Just Air? From Outside Air to Breathing Air in Your Diving Cylinder
What happens between outside air and a filled diving cylinder? An extensive look at compressors, cooling, drying, filtering, and the inspection of breathing air.
1. Just Air
During introductory dives, I almost always tell beginner divers the same thing: there is no pure oxygen in a diving cylinder, nor any special breathing gas. It simply contains air. The same air we breathe at the surface here and have been breathing our entire lives. Nothing special. The only remarkable thing is that we have compressed a large amount of it into a relatively small cylinder. That often reassures people. And essentially, it is true.
But this at the same time does some justice to the work that takes place at a filling station. Because much more is involved than just pressing air into a cylinder. Specialized and quite expensive equipment is required: not only a compressor but also various ancillary equipment to ensure that the air is ultimately suitable for breathing underwater.
Between the outside air that a compressor intakes and the breathing air that finally ends up in your diving cylinder under high pressure, quite a lot happens. That air must be compressed in several stages, cooled in between, stripped of water and oil, and ultimately passed through a filtering system. During this, we must prevent contaminants from the environment or from the compression process itself from entering the breathing air. And then we must also be able to check if the final product is actually clean enough for breathing underwater. Filling a diving cylinder is therefore indeed mostly just compressing a lot of ordinary air into a cylinder.
2. What is Air Actually?
Our entire life takes place in air. From our first breath shortly after birth to our last breath, we are continuously surrounded by it. We move in it, work in it, sleep in it, and breathe it in and out every few seconds. Precisely because air is always and everywhere present, we hardly ever contemplate it.
Diving is one of the exceptions. As soon as we go underwater, we leave the environment in which our body normally functions. Suddenly, we have to bring the air we need ourselves. The same applies, even more extremely, to space travel. Underwater and in space, something that is completely taken for granted at the surface suddenly becomes a vital necessity that we must organize technically.
But what exactly is air?
Air is not a single substance but a mixture of different gases. If we ignore water vapor, ordinary dry air consists of approximately 78% nitrogen and 21% oxygen. The small remainder mainly consists of argon, along with about 0.04% carbon dioxide (CO₂) and traces of other gases. That sounds like air is the same everywhere. And for the main components, that is surprisingly approximately true. The ratio between nitrogen and oxygen does not suddenly change whether you are in Curaçao, the Netherlands, or high in the mountains.
But air is never just that neat list of gases from the chemistry book. Outside air also contains water vapor, and the amount of that can vary enormously. The warmer the air, the more water vapor it can contain. In warm, humid tropical regions, such as the Caribbean, the Amazon, and Southeast Asia, water vapor can therefore constitute several percent of the air. In desert areas, such as around the Red Sea in Egypt and Saudi Arabia, the relative humidity of the air is usually much lower. Yet warm desert air can still contain quite a bit of water vapor in absolute terms. Conversely, icy cold polar air may have a relative humidity of 100% and still contain much less water than warm desert air. The colder the air, the less water vapor is needed to become saturated.
Additionally, outside air contains all sorts of small particles and substances that depend strongly on the environment. We often notice that without thinking. The air by the sea smells different than the air in a forest, and the air in the middle of a busy city smells different again. Sea air, for example, contains microscopic droplets and salt particles that come into the air through waves and splashing seawater. The typical smell of the sea does not actually come from the salt itself but mainly from volatile substances produced in and around the sea. In a forest, we find pollen, dust, and various organic substances coming from plants and trees.
In urban and industrial areas, very different substances can be added to that air. Exhaust gases, smoke, and combustion processes introduce fine particles, nitrogen oxides, and carbon monoxide into the air. Under certain conditions, such a mixture of air pollution can result in what we call smog. Solvents, paint, fuels, and other chemicals can also release volatile compounds into the ambient air. So air is everywhere largely the same mixture of nitrogen and oxygen, but that does not mean all outside air is equally clean. And for a compressor, that makes quite a difference. A compressor does not produce air. It uses the air that is already there. So good breathing air doesn't start at the compressor, but with the air the compressor draws in.
3. What is a diving compressor?
To compress air, you need a compressor. Simply put, that's a machine that draws in air and then compresses that air into an increasingly smaller volume, causing the pressure to rise. The air particles or gas molecules are thus brought increasingly closer together.
We encounter compressors everywhere. In a garage, compressed air is used for tools and for inflating tires. Refrigerators and air conditioners have compressors, and they also play an important role in various industrial processes. The basic principle is always the same: a gas is compressed so that its pressure increases. A diving compressor essentially does the same, but goes quite a bit further. A normal compressor you find at your local hardware store usually delivers a pressure of about 8 to 10 bar. Heavier workshop compressors can supply, for example, 10 to 15 bar. That is more than enough for a car tire or compressed air tools, but it's not enough for divers. We need a much higher pressure.
The air around us at sea level is at a pressure of about 1 bar. A diving cylinder is usually filled up to 200 bar, and some cylinders and storage systems work with 300 bar or slightly more. To get an idea of what that means: at 200 bar, we have, very simply put, compressed about 200 liters of outside air into a volume of one liter. In an 11-liter diving cylinder at 200 bar, there is roughly 2,200 liters of air, measured at normal atmospheric pressure.
That does not mean a compressor simply blows out air at 200 or 300 bar. The air is compressed step by step. In a typical high-pressure compressor for diving cylinders, this happens in three, four, or sometimes more compression stages. Each stage essentially consists of the same process. A quantity of air is drawn into a cylinder, a piston then reduces the available space, and the compressed air is sent to the next stage. There, that same air is compressed again into a smaller volume, causing the pressure to rise further.
This does not happen all at once, and there are good reasons for that. Air becomes hot when you compress it. At the same time, the outside air drawn in contains water vapor, some of which condenses during the compression and cooling process. Therefore, the air is cooled between the different compression stages, and condensate water is separated.
A diving compressor is thus not a single pump that brings air directly from 1 to 200 or 300 bar. It is more of a chain of successive compression steps in which the pressure is gradually raised and the air is cooled and treated in between. A compressor therefore consists of different ‘stages’, with cylinders and pistons that become progressively smaller. After each stage, the same amount of air occupies less volume. Consequently, less space is needed to compress that air further.
In a typical four-stage high-pressure compressor, for example, the cylinder of the first stage can have a diameter of approximately 110 mm. This first stage raises the intake of outside air from about 1 bar to about 3 bar. Then the air flows through cooling pipes, where part of the heat that arose during compression is dissipated.
The second cylinder is already considerably smaller, for example 60 mm in diameter, and further compresses the air to about 15 bar. Then, the air is cooled again and condensate is separated. The third stage is even smaller — for example 32 mm — and raises the pressure further to about 50 to 70 bar. Again, the air is cooled and condensate is separated before the air goes to the last stage. Only in the fourth stage, where in this example the cylinder is about 12 mm in diameter, is the air brought up to the final pressure of 200, 300 bar or a bit higher. Then a final cooling follows and condensate is separated again. This way, the same amount of air is compressed further in four steps and the cylinders become smaller and smaller. That is no coincidence. As the pressure increases, the volume of the same amount of air decreases further. Therefore, each subsequent stage has to process a much smaller volume of air than the previous one.
| 1st stage | 2nd stage | 3rd stage | 4th stage | |
|---|---|---|---|---|
| Compression | 1 bar → ±3 bar | ±3 bar → ±15 bar | ±15 bar → ±50–70 bar | ±50–70 bar → 200–300+ bar |
| Cylinders (diam.) | 110 mm | 60 mm | 32 mm | 12 mm |
By the way, you also know the word 'stages' from your regulator, which has a first and a second stage. Only there, the exact opposite happens. While the different stages of a compressor increase the pressure step by step, the two stages of your regulator reduce that high pressure back to ambient pressure. The first stage is the part that connects to the valve of your diving cylinder. This reduces the cylinder pressure, for example 200 bar, to an intermediate pressure of about 8 to 10 bar above ambient pressure. Next, the second stage — the part with the mouthpiece you breathe from — reduces that intermediate pressure further down to ambient pressure. That ambient pressure naturally depends on the depth at which you are diving. At the surface, it is about 1 bar, at 10 meters about 2 bar, and at 20 meters about 3 bar. Your regulator adapts to this and delivers the air at about the same pressure as the water surrounding you at that moment. In a certain sense, a compressor and a regulator do exactly the opposite: the compressor raises air pressure in stages from ambient pressure of about 1 bar to 200–300 bar, while the regulator reduces that pressure in two stages back from 200 (or 300) bar to ambient pressure.
4. What does the compressor draw in?
We saw earlier that a compressor does not make air. It draws air from its surroundings and compresses it. This means the quality of breathing air doesn’t only start at the filter system. Good breathing air starts with good intake air.
Before outside air enters the compressor, it first passes through an intake filter. This is primarily a particle filter: it stops dust, sand, pollen, and other small solid particles that we don’t want inside the compressor. In a dusty or salty environment, such a filter is important not only for air quality but also for the compressor’s lifespan. However, such an intake filter does not make polluted outside air clean. Gaseous contaminants like carbon monoxide (CO), carbon dioxide (CO₂), and vapors from fuels, paints, or solvents can pass through it. Therefore, it remains very important where the compressor takes its air from. For example, you wouldn’t place an air intake next to the exhaust of a car, truck, or diesel generator. You also want to keep smoke, paint, solvents, fuels, cleaning agents, and other volatile substances as far away as possible from the intake.
Combustion gases pose a particular risk. They can release carbon monoxide (CO): a colorless and odorless gas that can be dangerous at very low concentrations. If a running generator, car engine, or other combustion engine is near the air intake, the compressor can simply suck in those exhaust gases. Also, the exhaust of another machine or a poorly ventilated compressor room can affect the quality of the drawn-in air. After all, a compressor continuously moves large volumes of air. What seems harmless at some distance can therefore still reach the air intake. A filter behind the compressor is not a free pass to suck in bad air. Filters can do a lot, but not everything, and their capacity is limited. The cleaner the air entering the process at the start, the easier it is to produce reliable breathing air at the end.
There are also systems that further treat the drawn-in air before it even reaches the compressor. The standard intake filter mainly blocks dust, sand, pollen, and other solid particles, but there are many more specialized systems as well. For example, installations exist that reduce the CO₂ content of the intake air before it is compressed. Other systems use UV light to inactivate microorganisms such as bacteria, viruses, and fungi in the drawn-in air. Such systems can be useful when conditions make it difficult to obtain sufficiently clean intake air. The principle is interesting: instead of trying to remove an unwanted substance entirely at the end of the process from the compressed air, you address the problem already before the compressor. But even these systems do not change the basic principle: the cleaner the air you intake, the better.
Not only pollution plays a role. Temperature and humidity also strongly influence the work that a compressor and its filtration system must perform. We notice this, for example, in a tropical climate like Curaçao. Warm air can contain a lot of water vapor, and a compressor draws in hundreds of liters of this air every minute. All that water initially goes into the compressor as well. During compression, the air heats up, and during cooling, part of that water vapor condenses into liquid water. The warmer and more humid the intake air, the more water the system ultimately has to separate, and the heavier the drying system and filters are burdened. Therefore, it does matter where the compressor is located in a filling station and where its air intake is positioned.
5. Heat and Water
Anyone who watches a compressor running for a while quickly notices two things: it gets hot and surprisingly much water comes out. That may seem strange. After all, we only put air into the machine. So where do all that heat and water come from?
Why does air get hot when you compress it?
When we compress air, we bring the gas molecules closer together. This requires work. Part of that energy ends up as heat in the gas, causing the temperature of the air to rise significantly during compression. You can experience this principle at home as well. If you quickly pump up a tire with a bicycle pump, the pump gets warm. A diving compressor essentially does the same but continuously and at much higher pressures. For that reason, the air is cooled again after each stage of compression. The hot compressed air flows through cooling pipes or intercoolers where a fan blows outside air alongside, dissipating much of the heat. After the final stage of compression, the same happens in the aftercooler or final cooler. This cooling is important not only for the compressor but also has a second effect.
Where does all that water come from?
The water that comes out of a compressor is not produced by the compressor. It was already in the drawn-in outside air — but not as liquid water, instead as invisible water vapor. We saw earlier that warm air can hold much more water vapor than cold air. Therefore, as air cools down, a point can come where it can no longer hold all the water vapor present. The air becomes saturated. The temperature at which this happens is called the dew point. If the air cools down further below the dew point, the water vapor begins to condense into tiny droplets of liquid water. The same happens when dew lies on the grass in the morning, when a cold glass becomes wet on the outside, or when a mirror fogs up during a shower. The water does not appear out of nowhere; it was already present as water vapor in the air.
In a compressor, exactly the same happens but on a much larger scale. The compressor continuously draws in large amounts of outside air. In a warm and humid climate, every cubic meter contains quite a bit of water. During compression, the air heats up, and then it is cooled down repeatedly between compression stages. At each cooling, part of the water vapor condenses. The condensate separators remove the resulting liquid water from the air stream. This is not clean water, by the way. In an oil-lubricated compressor, this condensate contains not only the water that was originally water vapor in the outside air but also small amounts of compressor oil. The result is an oil-containing condensate that is collected separately and must be disposed of. Still, cooling and separation do not remove all the water from the air. After the last separator, the compressed air still contains water vapor. And it is precisely this remaining moisture that becomes important in the next step: drying and filtering our breathing air.
6. What should not be in your tank?
We have now seen that the outside air drawn into a compressor can contain quite a bit of water and that we must remove this moisture from the air as much as possible during and after compression. But water is not the only thing we prefer not to find in our final breathing air. Other substances can also enter the compressed air through the drawn-in outside air or during the compression process. Think, for example, of carbon monoxide, an increased concentration of carbon dioxide, oil, or volatile organic compounds. Some of these are unwanted even in very small amounts, especially when we inhale them under pressure and at depth.
That does not mean, by the way, that breathing air must be chemically completely pure. That would also be impossible. As we saw at the beginning, air itself is already a mixture of different gases and outside air always contains small amounts of other substances. What matters is which substances are present and at what concentration. So let's first look at the main substances that we do not want — or only want in very small amounts — in our scuba tank.
Carbon monoxide (CO)
Carbon monoxide is one of the most dangerous contaminants that can get into breathing air. The gas is colorless and odorless and is produced by incomplete combustion. Exhaust gases from cars, generators, and other combustion engines are therefore important possible sources. CO is dangerous because it binds much more strongly to hemoglobin in the blood than oxygen does. This reduces the blood's ability to carry oxygen. For a diver, this is particularly dangerous. Underwater, the partial pressure of a gas increases with the ambient pressure. A contaminant present in a very small concentration at the surface is thus inhaled at a higher partial pressure at depth. The best defense against CO is therefore primarily to prevent it from being drawn in. A good location of the air intake is essential. Some filter systems also contain a catalyst, such as hopcalite, which can convert CO into CO₂.
Carbon dioxide (CO₂)
Carbon dioxide is naturally present in outside air and is produced, among other things, by respiration and combustion. In open outside air, the concentration normally lies in the range of a few hundred parts per million (ppm), but in poorly ventilated spaces, this concentration can increase significantly. CO₂ is a normal part of our breathing: our bodies produce it continuously, and we exhale it again. However, a too high concentration in inhaled air makes it more difficult to remove CO₂ from the body. This can lead to headache, shortness of breath, increased breathing rate, and eventually more serious symptoms. For divers, CO₂ is also an important gas because exertion, high breathing resistance, and insufficient ventilation of the lungs can contribute to CO₂ buildup in the body. Therefore, we do not want to start with an elevated concentration in our breathing air as well. A standard breathing air filter does not necessarily remove CO₂ effectively. If a compressor draws air with an increased CO₂ content, this can eventually reappear in the produced breathing air. Hence, good ventilation and the placement of the air intake are again important. For situations where the drawn air structurally contains too much CO₂, there are special systems that remove CO₂ from the air before or after compression.
Water (moisture)
Water should also only be present in very limited amounts in breathing air. Moisture in a scuba tank can cause corrosion over time, especially in steel tanks. Moreover, water can freeze at very low temperatures and cause problems in valves and regulators. This is particularly relevant when compressed air cools down significantly during expansion. Therefore, throughout the entire compression process, a lot of attention is paid to cooling, condensate separation, and drying. The molecular sieve in the final filter removes a large part of the remaining water vapor.
Oil and hydrocarbons
Many high-pressure compressors are lubricated with oil. This oil should obviously not enter the breathing air in significant amounts. During the compression process, very small quantities of oil can be carried along with the airflow as droplets, aerosols, or vapor. Additionally, hydrocarbons can already be present in the drawn outside air, for example from fuels, solvents, or exhaust gases. Separators remove liquid oil and aerosols, while activated carbon plays an important role in removing oil vapors and other organic compounds. Good breathing air should ultimately be almost odorless and tasteless. A distinct oily or chemical smell or taste is therefore always a reason to doubt the air quality.
Oxygen
The oxygen content itself is also an important parameter. Ordinary outside air contains about 20.9% oxygen. With regular compressed air, we therefore expect to find approximately the same ratio. A noticeably different oxygen percentage can indicate that the air is mixed with another gas or that something else has happened in the production or filling process than intended. With Nitrox, the oxygen percentage is deliberately increased. But that is a different story, which we will return to in a separate chapter.
VOCs
Finally, various volatile organic compounds can be present in air, often designated by the English abbreviation VOCs: Volatile Organic Compounds. This is not one particular substance, but a large group of compounds that evaporate easily. They can, for example, come from gasoline and diesel, paint, glue, cleaning agents, and solvents. Which VOCs are present and how harmful they are varies greatly. Therefore, once again the first line of defense is simple: ensure that the compressor intakes as clean outside air as possible. Activated carbon can adsorb many organic vapors, but even here a filter is not a free pass to intake contaminated air. In the end, it always comes down to the same principle: what does not enter the intake air, you do not have to remove later.
7. From compressed air to breathing air
We now know which substances we prefer not to find in our scuba tank. Some of these can be kept out of the compressor by ensuring clean intake air. Another part is removed during the compression process when the air is cooled and condensate is separated. But even then, we are not quite done. After the last stage of compression, the compressed air still contains water vapor and may contain small amounts of oil vapor and other contaminants. Before we can store this air as breathing air in our tanks, it must therefore be further treated. A special filtering system is used for this.
Molecular sieve
One of the most important materials in a breathing air filter is a molecular sieve. This usually consists of zeolite: a hard, crystalline material mainly composed of silicon, aluminum, and oxygen. In a filter cartridge, it usually appears as small grains or beads. The special part is inside. Zeolite has a crystal structure with an enormous network of microscopic pores and channels. As a result, a handful of these grains actually has a surprisingly large internal surface to which water molecules can adhere.
This process is called adsorption. It resembles absorption, but there is an important difference. In absorption, a substance is taken up into another material, such as water in a sponge. In adsorption, molecules stick to the surface of a material. A molecular sieve thus removes the remaining water vapor from the compressed air and reduces the moisture content much further than cooling and mechanical separation alone would allow.
Activated carbon
A second commonly used filter material is activated carbon. This material also has an enormous internal surface due to its microscopic structure. Activated carbon adsorbs, among other things, oil vapors and various organic compounds. This helps not only to remove harmful substances but also unwanted odor and taste from the breathing air. The latter is very recognizable for a diver. Good breathing air should basically have no smell or taste. If you clearly taste or smell oil, exhaust fumes, or a chemical odor during a dive, that is a signal that something may be wrong.
Catalyst
Some filter systems also contain a catalyst, such as hopcalite. This material, a brown-black mixture of manganese and copper oxides that looks like coarsely ground coffee, is used to convert carbon monoxide (CO) into carbon dioxide (CO₂). A catalyst accelerates a chemical reaction without being consumed itself. In this case, carbon monoxide reacts with oxygen, producing carbon dioxide:
2 CO + O₂ → 2 CO₂
That might sound strange—after all, we also don't want carbon dioxide in excessive concentrations in our breathing air—but carbon monoxide is particularly dangerous even at very low concentrations. The conversion to CO₂ makes it possible to significantly reduce the CO content. Such a catalyst also only works well under the right conditions. Moisture is especially important: too much water vapor can greatly reduce the effectiveness. This is another reason why the air must be properly dried first.
A filter has a limited capacity
None of these materials work indefinitely. A molecular sieve eventually becomes saturated with water. Activated carbon becomes loaded with the substances it binds to. The action of a catalyst also depends on the conditions under which it is used. Therefore, filter cartridges have a limited service life. How long they last depends, among other things, on the amount of air flowing through them, the temperature of the compressed air, and especially the amount of moisture the filter must handle. This also explains why good cooling and water separation before the filter are so important. Every drop of water you can remove earlier from the system does not have to be captured by the filter material.
A filter is not a magic box
It is tempting to see the filtration system as the component that automatically turns any kind of compressed air into clean breathing air. It does not work like that. A filter can only remove what it is designed for and only as long as the filter material still has sufficient capacity. Some contaminants are hardly or not at all removed by a standard filter cartridge. Therefore, good breathing air starts much earlier in the process: with clean intake air, a well-maintained compressor, effective cooling and condensate separation, and timely replacement of filter material. The filter bank is thus a crucial final step in air treatment. But it cannot repair what fundamentally went wrong earlier in the process. A serious filling station therefore does not rely on a single filter. The air is treated in different steps, each addressing a different problem.
First, dust and other solid particles are kept out of the sucked-in air. During and after compression, liquid water and oil are separated. In more extensive installations, the compressed air can then be further cooled and dried (using refrigerated dryers), so that even more water condenses and can be drained. Then filter materials such as molecular sieve and activated carbon remove the remaining water vapor, oil vapors, and other contaminants. Additional filters or catalysts may be used for specific substances. The principle is always the same: remove each contaminant as early and as efficiently as possible from the air, so that the next step has as little to do as possible.
Linguistically, it is interesting that we conveniently call all these different processes "filtering." Technically, a lot is happening. Dust and other solid particles are blocked. Droplets of water and oil are separated. Water vapor and organic compounds are adsorbed onto materials such as molecular sieve and activated carbon. And some substances can even be chemically converted with the help of a catalyst. "Filtering" is therefore actually a surprisingly simple word for a whole range of different physical and chemical processes. Each technique tackles a different problem. A water droplet requires a different approach than water vapor, and a particle behaves very differently than a carbon monoxide molecule. That is precisely why a good breathing air system usually consists of different steps.
8. When is air actually 'breathing air'?
In the previous two paragraphs, we saw which substances we prefer not to find in our breathing air and how we filter them out. That does not mean breathing air must be chemically completely pure. That would also be impossible. Even clean outside air, for example, contains carbon dioxide and water vapor, and even after compression and filtering, very small amounts of oil, hydrocarbons, or other contaminants may remain. The question is therefore not only what is in the air but especially how much. Or more simply: how clean is clean enough?
There are standards for this. They define the requirements compressed air must meet before we can use it as breathing air. This includes, among other things, the oxygen content and the maximum allowed amounts of carbon monoxide, carbon dioxide, water, and oil. Strikingly, there is no universal worldwide standard for this. In Europe, EN 12021 is primarily used, while in the United States, CGA Grade E is an important reference for diving breathing air. Canada uses CSA Z180.1, and Australia and New Zealand AS/NZS 2299.1. China has its own national standard GB 18435-2007 that specifically relates to breathing gases for divers. Many smaller countries do not have their own separate standard for diving breathing air and in practice align with one of these larger standard systems or adopt similar requirements.
The values are expressed in percentages, ppm, and mg/m³. The unit ppm stands for parts per million: the number of parts of a particular substance per million parts of air. A concentration of 5 ppm CO means that in every million gas molecules, approximately five molecules of carbon monoxide are present. mg/m³, on the other hand, indicates how many milligrams of a substance are present in one cubic meter of air.
| Region | EU | USA | Canada | China | Australia/NZ |
|---|---|---|---|---|---|
| Standard | EN 12021 | CGA Grade E | CSA Z180.1 | GB 18435-2007 | AS/NZS 2299.1 |
| O₂ | 20–22% | 20–22% | 20–22% | 20–22% | 20–22% |
| CO₂ | ≤ 500 ppm | ≤ 1,000 ppm | ≤ 600 ppm | ≤ 500 ppm | ≤ 600 ppm |
| CO | ≤ 5 ppm | ≤ 10 ppm | ≤ 5 ppm | ≤ 10 ppm | ≤ 5 ppm |
| H₂O | ≤ 25 mg/m³ | ≤ 24 ppm* | ≤ 34 ppm* | dew point ≤ −21 °C | ≤ 50 mg/m³ |
| Oil | ≤ 0.5 mg/m³ | ≤ 5 mg/m³ | ≤ 1 mg/m³ | ≤ 5 mg/m³** | ≤ 0.5 mg/m³ |
| THC | — | ≤ 25 ppm | ≤ 10 ppm*** | — | — |
| Odor | None | None | Slight | None | None |
The first thing that stands out from the table is probably how many similarities there are. For the oxygen content, the five standards practically agree: regular breathing air should contain about 21% oxygen. They also all agree that carbon monoxide and carbon dioxide must only be present in very small concentrations. Yet, the limits are not exactly the same everywhere. For carbon monoxide, EN 12021, for example, allows a maximum of 5 ppm, while CGA Grade E and the Chinese standard allow up to 10 ppm. For carbon dioxide, the limit varies from 500 ppm in Europe and China to 1,000 ppm in CGA Grade E. The differences are also considerable for oil: from 0.5 mg/m³ in Europe and Australia/New Zealand to 5 mg/m³ in the American standard. The Chinese limit of 5 mg/m³ is not directly comparable, as oil mist and particles are grouped together there.
The way standards consider contaminants also differs. For example, the American and Canadian standards set a separate limit for THC, Total Hydrocarbons. Such substances can originate from oils and fuel vapors. Benzene, which occurs among other things in gasoline vapor, is an example of a hydrocarbon we definitely don't want in our breathing air. The European, Chinese, and Australian/New Zealand values in this comparison do not include a separate THC limit for regular breathing air. This does not mean hydrocarbons play no role there, but the standards use different ways to specify air quality.
Comparing moisture is even more complicated. Moisture can be expressed as a concentration in ppm or mg/m³, but also as dew point. The Chinese standard, for example, uses the latter and requires a dew point of a maximum of −21 °C. In short, there is no magic number below which compressed air suddenly becomes “breathing air.” The exact limits differ per standard, and sometimes the methods of measuring contaminants do as well. But the starting point is always the same: the air must contain sufficient oxygen and at the same time be clean and dry enough to be safely inhaled. A standard thus tells us how clean our breathing air must be. But it does not tell us how clean the air from our own compressor actually is. For that, testing is necessary.
9. Trust is good, measuring is better
As a diver, you are probably already familiar with an oxygen analyzer. Anyone who has ever dived with Nitrox has probably used such a device to check the oxygen percentage in a cylinder before a dive. For the other substances we want to monitor in breathing air, measuring equipment also exists. This can be done in various ways.
The most comprehensive test is a laboratory analysis. A sample of the produced breathing air is taken and examined by a specialized laboratory. This can very accurately determine how much CO, CO₂, water, oil, and other contaminants the air contains. Such an analysis provides a reliable picture of the air quality at the moment the sample was taken. But that is also a limitation. A laboratory analysis is a snapshot. If the air is excellent today, this does not automatically mean it will still be so next week. A filter gradually becomes saturated, a compressor can wear out, and the quality of the outside air drawn in can change. A generator that was not next to the air intake yesterday might suddenly be there tomorrow.
That is why a filling station must also regularly or even continuously monitor the air quality itself. There are sensors and analyzers available for oxygen (O₂), carbon monoxide (CO), carbon dioxide (CO₂), and moisture. More extensive systems can also measure, for example, VOCs (Volatile Organic Compounds). Air quality measuring devices are often very expensive, ranging from several hundred euros or dollars for a device that only measures oxygen, up to thousands of euros or dollars for a professional combined device that measures several of these parameters at once, and amounts in the five-digit range in euros or dollars for a continuous breathing air quality monitor.
Such a measurement system can make changes visible much faster than a periodic laboratory analysis. If the CO level suddenly begins to rise or the air coming out of the filter system becomes increasingly moist, that is an indication that something is changing somewhere. Perhaps polluted outside air is being drawn in, a separator is no longer functioning properly, or a filter cartridge is becoming saturated. The two methods therefore complement each other well. A laboratory analysis can periodically (for example monthly or quarterly) thoroughly check whether the produced breathing air meets the standard. Regular or continuous measurements can meanwhile monitor whether important values change between those analyses.
That measuring equipment usually contains multiple sensors, each measuring a certain substance in its own way. For oxygen (O₂) and carbon monoxide (CO), electrochemical sensors are often used, while carbon dioxide (CO₂) is usually measured with infrared light. Moisture can be determined with a sensor that reacts to water molecules, and for VOCs, for example, a Photoionization Detector (PID) can be used. A professional breathing air analyzer is thus actually a collection of small specialized measuring instruments in one device. Each sensor has its own accuracy and lifespan and can drift or become exhausted over time. Therefore, sensors must be checked regularly and, where necessary, calibrated or replaced.
This leads to an important principle for every filling station: You should not only trust your compressor and your filters. You must also check whether they actually do what you think they do. And ultimately, the same applies to the measuring equipment itself. Trust is good. Measuring is better. But then you must be able to trust your measurement.
10. Why don’t we fill directly from the compressor?
A diving cylinder contains a surprisingly large amount of air. A standard 11-liter cylinder filled to 200 bar contains the equivalent of over 2,000 liters of air. And at a busy dive center, sometimes dozens of such cylinders have to be filled one after another.
A compressor cannot produce that amount of air at the same rate it is needed during filling. For example, a compressor of 300 liters per minute produces about 18,000 liters of free air per hour; a machine of 450 liters per minute approximately 27,000 liters. Yet a filling station can supply much more air to a whole row of diving cylinders in a short time. This is because the compressor and the filling panel usually do not work directly with each other. The produced breathing air is first stored at high pressure in buffer tanks: large high-pressure cylinders that together form a supply of breathing air. Buffer tanks thus solve an important practical problem. A compressor produces air slowly but steadily, while a dive center consumes that air in peaks. Sometimes barely anything is filled for an hour, and shortly afterward, twenty or thirty empty cylinders are suddenly ready.
While it is quiet, the compressor can fill the buffer tanks. When the cylinders come in, the stored air can then be delivered much faster. Buffer tanks thus decouple production and consumption from each other. The compressor does not need to start every time someone wants to fill one cylinder, and the speed at which cylinders can be filled is no longer solely dependent on the amount of air the compressor produces at that same moment. That difference can be quite large. A compressor of 450 liters per minute produces approximately 27,000 liters of free air in an hour. In a buffer system, that air can be collected over a longer period and then made available to the filling panel in a much shorter time. The compressor may have taken hours to build up a large supply; the divers who come to fill their cylinders in the afternoon notice little of it.
Buffer tanks also have an important advantage: they make it possible to work with a cascade. In this system, the buffer tanks are not used as one large supply but distributed across different banks. And this brings us to a principle that every divemaster who regularly fills tanks should actually understand: having a buffer supply is one thing; using that supply efficiently is something else. For that, we use a cascade system.
Suppose we have an 11-liter diving cylinder that still contains 50 bar after the dive. We want to fill it back up to 200 bar. Behind our filling panel, there are three buffer banks. These can be three separate buffer tanks, but at larger filling stations, they can also be groups of multiple connected buffer tanks:
LOW → MID → HIGH
The names do not denote a fixed pressure. LOW is used for the first part of the filling, MID for the next part, and HIGH is reserved for the last bit. As air is used and buffers are refilled, the pressures can change, and in a manual cascade system, the roles of the different banks can also switch. As soon as we connect a buffer bank to a diving cylinder, air naturally flows from the place with the highest pressure to the place with the lowest pressure. This continues until both pressures equalize—or until we close the valve and switch to the next bank. A buffer bank can never fill a diving cylinder to a pressure higher than the pressure that remains in that bank itself.
That is why we start with LOW. As long as the pressure in LOW is higher than the pressure in the cylinder, we can still use that air. A buffer bank at, for example, 120 bar can therefore still be used very well to transfer air to a cylinder at 50 bar. It would be a waste to immediately tap into our precious high-pressure supply for that. That is the essence of a cascade: use low pressure as long as low pressure is sufficient, and save high pressure for the last part of the filling.
But how much difference does it actually make? What does a cascade actually yield?
Let’s take a somewhat larger filling station as an example. We have twenty buffer tanks of 50 liters each, all filled to 280 bar. At the same time, sixty diving cylinders arrive. They are 11-liter cylinders that, after the dive, still average 50 bar and need to be filled again to 200 bar. Each cylinder then needs 11 × (200 − 50) = 1,650 liters of air. For sixty cylinders, we therefore need a total of 60 × 1,650 = 99,000 liters of air. At first glance, that seems no problem. Our twenty buffer tanks together have a volume of 1,000 liters and are at 280 bar. So roughly 280,000 liters of free air are in there. More than enough, you would think.
But suppose now that we connect all twenty buffer tanks into one large bank. To be able to fill a cylinder to 200 bar, the pressure in that bank must remain above 200 bar. Of the 280 bar we have, we can therefore actually only use about 80 bar: 1,000 × (280 − 200) = 80,000 liters. With that, we cannot fill all sixty cylinders. We need 99,000 liters, but can only draw about 80,000 liters from the bank before the pressure drops to about 200 bar. The strange thing is that there is absolutely no shortage of air at that point. The buffer tanks still contain about 200,000 liters of air. Only that air is now at about 200 bar. So we still have a lot of air, but not enough pressure to fill our diving cylinders up to 200 bar. That is exactly why we divide the same twenty buffer tanks over different banks. For example:
- LOW: 10 × 50 L = 500 liters
- MID: 5 × 50 L = 250 liters
- HIGH: 5 × 50 L = 250 liters
All three start at 280 bar in our example. Now we use LOW for the first part of each filling. That bank may eventually fall well below 200 bar. That is no problem: as long as the pressure in the cylinder is still lower, we can keep using air from LOW. When LOW no longer has sufficient pressure difference, we switch to MID. And only for the last part of the filling do we use HIGH. We try to keep that bank at a high pressure as long as possible, because that high pressure is needed precisely to finally fill the cylinders up to 200 bar. With this cascade setup, we can utilize the same air supply much more effectively. In our simplified calculation example, we can fill all sixty diving cylinders from 50 to 200 bar.
So we started with exactly the same twenty buffer tanks and exactly the same amount of air. The only thing we have changed is the way we use that air supply. And that shows something important: a cascade system does not mean you have more air. It means you can actually use much more of the air you have. At a filling station, not only the quantity of air is valuable, but also the pressure at which that air is available. One hundred thousand liters of air at 100 bar can be particularly useful for the first part of a fill, but you can't fill a cylinder to 200 bar with it. For that, you need to keep part of your supply at a higher pressure.
A large pressure difference has another consequence. If we connect an almost empty diving cylinder to a high-pressure buffer bank and open the valve fully at once, a huge amount of air flows into the cylinder in a very short time. That might seem efficient, but it is not advisable. As we saw earlier with the compressor, air becomes warm when compressed. The same happens during filling a diving cylinder. The faster we fill, the less time that heat has to be released to the environment through the wall of the cylinder. As a result, the air inside the cylinder can become quite warm. This also affects the final fill pressure. The pressure of a gas is related to its temperature. A warm cylinder that immediately after filling shows 200 bar may only have 190 bar after cooling down, for example. You have not lost air; the same amount of air simply exhibits a lower pressure at a lower temperature.
That is why the valves of a buffer system must be opened slowly and gradually. This is especially true when the pressure difference is large. You are not trying to get the cylinder to pressure as fast as possible, but to fill it in a controlled manner and keep the heat generation within reasonable limits. Manufacturers can specify maximum or recommended fill rates for this purpose; these must always be followed. In the past, you often saw large water baths at filling stations in which the diving cylinders were placed during filling. The water could absorb part of the heat from the cylinder and thus help keep the temperature lower during filling. Such water baths are now much less used. A controlled fill rate usually makes them unnecessary, while a water bath also has disadvantages: the cylinder and valve remain wet continuously, and water can more easily enter the filling connection.
The most important principle therefore remains simple: do not fill faster than necessary. Allow the cylinder and the air time to release heat to the environment and keep in mind that a warm cylinder's pressure will still drop after filling when it cools down. As a guideline, a fill rate of about 20–40 bar per minute is often mentioned for diving cylinders. A cylinder that needs to be brought from 50 to 200 bar would thus take about 4 to 8 minutes at that rate. Technically, it is often possible to fill the same cylinder in two or three minutes, but that creates more heat and therefore a greater pressure drop when the cylinder cools again. Filling more slowly does not necessarily mean a filling station can process fewer cylinders per hour. Those who want to fill efficiently should better invest in more filling points than in a higher fill rate. A station where, for example, 16 or 20 cylinders can be connected simultaneously can quietly distribute the air over all those cylinders and still process a large number of cylinders per hour. A station with only four or six filling points reaches pressure more quickly to get each individual cylinder off the panel as fast as possible.
When the ambient temperature is very warm, as is the case in many typical diving destinations, especially in the afternoon, it can also be useful not to bring the tanks to 200 bar at once, but first to around 180 bar, then give them time to cool down and then top them up again. Efficient filling thus does not necessarily mean fast filling. It mainly means filling many cylinders simultaneously, in a controlled manner and at a sensible fill rate.
11. Finally the cylinder
After all the compression stages, cooling, separation, filtering, checks, and buffer tanks, we have finally arrived at the last part of the installation: the filling hose. For the person filling the cylinder, this may seem the simplest part of the entire process. You connect a cylinder, open a few valves and wait until the pressure gauge shows 200 bar. But precisely because high pressure is involved here, it is important to always follow the same fixed procedure.
Before you connect a cylinder, you first check whether it is actually allowed to be filled. Look at the cylinder, the valve, and the filling connection and check that there are no visible damages or other reasons not to fill the cylinder. Depending on the filling station, this should also include checking the inspection date and the permitted working pressure. A cylinder intended for 200 bar, of course, may not simply be filled up to 300 bar. Then you remove the dust cap and connect the filling hose correctly. The connection to the cylinder valve uses the same two systems you know from regulators: DIN and INT (yoke). For both, there are traditional screw or clamp connections, as well as quick couplings that make connecting large numbers of cylinders easier.
Only when everything is connected correctly do you gradually start the airflow. Especially with a large pressure difference, this must be done gradually. First open the air supply a little and let the pressure rise in a controlled manner. When filling multiple cylinders simultaneously, the air can be distributed over several filling points. Then open the valves further only as far as the desired filling speed allows.
During filling you monitor not only the pressure but also the temperature of the cylinder. A cylinder may warm up during filling, but when a cylinder quickly becomes very warm, that is a good reason to reduce the filling speed. Also keep in mind that the pressure will slightly drop again after filling as the cylinder cools. When filling large numbers of cylinders, it can be practical to first fill to, for example, 180 bar, let the cylinders cool down, and then gently top them up to 200 bar.
When the desired final pressure is reached, you close the air supply and the cylinder valve. Then you release the air from the filling hose via the designated vent. Only when the hose is completely depressurized do you disconnect the filling connection. Optionally check the final pressure once more and then replace the dust cap on the valve. At many filling stations, such a dust cap also serves a practical second function: cap off means the cylinder is filled and ready for use; cap off means the cylinder still needs to be filled. This allows at a glance to see which cylinders still need to be at the filling panel and which can go back to the dive site.
12. Working safely at the filling panel
A filling system operates at pressures of 200 to 300 bar or more. This means even a relatively small mistake can have major consequences. A filling hose that lets go under pressure can, for example, swing around with great force, and high-pressure air escaping through a small opening can exit at very high velocity. Therefore, always check before filling whether the filling connection is properly tightened and whether hoses, couplings, and valves look normal. Do not use a filling hose with visible damage, leakage, or a suspicious coupling until it has been checked. Also, do not lean your face over a valve, filling connection, or hose while filling. Stay as much as possible out of the direct line of couplings and connections. If something lets go or fails, you do not want to be right in front of it.
A connection must also never be loosened while there is still pressure on the filling hose. First close the cylinder valve and the air supply, then vent the filling hose via the designated vent, and check if the connection is pressureless before loosening it. If during filling you suddenly hear a loud leak, see a hose move, or something else abnormal happens, do not try to fix the problem with your hands while the system is still under pressure. First stop the air supply and depressurize the affected part of the system.
Anyone filling cylinders does well to protect themselves as well. Safety goggles protect the eyes if something unexpected goes wrong with a connection, O-ring, or coupling. In a compressor room or filling station, quite a lot of noise can also be produced. Venting of filling hoses can also cause a short but sharp noise. Hearing protection is therefore advisable, especially when working for extended periods at the filling station.
Work gloves can also be practical. Not so much because of the high pressure itself, but because during a busy filling day you constantly handle heavy cylinders, valves, hoses, and fittings. Your hands can get quite rough from that. However, make sure the gloves provide enough grip and feel to operate valves and fittings properly. Finally, sturdy, closed shoes, preferably safety shoes, are not an unnecessary luxury. A filled diving cylinder is heavy, and a cylinder that falls over or slips from your hands during transport can cause quite some damage to your feet. Slippers and other open shoes therefore really do not belong at a filling station.
Personal protective equipment does not automatically make the filling process safe. The most important protection remains a well-maintained installation, a fixed filling procedure, and an operator who knows what he is doing.
13. Just air?
We started this chapter with a reassuring message for beginner divers: in a diving cylinder there is actually nothing special. No pure oxygen, no mysterious breathing gas, just the same air we inhale every day. After everything we have now discussed, "just air" might sound a little different.
We now know that this air must first be drawn in at a suitable location and that even ordinary outside air can contain water vapor, dust, exhaust gases, and other contaminants. We have seen how a compressor raises that air in several stages from about 1 bar to 200 or 300 bar, why so much heat is generated in the process, and why surprisingly much water must be removed from the air along the way. We also now know that a compressor alone does not yet produce breathing air. For that, the air must be cooled, condensate separated, and the remaining water vapor, oil, and other contaminants reduced to safe levels. We have been introduced to molecular sieves, activated carbon, and catalysts and seen that behind the simple word filtering are various physical and chemical processes.
And we know that "clean" is not just a matter of feeling. For breathing air, there are standards with limit values for, among other things, oxygen, carbon monoxide, carbon dioxide, moisture, and oil. To know if our own air meets these standards, we have to measure — and even our measuring equipment must be checked regularly.
In addition, that clean breathing air still has to be stored and distributed. Buffer tanks not only provide us with a large air supply but a cascade system also ensures efficient use of the available pressure. Ultimately, that air reaches the cylinder through the filling panel and filling hose, where filling speed, temperature, final pressure, and safety again play a role.
Anyone who has read all this is actually no longer a layman. That does not mean you have to be a compressor mechanic and you do not have to be able to repair every part of a filling station yourself. But you do now understand what happens, why it happens, and where things can go wrong. That is perhaps the most important thing for a divemaster or instructor. Because every time you open a valve and breathe from your regulator, you trust that someone before you has done all those steps correctly. And in the end, there really is just air in your cylinder. But quite a lot is needed for that.
Note
This article is also part of the Pro Diving Guide of Porto Mari Dive Center, a series of in-depth articles for dive professionals and interested recreational divers.
