0L leaves almost no margin for error.

Many beginners confuse gauge pressure with absolute pressure. At the surface, the gauge reads 0 PSI, but that is only relative pressure. When calculating air consumption, you must add the 14.7 PSI of atmospheric pressure. At 33 feet (10 meters), you are actually breathing in an environment of 29.4 PSI.

A 1.9L (13 cu ft) cylinder lasts about 10 minutes at 10 meters. That may be barely enough for simple cleaning or photography at that depth. But as an emergency backup at a 30-meter wreck site, it provides less than 5 minutes of life-saving air.

Cylinder Water Capacity (L)	Total Air at 200 Bar	Duration at 10m (20L/min)	Duration at 20m (20L/min)	Duration at 30m (20L/min)
0.5 L	100 L	2.5 min	1.6 min	1.2 min
1.0 L	200 L	5.0 min	3.3 min	2.5 min
2.0 L	400 L	10.0 min	6.6 min	5.0 min
3.0 L	600 L	15.0 min	10.0 min	7.5 min

As depth increases, the partial pressure of nitrogen also rises. Although mini tanks do not hold enough gas to cause serious nitrogen narcosis, the higher pressure still increases the rate at which gas penetrates lung tissue. With every breath, pressure loading in the alveoli changes incrementally with each meter of depth.

Some divers overfill small tanks to 3300 PSI (227 Bar) to gain a bit more time. Even so, at a standard depth of 18 meters (2.8 ATA), that extra 10% pressure buys less than 40 additional seconds of breathing. Depth consumes air with brutal efficiency.

Water temperature also matters. Although cold water does not change the pressure formula, it lowers internal cylinder pressure through thermal contraction. In 10°C water, a cylinder originally filled to 200 Bar may read below 190 Bar. At 20 meters, that missing 10 Bar is equivalent to losing roughly 15 breaths.

A safe ascent requires adequate reserve pressure. Once tank pressure drops below 15–20 Bar, second-stage regulator performance deteriorates noticeably. In practice, a 1.0L cylinder only delivers about 180 liters of comfortably breathable air.

• 50 Bar (725 PSI): this is the recognized mandatory turnaround pressure and should never be used for staying at depth.

• Air used per breath: with every additional meter of depth, each breath consumes about 0.1% more cylinder pressure.

• Ascent speed: the longer the ascent takes, the longer depth-related pressure continues consuming the remaining gas.

For a 2.0L cylinder at 15 meters, the pressure factor is 2.5. If your surface air consumption (SAC) is 15 L/min, then your actual underwater breathing rate (RMV) becomes 37.5 L/min. Divide 400 liters by 37.5, and the theoretical limit is 10.6 minutes.

In real diving, however, most divers apply a safety discount and reduce that figure to about 70% of the theoretical value. You cannot guarantee that your breathing will stay that calm underwater. If you encounter current and have to swim against it, or if a loose tank mount makes you anxious, air consumption can double instantly and cut that 10-minute window down to 5 minutes.

Compressed air also has weight. Each liter of air weighs about 1.2 grams. When you breathe through the 400 liters in a 2.0L tank, the system becomes about 0.5 kg lighter. In small cylinders this buoyancy shift is not dramatic, but it still subtly affects trim and balance.

Actual Gas Capacity

A 0.5L mini tank filled to its rated pressure of 3000 PSI (207 Bar) contains 103.5 liters of breathable air by simple conversion. Many products on the market are labeled as 3 cu ft, but in reality hold only about 85 liters.

A 1.0L version filled to the same 207 Bar stores about 207 liters of air. In North American diving clubs, this size is commonly used as an emergency spare cylinder. A slightly larger 1.9L (13 cu ft) steel cylinder can provide close to 390 liters of compressed air at high pressure.

• 0.5 L (3 cu ft): actual supply is about 85–103 liters, enough for only about 5 minutes of surface breathing.

• 1.0 L (6 cu ft): actual supply is about 200 liters, enough for about 10 minutes at the surface.

• 1.9 L (13 cu ft): actual supply is about 370–400 liters, making it a much safer backup option.

• 2.7 L (19 cu ft): actual supply is about 540 liters, enough for more than 10 minutes at 10 meters.

• 4.4 L (30 cu ft): actual supply is about 850 liters, suitable for somewhat deeper single ascents.

The amount of air in a cylinder depends not only on its size, but also on fill pressure. If a dive shop fills only to 2000 PSI, a 1.9L tank holds only about 250 liters of air. A lower pressure reading on the gauge means about one-third fewer breaths underwater.

Pressure is also affected by temperature. A tank filled to 3000 PSI at 21°C (70°F) may drop to around 2700 PSI when taken into cold lake water at 4°C (40°F). This 10% pressure loss caused by temperature difference is simply a law of physics.

Many dive shops use rapid fills for speed, and the cylinder becomes very hot. Once that “hot fill” cools back down to water temperature, the gauge needle drops sharply. To get a full charge, the cylinder has to be topped off again after cooling.

• Regulator loss: every time the first stage is assembled and tested, about 1 liter of air is lost during purging.

• Hose volume: the high-pressure hose and second stage occupy about 0.5 liters of compressed gas space.

• Micro-leaks at the O-ring: an aging first-stage O-ring may quietly leak about 0.2 liters per minute.

When you account for these losses, a 0.5L cylinder usually performs worse underwater than the theoretical calculation suggests. At 10 meters (33 feet), ambient pressure is 2 ATA. Every breath removes twice as many air molecules from the tank as it would at the surface, so usable time is cut in half.

Aluminum 6061-T6 cylinders and steel cylinders also differ in storage logic. Aluminum tanks are lighter, but their walls are thicker, so they do not use internal volume as efficiently as steel cylinders of the same outside diameter. A 13 cu ft aluminum cylinder is typically noticeably bulkier than a steel tank of the same rated size.

The last 50 Bar (725 PSI) in the cylinder is not truly usable. As tank pressure approaches ambient pressure, regulator delivery becomes noticeably hard to breathe from. In practical calculations, that final 25 to 50 liters of air should be treated as unavailable.

• Gauge error: cheap miniature pressure gauges commonly have a reading deviation of 5% to 10%.

• Residual gas in the fill port: after disconnecting the air source, about 0.3 liters of gas is vented from the fill valve.

• Z factor: above 250 Bar, air molecules are so compressed that actual gas storage is lower than the ideal formula suggests.

Many people also overlook drysuit inflation. If you use a mini tank to inflate a drysuit, the suit consumes several liters of air every 10 meters of descent to offset squeeze. For a 0.5L bottle, that can easily consume the very last breath meant for survival.

Altitude changes the calculation as well. In high-altitude lakes, the starting pressure is not 1 ATA. Although the amount of gas stored in the cylinder stays the same, the lower surface pressure means the differential pressure is higher, so breathing time in shallow water may be slightly longer than it would be at sea.

It is also important to make sure there is no condensation inside the cylinder. Moisture takes up gas volume and, more importantly, accelerates internal oxidation under high pressure. If a 1.0L cylinder develops a thick oxide layer inside, actual gas capacity may shrink by 1% to 2%, and the air may develop a metallic taste.

Safety Reminder

A 0.5L cylinder at 200 Bar stores 100 liters of air. At 10 meters (2 ATA), ambient pressure doubles the air consumption rate relative to the surface. Usable gas then feels like only 50 liters at the surface. At a breathing rate of 20 L/min, that gives just 2.5 minutes of air. At 20 meters (3 ATA), supply from a 0.5L cylinder drops to 1.6 minutes.

Protection Against Pulmonary Barotrauma

At 10 meters, pressure is 2 ATA. If you take a full breath from a 0.5L mini tank at that depth, your lungs may contain about 6 liters of air. If you hold your breath and swim to the surface, that 6 liters will expand to 12 liters as pressure drops. Human lung capacity is usually only about 6 to 7 liters, so the excess volume can rupture the alveoli.

The alveolar wall is only about 0.2 microns thick, hundreds of times thinner than a human hair. If internal lung pressure exceeds external pressure by just 80 mmHg—about 0.1 atmosphere—the tissue can tear physically. That pressure difference can occur over a depth change of only 1.2 meters. For people playing in shallow water with micro tanks, this “shallow-water trap” is even more likely.

To understand air volume expansion during ascent more clearly, consider the following figures:

Starting Depth	Ending Depth	Pressure Change	Volume Expansion
10m (2 ATA)	0m (1 ATA)	down 50%	up 100%
20m (3 ATA)	10m (2 ATA)	down 33%	up 50%
30m (4 ATA)	20m (3 ATA)	down 25%	up 33%

The closer you are to the surface, the more violently pressure changes affect the lungs. From 10 meters to the surface, air volume doubles. That physical reality means you must keep breathing on the way up and never pause your breathing.

Arterial gas embolism (AGE) is the most serious consequence of breath-hold ascent. Once the lungs tear, escaping air can enter the bloodstream and become bubbles. These bubbles are often more than 10 microns in diameter and can travel throughout the body. If they lodge in blood vessels in the brain, they can cut off circulation. According to the U.S. Navy Diving Manual, symptoms of AGE often appear within 10 minutes after surfacing.

In addition to brain injury, escaped gas in the chest can cause several other conditions:

• Pneumothorax: air enters the space between the lung and chest wall, collapsing the lung and causing breathing difficulty.

• Mediastinal emphysema: bubbles collect between the lungs around the heart, compressing major blood vessels.

• Subcutaneous emphysema: air tracks under the skin of the neck, producing a crackling sensation when touched.

A 0.5L mini tank provides less than 3 minutes of air at 10 meters. As the tank runs low, people naturally feel distress and rush to get to the surface. That panic can easily trigger breath-holding while ascending. In reality, if the airway is kept open, expanding air in the lungs will escape automatically during ascent. That is what is meant by a slow exhalation ascent.

Controlling ascent speed is the physical way to prevent barotrauma. Organizations such as PADI recommend ascending no faster than 9 meters per minute. If you return from 10 meters, the ascent should take at least 67 seconds. Many mini-tank users do not wear dive computers and ascend by feel, often at speeds over 2 meters per second, which is about 13 times faster than the recommended limit.

Here are several practical underwater breathing safety metrics:

1. Resting air consumption: an adult typically uses about 20 liters of air per minute.

2. Ascent speed limit: should not exceed 0.15 meters per second.

3. Safety stop: remain at 5 meters for 180 seconds to allow microbubbles to off-gas.

4. Reserve pressure: ascent must begin when the gauge reaches 50 Bar (725 PSI).

The first-stage regulator usually reduces pressure to about 9 to 10 atmospheres above ambient. During ascent, if you are not breathing, that regulator pressure is also trapped in the airway. Combined with the natural expansion of air already in the lungs, it adds even more stress to the alveoli. Continuous, slow, deep breathing is the only safe way to relieve that pressure.

Some users deliberately slow their breathing or hold a breath after inhaling in order to “save air.” That is extremely dangerous in a 200 Bar system. Breath-holding increases carbon dioxide in the blood, and heart rate can rise above 120 beats per minute. That not only makes you consume air faster, but also increases the risk of losing consciousness underwater.

At 3 meters, one breath is already about 30% denser than it is at the surface. Your lungs are working with “heavier” air. If you remove the mouthpiece or try to speak underwater, never hold your breath. As long as the airway stays open, expanding gas can escape naturally through the mouth.

If the tank runs completely out of air, you should swim upward while making a long “ahhh” sound. This keeps the epiglottis open and ensures there is an exit path for expanding lung gas. Never clamp your lips shut, because even the last 0.5 meter of depth change can create tiny tears in lung tissue.

After surfacing, if you feel severe chest pain, cough up blood, or develop numbness on one side of the body, you must begin oxygen administration immediately and seek the nearest hyperbaric chamber. While waiting for rescue, the injured diver should lie flat rather than sit up or walk around. These details help prevent bubbles in the bloodstream from moving further toward the brain and buy valuable time for treatment.

Reserve Gas and Gas Quality

A 0.5L tank rated at 200 Bar contains only 100 liters of air in theory. In real diving, the 50 Bar reserve rule must be followed strictly. Once the gauge drops to 725 PSI, you should already be back at the surface. The remaining 25 liters are not for continued enjoyment; they are a survival reserve for problems such as water entering the second stage or an obstructed ascent.

Once you subtract that 50 Bar reserve, the actual usable gas in a 0.5L tank is only 75 liters. At a calm breathing rate of 20 L/min, that is just 3.75 minutes at the surface. At 10 meters (2 ATA), the consumption rate doubles to 40 L/min, shrinking your working window to less than 2 minutes. Many beginners ignore this mandatory 25% reserve, and end up facing complete gas depletion in roughly 90 seconds underwater.

A scuba cylinder must never be filled with ordinary industrial compressed air. Proper diving breathing gas must meet the CGA Grade E standard. Air from a regular tire inflator may contain fine oil mist, and concentrations above 5 mg/m³ can damage the lungs. At 2 ATA, air density increases, and those contaminants enter the bloodstream far faster than they do on land.

Carbon monoxide (CO) in breathing gas must never exceed 10 ppm. At the surface, that concentration may go unnoticed, but at 20 meters (3 ATA), the effect is comparable to breathing 30 ppm on land because of the increased partial pressure. That reduces the blood’s oxygen-carrying ability and can cause loss of consciousness underwater. Carbon dioxide (CO2) must also remain below 1000 ppm, otherwise it will cause rapid breathing and make you empty an already tiny tank in a very short time.

Professional dive shops use a four-stage filtration system that includes activated carbon, molecular sieve, and a carbon monoxide catalyst. If condensation appears inside the cylinder, the desiccant has already failed. If moisture content exceeds 67 ppm, internal corrosion accelerates in both steel and aluminum cylinders under pressure. Aluminum oxide powder from a corroded cylinder can clog the micron-level filter in the second stage, driving breathing resistance from a normal 1.2 joules per liter to the point where it becomes almost impossible to inhale.

Here are several indicators used to assess cylinder air quality and safety:

• Gas dew point: must be below -54°C so the high-pressure regulator will not freeze internally in cold water.

• Dust particles: must be smaller than 2 microns to avoid scratching the precision sealing surfaces in the pressure-reduction valve.

• Oxygen concentration: must remain at 21% ± 0.5%; too much deviation can cause oxygen toxicity or hypoxia.

• Cylinder inspection: hydrostatic testing should be performed every 5 years to check whether expansion exceeds 10%.

A cylinder should be stored with about 20 Bar of residual pressure. If it is completely emptied, outside moisture can flow back in. Seawater contains about 3.5% sodium chloride. Once it enters the cylinder, pitting corrosion can develop on the inner surface of the aluminum alloy. If a corrosion pit exceeds 0.5 mm in depth, stress concentration under 200 Bar pressure can increase more than threefold, creating a real risk of rupture.

Mini cylinders with Yoke fittings typically use an AS568-011 fluororubber O-ring. This seal can withstand 3000 PSI. If the O-ring ages and develops a 1 mm crack, pressure loss at 10 meters can reach 15 Bar per minute. For a 0.5L tank, that means losing one-third of its emergency air in just 3 minutes.

Many hand-operated high-pressure pumps claim to reach 3000 PSI, but while filling a 0.5L cylinder manually, pump temperature can quickly exceed 80°C. If the filter element is poor, that heat can break down lubricants and generate harmful gases. During filling, the cylinder should be cooled in water. By physical law, every 1°C rise in temperature increases pressure by about 0.3%, so the apparently high pressure during filling can drop sharply once the tank cools in the water.

When cylinder pressure falls below 30 Bar, second-stage breathing becomes noticeably stiff. That happens because the regulator spring can no longer properly overcome the intermediate pressure output. This sudden increase in breathing resistance can drive heart rate from 80 to 130 beats per minute. At that heart rate, air consumption may jump from 20 L/min to 60 L/min, and the final 15 liters of air at 10 meters may last only 15 seconds.

The table below compares usable gas among different mini-cylinder sizes when a standard 50 Bar reserve is kept:

Cylinder Water Capacity	Total Air at 200 Bar	50 Bar Reserve	Actual Usable Air
0.5 L	100 L	25 L	75 L
1.0 L	200 L	50 L	150 L
2.0 L	400 L	100 L	300 L

When filling from a small electric compressor, the air intake must be kept well away from the exhaust of any fuel engine. If the intake draws in even a tiny fraction of exhaust gas, the compressed carbon monoxide concentration can become lethal. When a diver feels dizzy or nauseated underwater, it is often not from lack of oxygen, but from invisible toxins intensified by compression.

If the mouthpiece on the second stage is slightly damaged, seawater can leak inward through the one-way valve. In a large tank that may be just annoying; in a 0.5L cylinder, every mouthful of seawater displaces precious breathing gas. Each purge uses about 2 to 3 liters of air, which is a major proportion of the total supply in a micro tank.

If you surface and find the gauge reading zero, that means you have already exceeded the safety reserve. At that point, the first stage must be checked for water intrusion and dried if necessary. The habit of checking the gauge before every dive and keeping residual pressure after every dive is the foundation for preventing these miniature life-support systems from failing when they are needed most.