Trimix (breathing gas) 31489 222473970 2008-06-29T15:07:53Z Nihiltres 236191 Merge two identical references (again) {{otheruses|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. 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. <ref name="palautz97">[http://www.bishopmuseum.org/research/treks/palautz97/phys.html Bishop Museum Diving Physics]</ref> The lower density of helium reduces breathing resistance at depth. <ref name="palautz97"/> Helium [[off-gas]]ses 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 [[drysuit]]s. Some divers suffer from [[hyperbaric]] [[arthralgia]] during descent.<ref>http://www.bordeninstitute.army.mil/published_volumes/harshEnv2/HE2ch31.pdf Military Diving Operations and Support p 980</ref> === Advantages of reducing oxygen in the mix === Lowering the oxygen content increases the depth and duration of the dive before which [[oxygen toxicity]] becomes a limiting factor. === Advantages of keeping some nitrogen in the mix === Nitrogen in trimix<ref>http://www.scuba-doc.com/HPNS.html Diving Medicine Online</ref> can prevent [[High Pressure Nervous Syndrome]], a problem when breathing [[heliox]] at depths below 130 meters (429 feet). === 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 (ppO<sub>2</sub> - 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 PPO<sub>2</sub> of 1.3 bar and an equivalent air depth of 40 meters (132 feet). In [[open-circuit scuba]], two classes of trimix are commonly used: "normoxic" trimix<ref name="exoticgases">[http://www.techdiver.ws/exotic_gases.shtml Tech Diver - Exotic Gases]</ref> - with a minimum PO2 at the surface of 0.18 and "hypoxic" trimix<ref name="exoticgases"/> - 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 ppO<sub>2</sub> is less than 0.18 bar. In fully closed circuit [[rebreather]]s that use trimix diluents, the mix can be "hyperoxic" in shallow water because the rebreather automatically adds oxygen to maintain a specific ppO<sub>2</sub>. 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. 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). ==References== {{reflist}} [[Category:Breathing gases]] [[Category:Helium]] [[bg:Тримикс]] [[cs:Trimix]] [[da:Trimix]] [[de:Trimix]] [[es:Trimix]] [[fr:Trimix]] [[it:Trimix]] [[nl:Trimix]] [[no:Trimix]] [[pl:Trimix]] [[ru:Тримикс (дайвинг)]] [[sv:Trimix]]