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Trimix diving

Is it not fascinating to dive at a wreck that could show you the past?

Often when you dive at the wreck you can see history and sense the presence of the elapsed time!

Trimix is a breathing gas, consisting of oxygen, helium and nitrogen, and is often used in deep 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.

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.

Because of its lower solubility, helium does not load tissues as heavily as nitrogen.

Lowering the oxygen content increases the maximum operating depth and duration of the dive before which oxygen toxicity becomes a limiting factor.

Retaining nitrogen in Trimix can contribute to the prevention of High Pressure Nervous Syndrome, a problem that can occur when breathing Trimix at depths below 130 to 180 meters.

Nitrogen is also much less expensive than helium.

Conventionally, the mix is named by its oxygen percentage, helium percentage and optionally the balance percentage, nitrogen.

For example, a mix named “trimix 10/70″ consisting of 10% oxygen, 70% helium, 20% nitrogen is suitable for a 100-meter dives.


Heliair is a breathing gas consisting of mixture of oxygen, nitrogen and helium and is often used during the deep phase of dives carried out using technical diving techniques.

It is easily blended from helium and air and so has a fixed 21:79 ratio of oxygen to nitrogen with the balance consisting of a variable amount of helium.

It is sometimes referred to as “poor man’s trimix”, because it is much easier to blend than trimix blends with variable oxygen content, since all that is required is to insert the requisite partial pressure of helium, and then top up with air from a conventional compressor.

The more complicated (and dangerous) step of adding pure oxygen at pressure required to blend trimix is absent when blending heliair.


Heliox is a breathing gas composed of a mixture of helium (He) and oxygen (O2).
Owing to the expense of helium, heliox is most likely to be used in deep commercial diving.
The proportion of oxygen in a diving mix depends on the maximum depth of the dive plan but is often hypoxic.