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 30 m (100 ft) dive, whilst breathing air, is a comfortable maximum. nitrogen in a gas mix is almost always obtained by adding air to the mix. helium 2% heliox storage quad. 2% oxygen by volume is sufficient at pressures exceeding 90 msw. 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 timbre 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. 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 timbre 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 (cxhy) 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. it 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 is decompressed 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 electro-galvanic fuel cell as used in a diving rebreather 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 air, oxygen and helium partial pressure gas blending system nitrox continuous blending compressor installation gas blending (or gas mixing) of breathing gases for diving is the filling of gas cylinders with non-air breathing gases. 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