Trimix
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Trimix is a breathing gas, consisting of oxygen, helium and nitrogen, and is often used in deep commercial diving and 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 on deep dives. 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 and duration of the dive before which oxygen toxicity becomes a limiting factor. Also, 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, nitrogen. For example, a mix named "trimix 10/70" consisting of 10% oxygen, 70% helium, 20% nitrogen is suitable for a 100 meters (330 feet) dive.
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 feet). 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 and cannot safely be breathed at shallow depths where the ppO2 is less than 0.17 bar. 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 transfer, the mix is allowed to cool, its pressure is measured and further gas is decanted until the correct pressure is achieved. This process often takes hours and is sometimes spread over days at busy blending stations.
