Heliox is a gas-mixture of helium and oxygen used mostly for saturation diving.
The exact mixture between the two is dependant on the pressure it will be breathed in (ie. working depth). Helium is used as a filler gas while the level of oxygen is based on a minimum for sustaining life and a maximum to prevent oxygen poisoning. A typical number for a dive to 100 m is 4 % oxygen and 96 % helium (regular air has 21% oxygen - all measurements by volume).
The reason for substituting the atmosphere's gases with helium is that nitrogen does bad things to you at high pressure.
Heliox is commonly used for dives greater than 200 fsw (feet of seawater). At over 300 feet there is a danger of high pressure nervous disorders - which can be eliminated by proper care and supervision (and funnily enough - by adding a small amount of nitrogen to the mix). Heliox also conducts heat better than regular air making saturation divers much more suspectible to changes in air temperature (Helium conducts heat 6 times better than nitrogen). Divers use hot water suits to heat their bodies up to approx 41°C while working.
As heliox also is a less dense gas-mixture than regular air it is also sometimes used in asthma treatment. The low density combined with high viscosity yields a low turbulent air stream which gives low flow resistance.
A side effect to this low density / high viscosity is that vocal chords move more easily in this mixture, making you speak in a high pitched voice.
Other gas-mixtures for diving are:
- Nitrox (For extended durations on 50-60 m dives)
- Trimix (For reduced nitrogen exposure, less expensive than Heliox)
More experimental gas mixtures include:
Much research is done to make diving both deeper and safer, but it mostly includes mixtures to be used during compression
. As with all current mixtures, if you decompress too rapidly
you are in danger of getting the bends
- heliox does not mitigate this.
When used for saturation diving, the gas-mix is recycled back to the ship where the CO2 is taken out of the mix and fresh O2 is added.