Breathing gas: Encyclopedia - Breathing gas

Breathing gas

Air is the most common and only natural breathing gas. Other artificial gases, either pure gases or mixtures of gases, are used in enclosed breathing environments such as SCUBA equipment, recompression chambers, submarines, space suits and anaesthetic machines.

A safe breathing gas has three essential features:

• it must contain sufficient oxygen to support the life, consciousness and work rate of the breather.
• it must not contain harmful gases. Carbon monoxide and carbon dioxide are common poisons in breathing gases. There are many others.
• 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.

Most breathing gases are a mixture of oxygen and one or more inert gases. The techniques used to fill diving cylinders with gases other than air are called gas blending.

Breathing gas - Common diving breathing gases

Common diving breathing gases are:

• Air is a mixture of 21% oxygen, 78% nitrogen, and approximately 1% other trace gases; to simplify calculations this last 1% is usually treated as if it were nitrogen. Being cheap and simple to use, it is the most common diving gas. As its nitrogen component causes nitrogen narcosis it has a safe depth limit of 40 metres (130 feet) for most divers.
• Pure oxygen is mainly used to speed the shallow decompression stops at the end of a technical dive. It was much used in frogmen's rebreathers.
• Nitrox is a mixture of oxygen and air, and generally refers to mixtures which are more than 21% oxygen. It is mainly used instead of air to accelerate decompression or to decrease the risk of Decompression sickness.
• Trimix is a mixture of oxygen, nitrogen and helium and is often used during the deep phase of a technical dive.
• Heliox is a mixture of oxygen and helium and is often used in the deep phase of a commercial deep dive.
• Heliair is a mixture of oxygen, nitrogen, and helium. It is suitable for in the deep phase of a technical dive. Blended from helium and air, it always has a 21:79 ratio of oxygen to nitrogen; the balance of the mix is helium.
• Neox is a mixture of oxygen and neon sometimes employed for technical dives. It is rarely used due to its cost.

Breathing gas - Individual component gases

Breathing gas - 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 results. The tissues and organs within the body (notably the heart and brain) are damaged if deprived of oxygen for much longer than four minutes.

The proportion of oxygen in a breathing gas determines the depth at which the mixture gas can safely be used:

• hypoxic mixes have lower proportion of oxygen than air, 21%, or more strictly less than 16% oxygen and are designed only to be breathed at depth as a "bottom gas". Trimix, Heliox and Heliair are used to create typical hypoxic mixes and are used in technical diving as deep breathing gases.
• normoxic mixes have the same proportion of oxygen as air, 21%. The maximum operating depth of a normoxic mix could be as shallow as 47 mtres/155 feet. Trimix is often described as normoxic even the level of oxygen is less than 21% but high enough to be safe to breath on the surface.
• hyperoxic mixes have a more oxygen than 21%. Nitrox is a typical hyperoxic breathing gas. Breathing Nitrox, as opposed to air, can result in less nitrogen narcosis. Hyperoxic mixtures, when compared to air, cause oxygen toxicity at shallower depths but can be used to shorten decompression stops by drawing the dissolved nitrogen come out of the body more quickly.

The minimum safe partial pressure of oxygen in a breathing gas is 16 kPa (0.16 bar). Below this partial pressure the diver risks unconsciousness and death due to hypoxia. When a hypoxic mix is breathed is shallow water it may not have a high enough ppO2 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 partial pressure of oxygen in a breathing gas depends on exposure time, but for dives of less than 3 hours is commonly considered to be 140 kPa (1.4 bar), although the U.S. Navy has been known to authorize dives with a partial oxygen pressure of as much as 180 kPa (1.8 bar). At high partial pressures or longer exposures, the diver risks oxygen toxicity including a seizure similar to an epileptic fit. Each breathing gas has a maximum operating depth that is determined by its oxygen content.

Oxygen analysers are used to measure the concentration of oxygen in the gas mix.

Filling a diving cylinder with pure oxygen costs around five times more than filling it with compressed air. As oxygen supports combustion and causes rust in diving cylinders, it should be handled with respect when gas blending.

"Divox" is oxygen. In the Netherlands, pure oxygen 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 "created" Divox and registered it as a trademark to circumvent the strict rules concerning medicinal oxygen thus making it easier for (recreational) scubadivers to obtain oxygen for blending their breathing gas.

Breathing gas - Nitrogen

Nitrogen (N₂) is an inert 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 often used to help design a breathing gas mix by determining the maximum nitrogen content for a particular depth of dive. Many divers find that the level of narcosis caused by a 30-metre (100-foot) dive, whilst breathing air, is a comfortable maximum. The partial pressure of nitrogen at this depth on air is 316 kPa (3.16 bar) (Fraction of nitrogen x absolute pressure = 0.79 x 400 kPa). So, what fraction of nitrogen would cause the same narcosis at 60 metres? The answer is 45% nitrogen. (316 kPa/700 kPa)

Breathing gas - Helium

Helium (He) is an inert gas that is less narcotic than nitrogen at diving pressures, so it is more suitable for deeper dives than nitrogen. But helium can still cause decompression sickness. It also causes High Pressure Nervous Syndrome. It is not very suitable for dry suit inflation due to its poor thermal insulation properties — helium is a very good conductor of heat, but air is a rather poor conductor of heat. Helium fills typically cost ten times more than an equivalent air fill. Helium also distorts the diver's voice, which may impede communication.

Breathing gas - 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.

Breathing gas - 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 complicated protocols to insure that oxygen is cleared from the lungs, the blood stream and the breathing equipment before breathing hydrogen starts. Like helium, it distorts the diver's voice. See Exotic diving gases. Tech Diver Web. URL accessed on Jan 9, 2005.

Breathing gas - Unwelcome components of breathing gases

Many gases are not suitable for use in diving breathing gases. Here is an incomplete list.

Breathing gas - Argon

Argon (Ar) is an inert gas that is more narcotic than nitrogen, so is not suitable as a diving breathing gas. It is used for dry suit inflation because of its good thermal insulation properties. Argon is much more expensive than air.

Breathing gas - Carbon dioxide

Carbon dioxide (CO₂) is produced by the metabolism of the human body and causes carbon dioxide poisoning.

Breathing gas - Carbon monoxide

Carbon monoxide (CO) is produced by incomplete combustion. Two common sources are:-

It causes carbon monoxide poisoning.

Breathing gas - Hydrocarbons

Hydrocarbons (C_xH_y) can occur due to compressor lubricants leaking, or due to incomplete combustion near the air intake, as for carbon monoxide.

Breathing gas - 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 divers 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 throat and making the diver thirsty. This problem is reduced with rebreathers because the soda lime reaction to remove carbon dioxide puts moisture back into the breathing gas. In a hot tropical climate, open circuit diving can accelerate heat exhaustion because of dehydration.

Breathing gas - Gas detection and measurement

Divers find it 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. Chemical and other types of gas detection methods are not often used in diving.

Adapted from the Wikipedia article "Breathing gas", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki