TRIMIX is one of the gas mixes used in technical diving below 40 m to 100 m and consists of oxygen, helium and nitrogen. The mixture of the three gases allows the diver to blend mixes suitable for different depths or purposed by adjusting the proportions of each gas to give the BEST MIX.
The proportions of the gas mix are found on the TRIMIX cylinder, thus, TX 16/40 means that it contains 16 percent oxygen, 40 percent helium and the rest is nitrogen.
TRIMIX is ideal for dives below 40 m with helium added to the mix to reduce the proportion of nitrogen and oxygen below those of air, allowing this gas mix to be used safely during deep dives and preventing toxicity.
Oxygen becomes toxic at a partial pressure of 1.6 bar, and nitrogen causes nitrogen narcosis at a partial pressure of 3.5 bar and above.
Due to the low percentage of oxygen, some TRIMIX mixes should not be used on the surface as the oxygen partial pressure is too low. For that purpose, divers use TRAVEL MIX, a blend that is used before and after deepwater dives.
For a dive from 60 to 100 m, at least three blends of TRIMIX are used – bottom mix, travel mix for the ascent, which is used after passing the halfway mark on the ascent, and deko mix, which is used at the surface.
TRIMIX usage and procedures vary from school to school; some diving schools use standard three blends of TRIMIX where the first mix is used from 40 to 60 m, the second from 60 to 80 m and the third from 80 to 100 m.