Trimix

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

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 at greater depth. A lower proportion of nitrogen is required to reduce nitrogen narcosis and other physiological effects of the gas at depth. Lowering the oxygen content increases the depth at which the oxygen can safely be breathed. The nitrogen in trimix can prevent High Pressure Nervous Syndrome, a problem when breathing heliox at depths below 130 meters / 429 feet.

Conventionally, the mix is named by its oxygen percentage, helium percentage and optionally the balance percentage, which is nitrogen. For example, a mix suitable for a 100 meters (330 ft) dive might be 10% oxygen, 70% helium, 20% nitrogen - this mix is named "trimix 10/70".

The ratio of gases in a particular mix is chosen to give a safe maximum operating depth and comfortable equivalent air depth for the planned dive. Safe limits for mix of gases in trimix are generally accepted to be a maximum partial pressure of oxygen (ppO₂ - see Dalton's law) of 1.0-1.2 bar and maximum equivalent air depth of 30 meters (100 ft). At 100 meters / 330 feet, "10/70" has a PPO₂ of 1.1 bar and an equivalent air depth of 18 meters /60 feet.

In open-circuit scuba, two classes of trimix are commonly used: "normoxic" trimix, such as "19/30", is used in the 30 meters / 100 feet to 60 meters / 200 feet depth range and "hypoxic" trimix, such as "10/50", is used for deeper diving, as a "bottom" gas only. In rebreathers that use trimix diluents, the mix will be "hyperoxic" in shallow water because the rebreather automatically adds oxygen to maintain a specific ppO₂. See breathing gas for more information on the composition and choice of gas blends.

Gas blending of trimix involves decanting oxygen and helium into the diving cylinder and then topping up the mix with air from a diving air compressor. To ensure an accurate mix, after each helium and oxygen decant, the mix is allowed to cool, its pressure is measured and further gas is decanted until the correct pressure is achieved. This process often take hours and is sometimes spread over days at busy blending stations.

Diving

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