Trimix (breathing gas)
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.
This is frequently used as a breathing gas in deep commercial diving operations, where it is often recycled to save the expensive helium component.
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.
[6] 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).
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.
[7][8] Helium conducts heat six times faster than air, so helium-breathing divers often carry a separate supply of a different gas to inflate drysuits.
Helium dissolves into tissues (this is called on-gassing) more rapidly than nitrogen as the ambient pressure is increased.
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.
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 130 metres (430 ft).
This allows diving throughout the usual recreational range, while decreasing decompression obligation and narcotic effects compared to air.
[19] GUE and UTD also promote hyperoxic trimix for this depth range, but prefer the term "TriOx".
[18] Gas blending of trimix generally involves mixing helium and oxygen with air to the desired proportions and pressure.
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 (preferably for both helium and oxygen) so that the fine adjustment to the intake gas flows can be made.
Drawbacks may be that the high heat of compression of helium results in the compressor overheating, especially in hot weather.
Although theoretically trimix can be blended with almost any combination of helium and oxygen, a number of "standard" mixes have evolved (such as 21/35, 18/45 and 15/55—see Naming conventions).
The basic distinction is that for hypoxic trimix diving the dive cannot be started on the bottom mix, and procedures for use of a travel mix for the first part of the descent, and gas switching during the descent to avoid oxygen toxicity are added to the required skills.
