Trimix is a breathing gas, consisting of oxygen, helium and nitrogen, and is often used in deep commercial diving and during the deep phase of dives carried out using technical diving techniques.
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.
Advantages of helium in the mix
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.
Because of its low molecular weight, helium leaves tissues more rapidly than nitrogen as the pressure is reduced (this is called off-gassing). Because of its lower solubility, helium does not load tissues as heavily as nitrogen.
Disadvantages of helium in the mix
Helium conducts heat six times faster than air; often helium breathing divers 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 stabilising jacket) still require a minimum of inflation to avoid "squeezing", i.e. damage to skin caused by pressurizing dry suit folds.
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