TecEvolution, More about Trimix Gas Mixes

Trimix is defined as any mix of Helium, Nitrogen, and Oxygen. It can be mixed either on a continuous flow trimix blending panel (expensive) or by partial pressure blending. When mixed by partial pressure, people usually mix heliair, because it is just partial pressure blending Helium and toping up the mix with air (you can fin a list of Heliair blends and more information on heliair here). Commercial divers and military divers frequently use Heliox (helium and oxygen), but this is seldom used in the Open Circuit Sport Technical Dive Community (slightly more in the closed circuit rebreather dive community) because of the cost of helium and because of a few other issues including but not limited to HPNS (High Pressure Nervous Syndrome) at depth over 120 metres of sea water (400 fsw) and a higher risk of isobaric counter diffusion (ICD).

It is used for deep dives (below 50 msw, shallower if diving in an overhead environment or task loaded missions) for two primary reasons:

• Helium is not narcotic as depth (avoids nitrogen narcosis)
• Helium has less breathing resistance at depth

Unfortunately, trimix have a few discards that include but are not limited to:

• It can be normoxic (21% O2) but also hypoxic and require a travel gas (if less than 16% O2 in the blend)
• ICD (Isobaric Counter Diffusion) if doing poor gas switches
• Longer Decompression Requirements
• High Cost
• HPNS (High Pressure Nervous Syndrome)
• Helium analysers are extremely expensive

Trimix Blends can be obtained from any Trimix Gas Blending Facility, and blending nitrox requires special training. The trimix cost is usually on a per litre basis. These facilities can use different methods to obtain the desired gas mix. The most common blending method in Thailand being partial pressure, blending method, with seldom places using a continuous flow trimix blending panel.