showing emergency gas supply cylinders|alt=Exterior view of a closed bell, showing the side door to the left, with a 50-litre oxygen cylinder and two 50-litre heliox cylinders mounted to the frame to the side of the door. When used by an
underwater diver, a breathing gas may also be referred to as a diving gas. A safe breathing gas for
hyperbaric use has four essential features: • It must contain sufficient oxygen to support life, consciousness and work rate of the breather. • It must not contain harmful contaminants.
Carbon monoxide and
carbon dioxide are common poisons which may contaminate breathing gases. There are many other possibilities. • It must not become toxic when being breathed at high
pressure such as when
underwater. Oxygen and
nitrogen are examples of gases that become
toxic under pressure. • It must not be too dense to breathe.
Work of breathing increases with
density and
viscosity. Maximum ventilation drops by about 50% when density is equivalent to air at 30
msw, and carbon dioxide levels rise unacceptably for moderate exercise with a gas density exceeding 6 g/litre. Breathing gas density of 10 g/litre or more may cause runaway
hypercapnia even at very low work levels, with potentially fatal effects. These common diving breathing gases are used: •
Air is a mixture of 21%
oxygen, 78%
nitrogen, and approximately 1% other trace gases, primarily
argon; to simplify calculations this last 1% is usually treated as if it were nitrogen. Being freely available and simple to use, it is the most common diving gas. As its nitrogen component causes
nitrogen narcosis, it is considered to have a safe depth limit of about for most divers, although the
maximum operating depth (MOD) of air taking an allowable oxygen partial pressure of 1,6 bar is . Breathing air is air meeting specified standards for contaminants. • Pure oxygen is mainly used to speed the shallow decompression stops at the end of a
military,
commercial, or
technical dive. Risk of acute
oxygen toxicity increases rapidly at pressures greater than 6 metres sea water. It was much used in
frogmen's rebreathers, and is still used by attack swimmers. • are mixtures of two or more gases specifically blended for use as breathing gas for divers. They may be mixed to the composition required before the dive, for open circuit use, or produced in the breathing circuit of a rebreather during the dive. Diving using mixed gases is referred to as mixed gas diving. •
Nitrox is a mixture of oxygen and nitrogen, either by mixing oxygen with air, or by removing nitrogen from air, and generally refers to mixtures which are more than 21% oxygen. It can be used as a tool to accelerate in-water decompression stops or to decrease the risk of
decompression sickness and thus prolong a dive (a common misconception is that the diver can go deeper; this is not true, owing to a shallower maximum operating depth than with conventional air). •
Trimix is a mixture of oxygen, nitrogen, and
helium and is often used at depth in
technical diving and
commercial diving instead of air to decrease the
work of breathing, reduce
nitrogen narcosis, and avoid the dangers of oxygen toxicity. •
Heliair is a form of trimix that is easily blended from helium and air without using pure oxygen. It always has a 21:79 ratio of oxygen to nitrogen; the balance of the mix is helium. •
Heliox is a mixture of oxygen and helium and is often used in the deep phase of a commercial deep dive to eliminate nitrogen narcosis and reduce density, to limit the
work of breathing. Heliox is the standard mixture type for deep offshore
saturation diving. •
Hydreliox is a mixture of oxygen, helium, and
hydrogen and is used for dives below 130 metres in commercial diving. Experimental work using hydreliox is also done in deep technical diving, where the hydrogen is used to reduce
HPNS. •
Hydrox, a gas mixture of
hydrogen and
oxygen, is used as a breathing gas in very
deep diving. •
Neox (also called neonox) is a mixture of oxygen and
neon sometimes employed in deep commercial diving. It is rarely used due to its cost. Also, DCS symptoms produced by neon ("neox bends") have a poor reputation; they are widely reported to be more severe than those produced by an exactly equivalent dive-table and mix with helium.
Breathing air Breathing air is atmospheric air with a standard of purity suitable for human breathing in the specified application. For hyperbaric use, the partial pressure of contaminants is increased in proportion to the absolute pressure, and must be limited to a safe composition for the depth or pressure range in which it is to be used.
Classification by oxygen fraction Breathing gases for diving are classified by oxygen
fraction. The boundaries set by authorities may differ slightly, as the effects vary gradually with concentration and between people, and are not accurately predictable. ;: where the oxygen content does not differ greatly from that of air and allows continuous safe use at atmospheric pressure. ;, or : where the oxygen content exceeds atmospheric levels, generally to a level where there is some measurable physiological effect over long term use, and sometimes requiring special procedures for handling due to increased fire hazard. The associated risks are oxygen toxicity at depth and fire, particularly in the breathing apparatus. ;: where the oxygen content is less than that of air, generally to the extent that there is a significant risk of measurable physiological effect over the short term. The immediate risk is usually hypoxic incapacitation at or near the surface.
Individual component gases Breathing gases for diving are mixed from a small number of component gases which provide special characteristics to the mixture which are not available from atmospheric air.
Oxygen Oxygen (O2) must be present in every breathing gas. This is because it is essential to the
human body's
metabolic process, which sustains life. The human body cannot store oxygen for later use as it does with food. If the body is deprived of oxygen for more than a few minutes, unconsciousness and death result. The
tissues and
organs within the body (notably the heart and brain) are damaged if deprived of oxygen for much longer than four minutes. Filling a diving cylinder with pure oxygen costs considerably more than filling it with compressed air. As oxygen supports combustion and causes rust in
diving cylinders, it should be handled with caution when
gas blending. Oxygen has historically been obtained by
fractional distillation of
liquid air, but is increasingly obtained by
non-cryogenic technologies such as
pressure swing adsorption (PSA) and
vacuum swing adsorption (VSA) technologies. The fraction of the oxygen component of a breathing gas mixture is sometimes used when naming the mix: •
hypoxic mixes, strictly, contain less than 21% oxygen, although often a boundary of 16% is used, and are designed only to be breathed at depth as a "bottom gas" where the higher pressure increases the
partial pressure of oxygen to a safe level.
Trimix,
Heliox and
Heliair are gas blends commonly used for hypoxic mixes and are used in professional and
technical diving as deep breathing gases. A may be assigned to a hypoxic gas mixture, based on the depth at which the partial pressure is equal to the minimum oxygen partial pressure acceptable to the person or organisation using the gas. •
normoxic mixes have the same proportion of oxygen as air, 21%. The maximum operating depth of a normoxic mix could be as shallow as . Trimix with between 17% and 21% oxygen is often described as normoxic because it contains a high enough proportion of oxygen to be safe to breathe at the surface. •
hyperoxic mixes have more than 21% oxygen.
Enriched Air Nitrox (EANx) is a typical hyperoxic breathing gas. Hyperoxic mixtures, when compared to air, cause
oxygen toxicity at shallower depths but can be used to shorten
decompression stops by drawing dissolved inert gases out of the body more quickly. The fraction of the oxygen determines the greatest depth at which the mixture can safely be used to avoid
oxygen toxicity. This depth is called the
maximum operating depth. The concentration of oxygen in a gas mix depends on the fraction and the pressure of the mixture. It is expressed by the
partial pressure of oxygen (PO2). The partial pressure of any component gas in a mixture is calculated as: :
partial pressure = total absolute pressure × volume fraction of gas component For the oxygen component, :PO2 = P × FO2 where: :PO2 = partial pressure of oxygen :P = total pressure :FO2 = volume fraction of oxygen content The minimum safe partial pressure of oxygen in a breathing gas is commonly held to be 16
kPa (0.16 bar). Below this partial pressure the diver may be at risk of unconsciousness and death due to
hypoxia, depending on factors including individual physiology and level of exertion. When a hypoxic mix is breathed in shallow water it may not have a high enough PO2 to keep the diver conscious. For this reason normoxic or hyperoxic "travel gases" are used at medium depth between the "bottom" and "decompression" phases of the dive. The maximum safe PO2 in a breathing gas depends on exposure time, the level of exercise and the security of the breathing equipment being used. It is typically between 100 kPa (1 bar) and 160 kPa (1.6 bar); for dives of less than three hours it is commonly considered to be 140 kPa (1.4 bar), although the U.S. Navy has been known to authorize dives with a PO2 of as much as 180 kPa (1.8 bar). At high PO2 or longer exposures, the diver risks oxygen toxicity which may result in a
seizure. Each breathing gas has a
maximum operating depth that is determined by its oxygen content. For therapeutic recompression and hyperbaric oxygen therapy partial pressures of 2.8 bar are commonly used in the chamber, but there is no risk of drowning if the occupant loses consciousness. For longer periods such as in
saturation diving, 0.4 bar can be tolerated over several weeks.
Oxygen analysers are used to measure the oxygen partial pressure in the gas mix.
Divox is breathing grade oxygen labelled for diving use. In the
Netherlands, pure oxygen for breathing purposes is regarded as medicinal as opposed to industrial oxygen, such as that used in
welding, and is only available on
medical prescription. The diving industry registered Divox as a
trademark for breathing grade oxygen to circumvent the strict rules concerning medicinal oxygen thus making it easier for (recreational)
scuba divers to obtain oxygen for blending their breathing gas. In most countries, there is no difference in purity in medical oxygen and industrial oxygen, as they are produced by exactly the same methods and manufacturers, but labeled and filled differently. The chief difference between them is that the record-keeping trail is much more extensive for medical oxygen, to more easily identify the exact manufacturing trail of a "lot" or batch of oxygen, in case problems with its purity are discovered. Aviation grade oxygen is similar to medical oxygen, but may have a lower moisture content.
Diluent gases Gases which have no metabolic function in the breathing gas are used to dilute the gas, and are therefore classed as diluent gases. Some of them have a reversible narcotic effect at high partial pressure, and must therefore be limited to avoid excessive narcotic effects at the maximum pressure at which they are intended to be breathed. Diluent gases also affect the density of the gas mixture and thereby the
work of breathing.
Nitrogen Nitrogen (N2) is a
diatomic gas and the main component of
air, the cheapest and most common breathing gas used for diving. It causes
nitrogen narcosis in the diver, so its use is limited to shallower dives. Nitrogen can cause
decompression sickness.
Equivalent air depth is used to estimate the decompression requirements of a
nitrox (oxygen/nitrogen) mixture.
Equivalent narcotic depth is used to estimate the narcotic potency of
trimix (oxygen/helium/nitrogen mixture). Many divers find that the level of narcosis caused by a dive, whilst breathing air, is a comfortable maximum. Nitrogen in a gas mix is almost always obtained by adding air to the mix.
Helium .
Helium (He) is an inert gas that is less narcotic than nitrogen at equivalent pressure (in fact there is no evidence for any narcosis from helium at all), and it has a much lower density, so it is more suitable for deeper dives than nitrogen. Helium is equally able to cause
decompression sickness. At high pressures, helium also causes
high-pressure nervous syndrome, which is a central nervous system irritation syndrome which is in some ways opposite to narcosis. Helium mixture fills are considerably more expensive than air fills due to the cost of helium and the cost of mixing and compressing the mix. Helium is not suitable for
dry suit inflation owing to its poor
thermal insulation properties – compared to air, which is regarded as a reasonable insulator, helium has six times the thermal conductivity. Helium's low molecular weight (monatomic MW=4, compared with diatomic nitrogen MW=28) increases the pitch of the breather's voice, which may impede communication. This is because the speed of sound is faster in a lower molecular weight gas, which increases the resonance frequency of the vocal cords. Helium leaks from damaged or faulty
valves more readily than other gases because atoms of helium are smaller allowing them to pass through smaller gaps in
seals. Helium is found in significant amounts only in
natural gas, from which it is extracted at low temperatures by fractional distillation.
Neon Neon (Ne) is an inert gas sometimes used in deep
commercial diving but is very expensive. Like helium, it is less narcotic than nitrogen, but unlike helium, it does not distort the diver's voice. Compared to helium, neon has superior thermal insulating properties. Neon has an atomic and molecular weight of approximately 20, compared to 14 for nitrogen and 4 for helium, but nitrogen gas (N2)has a molecular weight of 28.
Hydrogen Hydrogen (H2) has been used in deep diving gas mixes but is very explosive when mixed with more than about 4 to 5% oxygen (such as the oxygen found in breathing gas). This limits use of hydrogen to deep dives and imposes complicated protocols to ensure that excess oxygen is cleared from the breathing equipment before breathing hydrogen starts. Like helium, it raises the pitch of the diver's voice. The hydrogen-oxygen mix when used as a diving gas is sometimes referred to as
Hydrox. Mixtures containing both hydrogen and helium as diluents are termed
hydreliox.
Unwelcome components of breathing gases for diving Many gases are not suitable for use in diving breathing gases. Here is an incomplete list of gases commonly present in a diving environment:
Argon Argon (Ar) is an inert gas that is more narcotic than nitrogen, so is
not generally suitable as a diving breathing gas.
Argox is used for decompression research. It is sometimes used for
dry suit inflation by divers whose primary breathing gas is helium-based, because of argon's good thermal insulation properties. Argon is more expensive than air or oxygen, but considerably less expensive than helium. Argon is a component of natural air, and constitutes 0.934% by volume of the Earth's atmosphere.
Carbon dioxide Carbon dioxide (CO2) is produced by the
metabolism in the
human body and can cause
carbon dioxide poisoning. When breathing gas is recycled in a
rebreather or
life-support system, the carbon dioxide is removed by
scrubbers before the gas is re-used.
Carbon monoxide Carbon monoxide (CO) is a highly toxic gas that competes with dioxygen for binding to hemoglobin, thereby preventing the blood from carrying oxygen (see
carbon monoxide poisoning). It is typically produced by incomplete
combustion. Four common sources are: •
Internal combustion engine exhaust gas containing CO in the air being drawn into a
diving air compressor. CO in the intake air cannot be stopped by any filter. The exhausts of all internal combustion engines running on petroleum fuels contain some CO, and this is a particular problem on boats, where the intake of the compressor cannot be arbitrarily moved as far as desired from the engine and compressor exhausts. • Heating of
lubricants inside the compressor may vaporize them sufficiently to be available to a compressor intake or intake system line. • In some cases hydrocarbon lubricating oil may be drawn into the compressor's cylinder directly through damaged or worn seals, and the oil may (and usually will) then undergo combustion, being ignited by the immense compression ratio and subsequent temperature rise. Since heavy oils don't burn well – especially when not atomized properly – incomplete combustion will result in carbon monoxide production. • A similar process is thought to potentially happen to any particulate material, which contains "organic" (carbon-containing) matter, especially in cylinders which are used for hyperoxic gas mixtures. If the compressor air filter(s) fail, ordinary
dust will be introduced to the cylinder, which contains organic matter (since it usually contains
humus). A more severe danger is that air particulates on boats and industrial areas, where cylinders are filled, often contain carbon-particulate combustion products (these are what makes a dirt rag black), and these represent a more severe CO danger when introduced into a cylinder. Carbon monoxide is generally avoided as far as is reasonably practicable by positioning of the air intake in uncontaminated air, filtration of particulates from the intake air, use of suitable compressor design and appropriate lubricants, and ensuring that running temperatures are not excessive. Where the residual risk is excessive, a
hopcalite catalyst can be used in the high pressure filter to convert carbon monoxide into carbon dioxide, which is far less toxic.
Hydrocarbons Hydrocarbons (C
xH
y) are present in compressor lubricants and
fuels. They can enter diving cylinders as a result of contamination, leaks, or due to incomplete combustion near the air intake. • They can act as a
fuel in combustion increasing the risk of
explosion, especially in high-oxygen gas mixtures. • Inhaling oil mist can damage the
lungs and ultimately cause the lungs to degenerate with severe
lipid pneumonia or
emphysema.
Moisture content The process of
compressing gas into a diving cylinder removes moisture from the gas. This is good for
corrosion prevention in the cylinder but means that the diver inhales very dry gas. The dry gas extracts moisture from the diver's lungs while underwater contributing to
dehydration, which is also thought to be a predisposing risk factor of
decompression sickness. Evaporation in the lungs causes a significant heat loss. Dry gas is also uncomfortable, causing a dry mouth and throat and making the diver thirsty. This problem is reduced in
rebreathers because the
soda lime reaction, which removes carbon dioxide, also puts moisture back into the breathing gas, and the relative humidity and temperature of exhaled gas is relatively high and there is a cumulative effect due to rebreathing. In hot climates, open circuit diving can accelerate
heat exhaustion because of dehydration. Another concern with regard to moisture content is the tendency of moisture to condense as the gas expands while passing through the regulator; this coupled with the extreme reduction in temperature, also due to the decompression, can cause the moisture to solidify as ice. This
icing up in a regulator can cause moving parts to seize and the regulator to fail or free flow. This is one of the reasons that scuba regulators are generally constructed from brass, and chrome plated (for protection). Brass, with its good thermal conductive properties, quickly conducts heat from the surrounding water to the cold, newly decompressed air, helping to prevent icing up.
Gas analysis Gas mixtures must generally be analysed either in process or after blending for quality control. This is particularly important for breathing gas mixtures where errors can affect the health and safety of the end user. It is difficult to detect most gases that are likely to be present in diving cylinders because they are colourless, odourless and tasteless. Electronic sensors exist for some gases, such as
oxygen analysers,
helium analyser,
carbon monoxide detectors and
carbon dioxide detectors. Oxygen analysers are commonly found underwater in
rebreathers. Oxygen and helium analysers are often used on the surface during
gas blending to determine the percentage of oxygen or helium in a breathing gas mix. Chemical and other types of gas detection methods are not often used in recreational diving, but are used for periodic quality testing of compressed breathing air from diving air compressors.
Breathing gas standards Standards for breathing gas quality are published by national and international organisations, and may be enforced in terms of legislation. In the UK, the Health and Safety Executive indicate that the requirements for breathing gases for divers are based on the BS EN 12021:2014. The specifications are listed for oxygen compatible air, nitrox mixtures produced by adding oxygen, removing nitrogen, or mixing nitrogen and oxygen, mixtures of helium and oxygen (heliox), mixtures of helium, nitrogen and oxygen (trimix), and pure oxygen, for both open circuit and reclaim systems, and for high pressure and low pressure supply (above and below 40 bar supply). Oxygen content is variable depending on the operating depth, but the tolerance depends on the gas fraction range, being ±0.25% for an oxygen fraction below 10% by volume, ±0.5% for a fraction between 10% and 20%, and ±1% for a fraction over 20%. Water content is limited by risks of
icing of control valves, and corrosion of containment surfaces – higher humidity is not a physiological problem – and is generally a factor of
dew point. Other specified contaminants are carbon dioxide, carbon monoxide, oil, and volatile hydrocarbons, which are limited by toxic effects. Other possible contaminants should be analysed based on risk assessment, and the required frequency of testing for contaminants is also based on risk assessment. In Australia breathing air quality is specified by Australian Standard 2299.1, Section 3.13 Breathing Gas Quality.
Diving gas blending Gas blending (or gas mixing) of breathing gases for diving is the filling of
gas cylinders with non-
air breathing gas mixtures. Filling cylinders with a mixture of gases has dangers for both the filler and the diver. During filling there is a risk of fire due to use of oxygen and a risk of explosion due to the use of high-pressure gases. The composition of the mix must be safe for the depth and duration of the planned dive. If the concentration of oxygen is too lean the diver may lose consciousness due to
hypoxia and if it is too rich the diver may develop
oxygen toxicity. The concentration of inert gases, such as nitrogen and helium, are planned and checked to avoid nitrogen narcosis and decompression sickness.
Methods used include batch mixing by partial pressure or by mass fraction, and continuous blending processes. Completed blends are analysed for composition for the safety of the user. Gas blenders may be required by legislation to prove competence if filling for other persons.
Density Excessive density of a breathing gas can raise the work of breathing to intolerable levels, and can cause carbon dioxide retention at lower densities. Helium is used as a component to reduce density as well as to reduce narcosis at depth. Like partial pressure, density of a mixture of gases is in proportion to the volumetric fraction of the component gases, and absolute pressure. The ideal gas laws are adequately precise for gases at respirable pressures. The density of a gas mixture at a given temperature and pressure can be calculated as: :ρm = (ρ1 V1 + ρ2 V2 + .. + ρn Vn) / (V1 + V2 + ... + Vn) where :ρm = density of the gas mixture :ρ1 ... ρn = density of each of the components :V1 ... Vn = partial volume of each of the component gases Since gas fraction Fi (volumetric fraction) of each gas can be expressed as Vi / (V1 + V2 + ... + Vn ) by substitution, :ρm = (ρ1 F1 + ρ2 F2 + .. + ρn Fn) == Hypobaric breathing gases ==