Trimix (breathing gas)

Trimix is a breathing gas consisting of oxygen, helium, and nitrogen. It is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.
The helium is included as a substitute for some of the nitrogen, to reduce the narcotic effect of the breathing gas at depth and to reduce the work of breathing. 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 and helium.
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.

Applications

Trimix is used as an ambient pressure breathing gas for underwater diving. It has been used in both scuba and surface-supplied applications, for professional and recreational diving, and for surface oriented and saturation diving. The most common use is in recreational technical diving.
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. A normoxic mix such as "19/30" is used in the 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 in shallow water, because the rebreather automatically adds oxygen to maintain a specific partial pressure of oxygen. Hyperoxic trimix is also sometimes used on open circuit scuba, to reduce decompression obligations.

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. 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. A lower proportion of oxygen reduces the risk of oxygen toxicity on deep dives.
The lower density of helium reduces breathing resistance at depth. 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.
Because of its low molecular weight, helium enters and leaves tissues by diffusion more rapidly than nitrogen as the pressure is increased or reduced. 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.

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. Dry suits still require a minimum of inflation to avoid "suit squeeze", i.e. injury to skin caused by pinching by tight dry suit folds.
Helium diffuses into tissues more rapidly than nitrogen as the ambient pressure is increased. A consequence of the higher loading in some tissues is that some 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.
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. 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 helium 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 to 1.4 bar and may reduce the P_O₂ further to 1.3 bar or 1.2 bar depending on the depth, the duration and the kind of breathing system used. 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, 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. 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 dive. Hyperoxic trimix is sometimes referred to as Helitrox, TriOx, or HOTx with the "x" in HOTx representing the mixture's fraction of helium as a percentage.
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 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 and has a maximum operating depth of, where it has an equivalent narcotic depth of. This allows diving throughout the usual recreational range, while decreasing decompression obligation and narcotic effects compared to air. GUE and UTD also promote hyperoxic trimix for this depth range, but prefer the term "TriOx".
The use of trimix as a breathing gas while diving is called trimix diving, and is a sub-category of mixed gas diving, also sometimes referred to simply as gas diving.

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.
A second method called 'continuous blending' is done by mixing oxygen and helium into the intake air of a compressor. 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 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.