Trimix (breathing gas)

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.cite book |title=Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. |last=Brubakk |first=A. O. |coauthors=T. S. Neuman |year=2003 |publisher=Saunders Ltd. |location=United States |isbn=0702025712 |pages=800 ] cite journal |author=Gernhardt, ML |title=Biomedical and Operational Considerations for Surface-Supplied Mixed-Gas Diving to 300 FSW. |journal=In: Lang, MA and Smith, NE (eds). Proceedings of Advanced Scientific Diving Workshop |location=Smithsonian Institution |place=Washington, DC |date=2006 |url=http://archive.rubicon-foundation.org/4655 |accessdate=2008-08-28 ]

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. 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.cite web |url=http://www.bishopmuseum.org/research/treks/palautz97/phys.html |publisher=Bishop Museum |title=Diving Physics and "Fizzyology" |year=1997 |accessdate=2008-08-28 ]

The lower density of helium reduces breathing resistance at depth.

Helium off-gasses rapidly and it does not enter slow tissues as readily 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.

Some divers suffer from hyperbaric arthralgia during descent. [cite journal |author=Vann RD and Vorosmarti J |url=http://www.bordeninstitute.army.mil/published_volumes/harshEnv2/HE2ch31.pdf |title=Military Diving Operations and Support |page=p980 |journal=Medical Aspects of Harsh Environments, Volume 2 |publisher=Borden Institute |year=2002 |accessdate=2008-08-28 ]

Helium on-gasses rapidly and usually requires 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.cite journal |last=Acott |first=C. |title=Oxygen toxicity: A brief history of oxygen in diving |journal=South Pacific Underwater Medicine Society journal |volume=29 |issue=3 |date=1999 |issn=0813-1988 |oclc=16986801 |url=http://archive.rubicon-foundation.org/6014 |accessdate=2008-08-28 ] [cite journal |author=Gerth, WA |title=Decompression Sickness and Oxygen Toxicity in US Navy Surface-Supplied He-O2 Diving. |journal=In: Lang, MA and Smith, NE (eds). Proceedings of Advanced Scientific Diving Workshop |location=Smithsonian Institution |place=Washington, DC |date=2006 |url=http://archive.rubicon-foundation.org/4654 |accessdate=2008-08-28 ]

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). [cite journal |last=Hunger Jr |first=W. L. |coauthors=P. B. Bennett. |title=The causes, mechanisms and prevention of the high pressure nervous syndrome |journal=Undersea Biomed. Res. |volume=1 |issue=1 |pages=1–28 |date=1974 |issn=0093-5387 |oclc=2068005 |pmid=4619860 |url=http://archive.rubicon-foundation.org/2661 |accessdate=2008-08-28 ] [cite journal |last=Bennett |first=P. B. |coauthors=R. Coggin; M. McLeod. |title=Effect of compression rate on use of trimix to ameliorate HPNS in man to 686 m (2250 ft) |journal=Undersea Biomed. Res. |volume=9 |issue=4 |pages=335–51 |date=1982 |issn=0093-5387 |oclc=2068005 |pmid=7168098 |url=http://archive.rubicon-foundation.org/2920 |accessdate=2008-04-07 ] [cite web |url=http://www.scuba-doc.com/HPNS.html |title=High Pressure Nervous Syndrome |publisher=Diving Medicine Online |author=Campbell, E |accessdate=2008-08-28 ]

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 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 1.0-1.6 bar and maximum equivalent air depth of 30 to 45 meters (100 to 150 feet). At 100 meters (330 feet), "12/52" has a PPO₂ of 1.3 bar and an equivalent air depth of 43 meters (140 feet).

In open-circuit scuba, two classes of trimix are commonly used: "normoxic" trimixcite web |url=http://www.techdiver.ws/exotic_gases.shtml |author=Tech Diver |title=Exotic Gases |accessdate=2008-08-28] - with a minimum PO2 at the surface of 0.18 and "hypoxic" trimix - with a PO2 less than 0.18 at the surface. A Normoxic mix, such as "19/30", is used in the 30 meters (100 feet) to 60 meters (200 feet) depth range and 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.cite journal |author=Richardson, D; Menduno, M; Shreeves, K. (eds). |title=Proceedings of Rebreather Forum 2.0. |journal=Diving Science and Technology Workshop. |volume= |date=1996 |pages=286 |url=http://archive.rubicon-foundation.org/7555 |accessdate=2008-08-28 ]

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.cite book |author=Harlow, V |title=Oxygen Hacker's Companion |publisher=Airspeed Press |year=2002 |isbn=0967887321 ]

A second method called 'continuous blending' is now gaining favor. 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). [cite web |url=http://shadowdweller.skynetblogs.be/post/3924720/continuous-trimix-blending-with-2-nitrox-stic |title=Continuous trimix blending with 2 nitrox sticks (English) |author= |publisher=The shadowdweller |year=2006 |accessdate=2008-08-28 ]

History of trimix as a diving gas

;1919: Professor Elihu Thompson speculates that helium could be used instead of nitrogen to dilute the oxygen content of a breathing mix and thus reduce narcosis, but because of high prices of helium at that time, the idea remained stictly hypothetical.

;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 Sinkis (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: Tom Mount developes first trimix training standards (IANTD).

;1994: Combined UK/USA team, including leading wreck divers John Chatterton and Gary Gentile, succesfully complete a series of wreck dives on the "RMS Lusitania" expedition to a depth of 100 meters using trimix.

"Source: citeweb|url=http://www.techdiver.ws/trimix_eng.shtml|title=Trimix and heliox diving|date=February 14 2002|accessdate=2008-10-07"

ee also

*Argox (scuba)
*Enriched Air Nitrox
*Heliox
*Hydreliox
*Hydrox

References