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Trimix and Heliox
 
What are trimix and heliox?
 
Trimix means a mix of three components ("tri" and "mix"), and usually when people talk about trimix, they mean the mix of oxygen, nitrogen and helium. Trimix is used in very deep dives instead of air to reduce the partial pressure of oxygen (to avoid oxygen toxicity) and nitrogen (to avoid nitrogen narcosis). Heliox is a mix of helium and oxygen ("heli" and "ox").
 
The percentages of gas components vary depending on the dive. The deeper one go, the less there will be oxygen and nitrogen, and the more there will be helium.
Trimix mixes are labeled for example as "Trimix 10 50" or "Trimix 10/50", where 10 represents the percentage of oxygen in the mix, and 50 is the percentage of helium.
 

History of trimix and heliox 
 
1919:
Professor Elihu Thompson speculated 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 was mainlu hypothetic.
 
1925:
The US Navy began 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% O2, 80%He) had been successfully decompressed from deep dives.
 
1937:
Several test dives with helium mixtures, including Max Nohl's 127 m dive.
 
1939:
US Navy used heliox in USS Squalus salvage operation.
 
1965:
Saturation dives started using heliox.
 
1970:
Hal Watts performes dual body recovery at Mystery Sinkis (126 m). Several cave divers (ie. Sheck Excley and Jochen Hasemayer) use heliox on even 212 m of depth.
 
1987:
First mass use of trimix and heliox: Wakulla Springs Project.  Exley teaches non-commercial divers to trimix usage at caves.
 
1991:
Tom Mount developes first trimix training standards (IANTD).
 
1994:
UK/USA team succesfully performes the Lucitania project (100 m).
 

Why one should dive deep dives with trimix? 
 
1. Nitrogen narcosis can be avoided by replacing nitrogen with helium. Helium is not as narcotic as nitrogen.
 
2. By decreasing the percentage of oxygen in the mix, one can dive deeper without a danger of oxygen toxicity.
 
It is wrong to think that the high price of helium mix is an excuse to dive deep with air. If you can't afford it, then you don't dive deep. Nitrogen narcosis can be very dangerous, and oxygen toxicity even worse. Remember still that trimix diving without adequate training is not correct way to dive!
 

Gas filling
 
Gas filling of trimix is usually done like this:
- empty cylinders are filled partially with helium
- top up fill cylinders with air or nitrox
- oxygen and helium percentage calculations or measurements.
(helium topped with air is sometimes referred as heliair
 
Another way:
- let some helium flow to compressor inlet
- possibly some oxygen too to compressor inlet
- oxygen and helium percentage calculations or measurements.
 
Yet another way:
- empty cylinders are filled partially with helium
- then the gas blender fills some oxygen to cylinders (requires oxygen clean gears)
- top up fill cylinders with air or nitrox
- oxygen and helium percentage calculations or measurements.
 
Also used trimix cylinders are often topped to save the remaining trimix mix. This might cause some uncertainty or difficulties to gas measurements, unless accurate analyzers are available.
 
NOTE! Gas mixing requires blender training and these examples don't take into account for example thermal expansion or gas compressibility.
 

 
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