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 (which is cheap but narcotic) and helium (which is not narcotic and reduces work of breathing, but is more expensive and can increase 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.
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 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. 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 (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.
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 (if used together with a buoyancy compensator) 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 (called ingassing) 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.
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