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[Enlarge image] Trimix and heliox diving   Author and copyright owner: Matti Anttila. Copying etc. prohibited without prior written permission by the author. Comments: matti@antti.la. Last update 14.2.2002. In case the reader of this page has comments, questions etc, I'm happy to receive them by e-mail or web form.   [suomeksi ]

Contents:

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.

Milestones: 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). More history: http://www.mindspring.com/~divegeek/mixhistory.htm

The answer is simple and brief: 1. Nitrogen narcosis can be avoided by replacing nitrogen with helium. Helium is not as narcotic as nitrogen. Take a look at http://techdiver.ws/exotic_gases.shtml#9. 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!

When people talk about the advantages of trimix, it is usually the easiest way to demonstrate them with some real diving examples. I have gathered few examples below, which are dangerous or even impossible to dive using regular air instead of trimix. IMPORTANT! These examples are not dive plans to anyone. They don't take into account personal features, gas calculations or CNS/OTU tracks etc. Dive 1: 60 m Cave dive performed by multilevel dive plan: During the first 10 minutes the maximum depth will be 30 meters, and after that divers are changing their cave/tunnel to a deeper level, in which the maximum depth will be 58 msw. However, there are chances that divers may have to dip down to 60 msw in some point of this level. Dive time of this level is 12 minutes. After this, divers ascend to 40 msw level, which guides divers to cave exit (20 msw). Rest of dive is open water and decompression time. Dive information and decompression time table: Depth / msw Time / min Run time / min Gas 30 10 10 EAN34 60 12 22 Trimix 20 35 40 1 25 Trimix 20 35 21 1 28 EAN50 12 3 32 EAN50 9 4 36 EAN50 6 15 51 Oxygen       Dive 2: 82 m Cave dive performed by multilevel dive plan: It will take at most 8 minutes to reach the depth of 42 msw from surface. From this point, divers may begin their direct descent to the bottom of this cave hall (76-77 msw). The bottom is not flat, and there will be an interesting thing to take a look at 82 msw. Bottom time from 42 msw level will be 10 minutes in the main hall before they start ascent. Returning will be the same as in example 1 above. Dive information and decompression time table: Depth / msw Time / min Run time / min Gas 42 8 8 EAN26 82 10 18 Trimix 11 47 33 1 22 EAN26 30 1 23 EAN26 27 1 24 EAN26 24 1 25 EAN26 21 1 27 EAN50 18 2 29 EAN50 15 3 32 EAN50 12 4 36 EAN50 9 7 44 EAN50 6 22 66 Oxygen       Dive 3: 110 m Wreck dive, which is performed from a boat on the sea. Direct descent and ascent. Maximum depth is 110 msw. Dive information and decompression time table: Depth / msw Time / min Run time / min Gas 50 3 3 Air 110 12 15 Trimix 11 65 50 2 20 Air 39 1 22 Air 36 1 23 Air 33 1 24 Air 30 1 26 Air 27 2 28 Air 24 3 31 Air 21 3 34 EAN50 18 4 38 EAN50 15 5 44 EAN50 12 7 51 EAN50 9 12 63 EAN50 6 43 106 Oxygen       Dive 4: 282 m This example is an over-killer, but I mention it just for interest. Dive information available on http://www.onr.com/user/zacaton/Zacaton.html. This was the world record deep dive with scuba equipment. The current record is 308 m (1010 feet, John Bennett, 2001): http://www.about-scuba-diving.com/scuba-diving/history.htm.

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, such as the Atomox helium analyzer. NOTE! Gas mixing requires blender training and these examples don't take into account for example thermal expansion or gas compressibility.

Trimix training is very advanced training and prior that one should have good knowledge and broad experience about diving generally, diving with nitrox and decompression gas usage.

Training agencies for nitrox and/or trimix diving:

GUE (http://www.gue.com):
For example: Tech Diver 1-3 courses.

IANTD (http://www.iantd.com):
Basic Nitrox Diver, Advanced Nitrox Diver, Technical Diver, Normoxic Trimix Diver and Trimix Diver courses.

DSAT (http://www.padi.com):
Tec Deep Diver course, trimix course will be available 2002(?). DSAT Gas Blender: http://www.padi.com/courses/tecrec/gasblender.asp

PADI (http://www.padi.com):
Enriched Air Nitrox Diver Specialty course.

CMAS (http://www.cmas.org):

NAUI

TDI

etc.