Trimix (breathing gas)

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Trimix scuba cylinder label Trimix label.png
Trimix scuba cylinder label
IMCA Trimix cylinder shoulder colour code IMCA Trimix shoulder.svg
IMCA Trimix cylinder shoulder colour code
alternative IMCA Trimix cylinder shoulder colour code IMCA Trimix shoulder quartered.svg
alternative IMCA Trimix cylinder shoulder colour code

Trimix is a breathing gas consisting of oxygen, helium and nitrogen and is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, [1] [2] and in advanced recreational diving. [3] [4]

Contents

The helium is included as a substitute for some of the nitrogen, to reduce the narcotic effect of the breathing gas at depth. With a mixture of three gases it is possible to create mixes suitable for different depths or purposes by adjusting the proportions of each gas. Oxygen content can be optimised for the depth to limit the risk of toxicity, and the inert component balanced between nitrogen (which is cheap but narcotic) and helium (which is not narcotic and reduces work of breathing, but is more expensive and increases heat loss).

The mixture of helium and oxygen with a 0% nitrogen content is generally known as heliox. This is frequently used as a breathing gas in deep commercial diving operations, where it is often recycled to save the expensive helium component. Analysis of two-component gases is much simpler than three-component gases.

Function of the helium

The main reason for adding helium to the breathing mix is to reduce the proportions of nitrogen and oxygen below those of air, to allow the gas mix to be breathed safely on deep dives. [1] A lower proportion of nitrogen is required to reduce nitrogen narcosis and other physiological effects of the gas at depth. Helium has very little narcotic effect. [5] A lower proportion of oxygen reduces the risk of oxygen toxicity on deep dives.

The lower density of helium reduces breathing resistance at depth. [1] [5] Work of breathing can limit the use of breathing gas mixtures in underwater breathing apparatus, as with increasing depth a point may be reached where work of breathing exceeds the available effort from the diver. Beyond this point accumulation of carbon dioxide will eventually result in severe and debilitating hypercapnia, which, if not corrected quickly, will cause the diver to attempt to breathe faster, exacerbating the work of breathing, which will lead to loss of consciousness and a high risk of drowning. [6]

Because of its low molecular weight, helium enters and leaves tissues by diffusion more rapidly than nitrogen as the pressure is increased or reduced (this is called on-gassing and off-gassing). Because of its lower solubility, helium does not load tissues as heavily as nitrogen, but at the same time the tissues can not support as high an amount of helium when super-saturated. In effect, helium is a faster gas to saturate and desaturate, which is a distinct advantage in saturation diving, but less so in bounce diving, where the increased rate of off-gassing is largely counterbalanced by the equivalently increased rate of on-gassing.

Some divers suffer from compression arthralgia during deep descent, and trimix has been shown to help avoid or delay the symptoms of compression arthralgia. [7] [8]

Disadvantages of the helium

Helium conducts heat six times faster than air, so helium-breathing divers often carry a separate supply of a different gas to inflate drysuits. This is to avoid the risk of hypothermia caused by using helium as inflator gas. Argon, carried in a small, separate tank connected only to the inflator of the drysuit, is preferred to air, since air conducts heat 50% faster than argon. [9] Dry suits (if used together with a buoyancy compensator) still require a minimum of inflation to avoid "squeezing", i.e. damage to skin caused by pinching by tight dry suit folds.

Helium dissolves into tissues (this is called on-gassing) more rapidly than nitrogen as the ambient pressure is increased. A consequence of the higher loading in some tissues is that many decompression algorithms require deeper decompression stops than a similar pressure exposure dive using air, and helium is more likely to come out of solution and cause decompression sickness following a fast ascent. [10]

In addition to physiological disadvantages, the use of trimix also has economic and logistic disadvantages. The price of helium increased by over 51% between the years 2000 and 2011. [11] This price increase affects open-circuit divers more than closed-circuit divers due to the larger volume of helium consumed on a typical trimix dive. Additionally, as trimix fills require more expensive analysis equipment than air and nitrox fills, there are fewer trimix filling stations. The relative scarcity of trimix filling stations may necessitate going far out of one's way in order to procure the necessary mix for a deep dive that requires the gas.

Advantages of controlling the oxygen fraction

Lowering the oxygen content of a breathing gas mixture increases the maximum operating depth and duration of the dive before which oxygen toxicity becomes a limiting factor. Most trimix divers limit their working oxygen partial pressure [PO2] to 1.4 bar and may reduce the PO2 further to 1.3 bar or 1.2 bar depending on the depth, the duration and the kind of breathing system used. [1] [2] [12] [13] A maximum oxygen partial pressure of 1.4 bar for the active sectors of the dive, and 1.6 bar for decompression stops is recommended by several recreational and technical diving certification agencies for open circuit, [14] and 1.2 bar or 1.3 bar as maximum for the active sectors of a dive on closed-circuit rebreather. Increasing the oxygen fraction in a trimix to be used as a decompression gas can accelerate decompression with a lowered risk of isobaric counter diffusion complications.

Advantages of keeping some nitrogen in the mix

Retaining nitrogen in trimix can contribute to the prevention of High Pressure Nervous Syndrome, a problem that can occur when breathing heliox at depths beyond about 130 metres (430 ft). [1] [15] [16] [17] Nitrogen is also much less expensive than helium.

Naming conventions

The term trimix implies that the gas has three functional components, which are helium, nitrogen and oxygen. Since the nitrogen and all or part of the oxygen is usually provided from air, the other components of ordinary atmospheric air are generally ignored. Conventionally, the composition of a mix is specified by its oxygen percentage, helium percentage and optionally the balance percentage, nitrogen, in that order. For example, a mix named "trimix 10/70" or trimix 10/70/20, consisting of 10% oxygen, 70% helium, 20% nitrogen is suitable for a 100-metre (330 ft) dive. Hyperoxic trimix is sometimes referred to as Helitrox, TriOx, or HOTx (High Oxygen Trimix) with the "x" in HOTx representing the mixture's fraction of helium as a percentage. [18]

The basic term Trimix is sufficient, modified as appropriate with the terms hypoxic, normoxic and hyperoxic, and the usual forms for indicating constituent gas fraction, to describe any possible ratio of gases, but the National Association of Underwater Instructors (NAUI) uses the term "helitrox" for hyperoxic 26/17 Trimix, i.e. 26% oxygen, 17% helium, 57% nitrogen. Helitrox requires decompression stops similar to Nitrox-I (EAN32) and has a maximum operating depth of 44 metres (144 ft), where it has an equivalent narcotic depth of 35 metres (115 ft). This allows diving throughout the usual recreational range, while decreasing decompression obligation and narcotic effects compared to air. [19] GUE and UTD also promote hyperoxic trimix for this depth range, but prefer the term "TriOx".

Applications

In open-circuit scuba, two classes of trimix are commonly used: normoxic trimix—with a minimum PO2 at the surface of 0.18 and hypoxic trimix—with a PO2 less than 0.18 at the surface. [20] A normoxic mix such as "19/30" is used in the 30 to 60 m (100 to 200 ft) depth range; a hypoxic mix such as "10/50" is used for deeper diving, as a bottom gas only, and cannot safely be breathed at shallow depths where the PO2 is less than 0.18 bar.

In fully closed-circuit rebreathers that use trimix diluents, the mix in the breathing loop can be hyperoxic (meaning more oxygen than in air, as in enriched air nitrox) in shallow water, because the rebreather automatically adds oxygen to maintain a specific partial pressure of oxygen. [21] Hyperoxic trimix is also sometimes used on open circuit scuba, to reduce decompression obligations. [18]

Blending

Partial pressure gas blending equipment for scuba diving Gas blending equipment.JPG
Partial pressure gas blending equipment for scuba diving
Gas blending oxygen and helium analyser Gas blending oxygen and helium analyser.JPG
Gas blending oxygen and helium analyser

Gas blending of trimix generally involves mixing helium and oxygen with air to the desired proportions and pressure. Two methods are in common use:

Partial pressure blending is done by decanting oxygen and helium into the diving cylinder and then topping up the mix with air from a diving air compressor. To ensure an accurate mix, after each helium and oxygen transfer, the mix is allowed to cool, its pressure is measured and further gas is decanted until the correct pressure is achieved. This process often takes hours and is sometimes spread over days at busy blending stations. Corrections can be made for temperature effect, but this requires accurate monitoring of the temperature of the mixture inside the cylinder, which is generally not available. [22]

A second method called 'continuous blending' is done by mixing oxygen and helium into the intake air of a compressor. [22] The oxygen and helium are fed into mixing tubes in the intake air stream using flow meters or analysis of the oxygen content after oxygen addition and before and after the helium addition, and the oxygen and helium flows adjusted accordingly. On the high pressure side of the compressor a regulator or bleed orifice is used to reduce pressure of a sample flow and the trimix is analyzed (preferably for both helium and oxygen) so that the fine adjustment to the intake gas flows can be made. The benefit of such a system is that the helium delivery tank pressure need not be as high as that used in the partial pressure method of blending and residual gas can be 'topped up' to best mix after the dive. This is important mainly because of the high cost of helium. Drawbacks may be that the high heat of compression of helium results in the compressor overheating, especially in hot weather. Temperature of the trimix entering the analyser should be kept constant for best reliability of the analysis, and the analyser should be calibrated at ambient temperature before use. The mixing tube is a very simple device, and DIY versions of the continuous blend units can be made for a relatively low cost compared to the cost of analysers and compressor. [22] [23]

Choice of mixture composition

The ratio of gases in a particular mix is chosen to give a safe maximum operating depth and comfortable equivalent narcotic depth for the planned dive. Safe limits for mix of gases in trimix are generally accepted to be a maximum partial pressure of oxygen (PO2—see Dalton's law) of 1.0 to 1.6 bar and maximum equivalent narcotic depth of 30 to 50 m (100 to 160 ft). At 100 m (330 ft), "12/52" has a PO2 of 1.3 bar and an equivalent narcotic depth of 43 m (141 ft).

"Standard" mixes

Although theoretically trimix can be blended with almost any combination of helium and oxygen, a number of "standard" mixes have evolved (such as 21/35, 18/45 and 15/55—see Naming conventions). Most of these mixes originated from starting by decanting a given pressure of helium into an empty cylinder, and then topping up the mix with 32% nitrox. The "standard" mixes evolved because of three coinciding factors — the desire to keep the equivalent narcotic depth (END) of the mix at approximately 34 metres (112 ft), the requirement to keep the partial pressure of oxygen at 1.4 ATA or below at the deepest point of the dive, and the fact that many dive shops stored standard 32% nitrox in banks, which simplifies mixing. [24] The use of standard mixes makes it relatively easy to top up diving cylinders after a dive using residual mix — only helium and banked nitrox are needed to top up the residual gas from the last fill.

The method of mixing a known nitrox mix with helium allows analysis of the fractions of each gas using only an oxygen analyser, since the ratio of the oxygen fraction in the final mix to the oxygen fraction in the initial nitrox gives the fraction of nitrox in the final mix, hence the fractions of the three components are easily calculated. It is demonstrably true that the END of a nitrox-helium mixture at its maximum operating depth (MOD) is equal to the MOD of the nitrox alone.

Heliair

Heliair is a breathing gas consisting of mixture of oxygen, nitrogen and helium and is often used during the deep phase of dives carried out using technical diving techniques. This term, first used by Sheck Exley, [25] is mostly used by Technical Diving International (TDI).

It is easily blended from helium and air and so has a fixed 21:79 ratio of oxygen to nitrogen with the balance consisting of a variable amount of helium. It is sometimes referred to as "poor man's trimix", [25] [26] because it is much easier to blend than trimix blends with variable oxygen content, since all that is required is to insert the requisite partial pressure of helium, and then top up with air from a conventional compressor. The more complicated (and dangerous) step of adding pure oxygen at pressure required to blend trimix is absent when blending heliair.

Heliair blends are similar to the standard Trimix blends made with helium and Nitrox 32, but with a deeper END at MOD. Heliair will always have less than 21% oxygen, and will be hypoxic (less than 17% oxygen) for mixes with more than 20% helium.

History as a diving gas

Training and certification

CMAS-ISA Normoxic Trimix diver certification card CMAS-ISA Normoxic Trimix diver certification card PC160020.jpg
CMAS-ISA Normoxic Trimix diver certification card

Technical diver training and certification agencies may differentiate between levels of trimix diving qualifications, The usual distinction is between normoxic trimix and hypoxic trimix, sometimes also called full trimix. The basic distinction is that for hypoxic trimix diving the dive cannot be started on the bottom mix, and procedures for use of a travel mix for the first part of the descent, and gas switching during the descent to avoid oxygen toxicity are added to the required skills. Longer decompression using a larger variety of mixtures may also complicate procedures. In closed circuit rebreather diving, use of a hypoxic diluent prevents the diver from conducting a diluent flush at shallow depths while breathing from the loop, so that it remains possible at the maximum depth of the dive, where it may be more critical.

See also

Related Research Articles

Nitrox refers to any gas mixture composed of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. In the usual application, underwater diving, nitrox is normally distinguished from air and handled differently. The most common use of nitrox mixtures containing oxygen in higher proportions than atmospheric air is in scuba diving, where the reduced partial pressure of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness.

<span class="mw-page-title-main">Nitrogen narcosis</span> Reversible narcotic effects of respiratory nitrogen at elevated partial pressures

Narcosis while diving is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain gases at high partial pressure. The Greek word νάρκωσις (narkōsis), "the act of making numb", is derived from νάρκη (narkē), "numbness, torpor", a term used by Homer and Hippocrates. Narcosis produces a state similar to drunkenness, or nitrous oxide inhalation. It can occur during shallow dives, but does not usually become noticeable at depths less than 30 metres (98 ft).

Heliox is a breathing gas mixture of helium (He) and oxygen (O2). It is used as a medical treatment for patients with difficulty breathing because this mixture generates less resistance than atmospheric air when passing through the airways of the lungs, and thus requires less effort by a patient to breathe in and out of the lungs. It is also used as a breathing gas diluent for deep ambient pressure diving as it is not narcotic at high pressure, and for its low work of breathing.

<span class="mw-page-title-main">Technical diving</span> Extended scope recreational diving

Technical diving is scuba diving that exceeds the agency-specified limits of recreational diving for non-professional purposes. Technical diving may expose the diver to hazards beyond those normally associated with recreational diving, and to a greater risk of serious injury or death. Risk may be reduced via appropriate skills, knowledge, and experience. Risk can also be managed by using suitable equipment and procedures. The skills may be developed through specialized training and experience. The equipment involves breathing gases other than air or standard nitrox mixtures, and multiple gas sources.

<span class="mw-page-title-main">Deep diving</span> Underwater diving to a depth beyond the norm accepted by the associated community

Deep diving is underwater diving to a depth beyond the norm accepted by the associated community. In some cases this is a prescribed limit established by an authority, while in others it is associated with a level of certification or training, and it may vary depending on whether the diving is recreational, technical or commercial. Nitrogen narcosis becomes a hazard below 30 metres (98 ft) and hypoxic breathing gas is required below 60 metres (200 ft) to lessen the risk of oxygen toxicity.

<span class="mw-page-title-main">Breathing gas</span> Gas used for human respiration

A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed habitats. Oxygen is the essential component for any breathing gas. Breathing gases for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression, reducing nitrogen narcosis or allowing safer deep diving.

<span class="mw-page-title-main">Gas blending for scuba diving</span> Mixing and filling cylinders with breathing gases for use when scuba diving

Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.

<span class="mw-page-title-main">Scuba diving</span> Swimming underwater, breathing gas carried by the diver

Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface breathing gas supply, and therefore has a limited but variable endurance. The name scuba is an anacronym for "Self-Contained Underwater Breathing Apparatus" and was coined by Christian J. Lambertsen in a patent submitted in 1952. Scuba divers carry their own source of breathing gas, usually compressed air, affording them greater independence and movement than surface-supplied divers, and more time underwater than free divers. Although the use of compressed air is common, a gas blend with a higher oxygen content, known as enriched air or nitrox, has become popular due to the reduced nitrogen intake during long or repetitive dives. Also, breathing gas diluted with helium may be used to reduce the effects of nitrogen narcosis during deeper dives.

Hydreliox is an exotic breathing gas mixture of hydrogen, helium, and oxygen. For the Hydra VIII mission at 50 atmospheres of ambient pressure, the mixture used was 49% hydrogen, 50.2% helium, and 0.8% oxygen.

Argox is the informal name for a scuba diving breathing gas consisting of argon and oxygen. Occasionally the term argonox has been used to mean the same mix. The blend may consist of varying fractions of argon and oxygen, depending on its intended use. The mixture is made with the same gas blending techniques used to make nitrox, except that for argox, the argon is added to the initial pure oxygen partial-fill, instead of air.

<span class="mw-page-title-main">Technical Diving International</span> Technical diver training and certification agency

Technical Diving International (TDI) claims to be the largest technical diving certification agency in the world, and one of the first agencies to offer mixed gas and rebreather training. TDI specializes in more advanced Scuba diving techniques, particularly diving with rebreathers and use of breathing gases such as trimix and heliox.

In physiology, isobaric counterdiffusion (ICD) is the diffusion of different gases into and out of tissues while under a constant ambient pressure, after a change of gas composition, and the physiological effects of this phenomenon. The term inert gas counterdiffusion is sometimes used as a synonym, but can also be applied to situations where the ambient pressure changes. It has relevance in mixed gas diving and anesthesiology.

Hydrox, a gas mixture of hydrogen and oxygen, is occasionally used as an experimental breathing gas in very deep diving. It allows divers to descend several hundred metres. Hydrox has been used experimentally in surface supplied, saturation, and scuba diving, both on open circuit and with closed circuit rebreathers.

Equivalent narcotic depth (END) (historically also equivalent nitrogen depth) is used in technical diving as a way of estimating the narcotic effect of a breathing gas mixture, such as nitrox, heliox or trimix. The method is used, for a given breathing gas mix and dive depth, to calculate the equivalent depth which would produce about the same narcotic effect when breathing air.

<span class="mw-page-title-main">Scuba gas planning</span> Estimation of breathing gas mixtures and quantities required for a planned dive profile

Scuba gas planning is the aspect of dive planning and of gas management which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive. It may assume that the dive profile, including decompression, is known, but the process may be iterative, involving changes to the dive profile as a consequence of the gas requirement calculation, or changes to the gas mixtures chosen. Use of calculated reserves based on planned dive profile and estimated gas consumption rates rather than an arbitrary pressure is sometimes referred to as rock bottom gas management. The purpose of gas planning is to ensure that for all reasonably foreseeable contingencies, the divers of a team have sufficient breathing gas to safely return to a place where more breathing gas is available. In almost all cases this will be the surface.

<span class="mw-page-title-main">Scuba gas management</span> Logistical aspects of scuba breathing gas

Scuba gas management is the aspect of scuba diving which includes the gas planning, blending, filling, analysing, marking, storage, and transportation of gas cylinders for a dive, the monitoring and switching of breathing gases during a dive, efficient and correct use of the gas, and the provision of emergency gas to another member of the dive team. The primary aim is to ensure that everyone has enough to breathe of a gas suitable for the current depth at all times, and is aware of the gas mixture in use and its effect on decompression obligations, nitrogen narcosis, and oxygen toxicity risk. Some of these functions may be delegated to others, such as the filling of cylinders, or transportation to the dive site, but others are the direct responsibility of the diver using the gas.

<span class="mw-page-title-main">John Morgan Wells</span> Physiologist, aquanaut and researcher (1940–2017)

John Morgan Wells was a marine biologist, and physiologist involved in the development of decompression systems for deep diving, and the use of nitrox as a breathing gas for diving. He is known for developing the widely used NOAA Nitrox I and II mixtures and their decompression tables in the late 1970s, the deep diving mixture of oxygen, helium, and nitrogen known as NOAA Trimix I, for research in undersea habitats, where divers live and work under pressure for extended periods, and for training diving physicians and medical technicians in hyperbaric medicine.

<span class="mw-page-title-main">Outline of underwater diving</span> Hierarchical outline list of articles related to underwater diving

The following outline is provided as an overview of and topical guide to underwater diving:

Diving support equipment is the equipment used to facilitate a diving operation. It is either not taken into the water during the dive, such as the gas panel and compressor, or is not integral to the actual diving, being there to make the dive easier or safer, such as a surface decompression chamber. Some equipment, like a diving stage, is not easily categorised as diving or support equipment, and may be considered as either.

<span class="mw-page-title-main">Diving rebreather</span> Closed or semi-closed circuit scuba

A Diving rebreather is an underwater breathing apparatus that absorbs the carbon dioxide of a diver's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the diver. This differs from open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, and, for covert military use by frogmen or observation of underwater life, to eliminate the bubbles produced by an open circuit system. A diving rebreather is generally understood to be a portable unit carried by the user, and is therefore a type of self-contained underwater breathing apparatus (scuba). A semi-closed rebreather carried by the diver may also be known as a gas extender. The same technology on a submersible or surface installation is more likely to be referred to as a life-support system.

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