Benefits of Helium
Helium is a great gas for diving and we have Stunning Gas Prices. It's origins can be traced back to one of the fundamental elements that was created in the Big Bang when the Universe was created. Helium is the second most abundant and second lightest element in the Universe and today almost all Helium is now created in the Univese by nuclear fusion in the Stars of Hydrogen.
Interestingly Radioactive decay that emits Alpha Particles which are in fact Helium Nuclei. On Earth the majority of the Helium we find today is as a result of such Natural Radio Activity (heavier elements decaying into other elements and a high proportion of Helium). In Natural Gas (Methane) you often find about 7% of the Gas is actually Helium. The US, Europe, QTAR and USSR has great large Gas fields where today we get the majority of our Helium from. Essentially it is separated using temperature distillation techniques (called Fractional Distillation). The worlds 2nd largest production area is Arzew in Algeria, US is the first largest producer (in Texas/Kansas).
Without going too deep into the Quantum Mechanics and Nuclear Physics, this element exhibits some very interesting properties. It is odourless, colourless and tasteless. It is non-toxic and nearly inert (generally treated as inert). It heads the periodic table for the noble gases and has the atomic number 2. You need extreme conditions to create compounds with Helium (all of which are unstable) and it is extremely rare to find stable isotopes of Helium. It has a melting point (-272.2 °C) just above Absolute Zero (-273.15 °C) and a boiling point (-268.93 °C), It's heat capacity at 25 °C is 20.786 Jmol-1K-1. A very interesting point is that due to Helium's small molecule, it has a diffusion rate through solids three times that of air and around 65% of that of Hydrogen. Very significantly for the diver is that Helium is less water soluable than any other gas known, and Helium has a negative Joule-Thomson coefficient at normal ambient Temparatures. The effect of this is it heats up when allowed to freely expand - rather than the conventional cooling upon free expansion. (It only does this below its inversion temperature of -233.15 °C)
All good stuff and interesting Physics, but how is this useful to diving... well...
The Deeper you go the density of the gas that you breathe goes up proportional to depth. Also occurring is Nitrogen Narcosis in great effect again proportional to depth - from around 24msw down this is sub clinically affecting you - it may be verifiable that you have considerably slower mental and motor capacity. At around 50msw this is getting dangerous (like alcohol you can condition yourself to 'tolerate' this narcosis to a point).
The other problem is that air is getting so dense that you are working much harder to breathe, this is producing dangerous levels of CO2 which can creep up and bite you (often in part the cause as to why panic may suddenly occur in skilled divers). Air can be safely used down to 66msw based on the toxicity of Oxygen, however its use as a breathing gas is questionable below 45msw due to its narcosis effects.
By using a less narcotic and lighter gas to replace some of the Nitrogen it the breathing gas can abate some of the effects of the work of breathing and narcosis. There are downsides with effects such as High Pressure Nervous Syndrome becoming more prevalant as the pressure of Helium increases to replace nearly all of the Nitrogen. This is a major problem to be very careful with on gases such as Heliox (Oxygen and Helium only). Often the price of adding a lighter inert gas to that of Nitrogen is slightly longer decompression and more care in the rate of ascent and more accuracy in maintaining stop depths. Whilst Hydrogen (with Oxygen <= 4%) and Helium can replace the Nitorgen, care must be exercised with Neon (more HPNS and other issues), Argon (far too dense and highly narcotic at shallow depths) and Krypton/Xenon (seriously not to be used with diving). Xenon is 25 times more narcotic than Nitrogen. Putting price aside take care with Neon it is not narcotic shallower than depths of 360msw, however it is denser than Helium.
Why add gases like Helium to replace the Nitrogen in the resultant blend or Gas mix to provide less narcosis and with that comes a clearer head, better motor function and hence skills and reflexes and hence a better chance of enjoying and surviving such a deep dive.
Common mixes at the top end of the Technical Diving (depth ranges are effective) are:-
- Hydrox (Hydrogen + Oxygen) >250msw < 500msw
- Neox (Neon + Oxygen) > 150msw < 360msw
- Hydreliox (Hydrogen + Oxygen + Helium) >125msw < 250msw
- Heliox (Helium + Oxygen) > 95msw < 175msw
- Trimix (Helium + Oxygen + Nitrogen) > 40msw < 100msw
- Nitrox > 0msw < 60msw
- Heliox (deco 80/20) >0msw < 10msw
- Oxygen > 0msw < 6msw
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