Trimix

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.^[1][2]

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.

Mixes

Advantages of helium in the mix

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.^[1] 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.^[3] A lower proportion of oxygen reduces the risk of oxygen toxicity on deep dives.

The lower density of helium reduces breathing resistance at depth.^[1][3]

Because of its low molecular weight, helium leaves tissues more rapidly than nitrogen as the pressure is reduced (this is called off-gassing). Because of its lower solubility, helium does not load tissues as heavily as nitrogen.

Disadvantages of helium in the mix

Helium conducts heat 5 times faster than air; often helium breathing divers carry separate gas supplies to inflate drysuits.^{[clarification needed]}

Some divers suffer from hyperbaric arthralgia during descent.^[4]

Helium dissolves into tissues more rapidly than nitrogen as the ambient pressure is increased (this is called on-gassing). A consequence of the higher loading in some tissues is that many decompression algorithms require deeper decompression stops than a similar decompression dive using air.

Advantages of reducing oxygen in the mix

Lowering the oxygen content increases the maximum operating depth and duration of the dive before which oxygen toxicity becomes a limiting factor.^[1][2][5][6]

Advantages of keeping some nitrogen in the mix

Retaining nitrogen in trimix can contribute to the prevention of High Pressure Nervous Syndrome, a problem that can occur when breathing heliox at depths below 130 meters (429 feet).^[1][7][8][9] Nitrogen is also much less expensive than helium.

Naming

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-metre (330 ft) dive.

The ratio of gases in a particular mix is chosen to give a safe maximum operating depth and comfortable equivalent narcotic 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 1.0–1.6 bar and maximum equivalent narcotic depth of 30 to 50 m (100 to 160 ft). At 100 m (330 ft), "12/52" has a PPO₂ of 1.3 bar and an equivalent narcotic depth of 43 m (140 ft).

In open-circuit scuba, two classes of trimix are commonly used: normoxic trimix - with a minimum PO2 at the surface of 0.18 and hypoxic trimix - with a PO2 less than 0.18 at the surface.^[10] A normoxic mix such as "19/30" is used in the 30 to 60 m (100 to 200 ft) depth range; a hypoxic mix 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 0.18 bar.

In fully closed circuit rebreathers that use trimix diluents, the mix can be hyperoxic in shallow water because the rebreather automatically adds oxygen to maintain a specific ppO₂.^[11] Less commonly, hyperoxic trimix is sometimes used on open circuit scuba. Hyperoxic trimix is sometimes referred to as Helitrox or TriOx.

See breathing gas for more information on the composition and choice of gas blends.

Blending

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.^[12]

A second method called 'continuous blending' is now gaining favor.^[12] Oxygen, helium and air are blended on the intake side of a compressor. The oxygen and helium are fed into the air stream using flow meters, so as to achieve the rough mix. The low pressure air is analyzed for oxygen content and the oxygen (and helium) flows adjusted accordingly. On the high pressure side of the compressor a regulator is used to reduce pressure and the trimix is metered through an analyzer (preferably helium and oxygen) so that the fine adjustment to the intake gas flows can be made.

The benefit of such a system is that the helium delivery tank pressure need not be as high as that used in the partial pressure method of blending and residual gas can be 'topped up' to best mix after the dive.

Drawbacks may be that the increased compressibility of helium results in the compressor over-heating (especially in tropical climates) and that the hot trimix entering the analyzer on the high pressure side can affect the reliability of the analysis. DIY versions of the continuous blend units can be made for as little as $200 (excluding analyzers).^[12][13]

"Standard" mixes

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). Most of these mixes originated from filling the cylinders with a certain percentage of helium, and then topping the mix with 32% enriched air nitrox. The "standard" mixes evolved because of three coinciding factors - the desire to keep that equivalent narcotic depth of the mix at approximately 100 feet, the requirement to keep the partial pressure of oxygen at 1.4 ATA or below at the deepest point of the dive, and the fact that many dive shops stored standard 32% enriched air nitrox in banks, which simplified mixing.^[14] The use of standard mixes makes it relatively easy to top up diving cylinders after a dive using residual mix - only helium and banked nitrox needs to be used to top up the residual gas from the last fill.

Helitrox/TriOx

The National Association of Underwater Instructors (NAUI) uses the term "helitrox" for 26/17 Trimix, i.e. 26% oxygen, 17% helium, 57% nitrogen. Helitrox requires decompression stops similar to Nitrox-I (EAN28) and has a maximum operating depth of 44 metres (140 ft), where it has an equivalent narcotic depth of 35 metres (110 ft). This allows diving throughout the usual recreational range, while decreasing decompression obligation and narcotic effects compared to air.^[15]

GUE and UTD also promote hyperoxic trimix, but prefer the term "TriOx".

History of trimix as a diving gas

1919

Professor Elihu Thompson speculates that helium could be used instead of nitrogen to reduce the breathing resistance at great depth.^[16] The effects from narcosis was not proven until the salvage of the USS Squalus in 1939.^[16] Heliox was used with air tables resulting in a high incidence of decompression sickness so the use of helium was discontinued.^[17]

1925

The US Navy begins examining helium's potential usage and by the mid 1920's lab animals were exposed to experimental chamber dives using heliox. Soon, human subjects breathing heliox 20/80 (20% oxygen, 80% helium) had been successfully decompressed from deep dives.

1937

Several test dives are conducted with helium mixtures, including salvage diver's Max "Gene" Nohl's dive to 127 meters.

1939

US Navy used heliox in USS Squalus salvage operation.

1965

First saturation dives using heliox.

1970

Hal Watts performs dual body recovery at Mystery Sink (126 m). Cave divers Sheck Exley and Jochen Hasenmayer use heliox to a depth of 212 meters.

1987

First mass use of trimix and heliox: Wakulla Springs Project. Exley teaches non-commercial divers in relation to trimix usage in cave diving.

1991

Billy Deans commences teaching of trimix diving for recreational diving. Tom Mount develops first trimix training standards (IANTD). Use of trimix spreads rapidly to North East American wreck diving community.

1994

Combined UK/USA team, including leading wreck divers John Chatterton and Gary Gentile, successfully complete a series of wreck dives on the RMS Lusitania expedition to a depth of 100 meters using trimix.

2001

John Bennett (diver) The Guinness Book of records recognises John Bennet as the first scuba diver to dive to 1000ft, using Trimix.

2005

David Shaw sets depth record for using a trimix rebreather, tragically dying while repeating the dive.^[18][19]

Source: "Trimix and heliox diving". February 14, 2002. http://www.techdiver.ws/trimix_eng.shtml. Retrieved 2008-10-07. 

See also

• Argox
• Nitrox
• Heliox
• Hydreliox
• Hydrox

References