What are the differences between trimix and air?

Answer 1

"Trimix" is a term used in commercial and technical Scuba diving to refer to a breathing gas composed primarily of three gases: helium, nitrogen and oxygen. To understand why divers use this mixture of gases as a breathing gas, we'll need to take a look at the typical gas used in scuba: air.

Air is comprised of nominally 21% oxygen and 79% nitrogen. All other gases contained in air comprise well under one percent of the total mix. Nitrogen has some negative side effects when breathed at the pressures required for diving. In order to reduce the effects of nitrogen in the breathing gas, some percentage of helium is often used in place of nitrogen.

Helium is considered an "inert" gas, in that it isn't used in the metabolic breathing cycle. Since it is a small molecule gas (it has 28% of the molecular weight of nitrogen), a diver's body tissues absorb and release helium much more quickly than they do nitrogen. This reduces the risk of decompression illness when compared to a nitrogen-rich gas like air. Helium also has the advantage of not being narcotic. Nitrogen, at the elevated pressures at depth, can cause intoxication: symptom called nitrogen narcosis.

The net result of these two factors is that divers using helium in place of nitrogen can dive to deeper depths for longer periods. Deep commercial "saturation" dives and deep "technical" dives are often performed using a mix of helium and oxygen called heliox.

There are also disadvantages to using helium. The most significant of these is its scarcity and resulting high cost. World supplies of helium are critically low and the cost continues to rise. As a result, helium is most often used by divers using rebreathers and in surface supplied and deep saturation diving, all of which conserve breathing gas.

Helium also conducts heat six times more efficiently than air. This introduces the risk of hypothermia if a diver uses their heliox breathing gas to inflate their dry suit. Commercial and technical divers using heliox as a breathing gas often carry a separate cylinder of argon to keep their dry suit inflated at depth.

Trimix, consisting of oxygen, helium and nitrogen is often used in place of pure heliox. This achieves some of the advantages of heliox at a greatly reduced cost. A typical trimix mixture used in deeper diving is trimix 10/70. This consists of 10% oxygen, 70% helium and 20% nitrogen. This gas mix won't support consciousness at the surface (10% oxygen is considered hypoxic), but it does allow diving to 100 meters. Most trimix gas mixes are bespoke, with the mix percentages being designed for a particular dive profile.