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Thread: Heliox Diving

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    Heliox Diving

    I have read with some interest that a number of our members are going to start diving Heliox.

    I am fascinated - I wonder if you guys could explain the background for this choice as with my very limited understanding i thought this was now out of favour....

    Obviously high He %'s cause HPNS - which is nasty - we typically control it by making sure we have some Nitrogen in the mix which acts as a depressant on the nervous system....

    Love to learn more...

    Stuart

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    There is a bit of a trade off here in that elimination of N2 is a good thing however high helium can create it's own problems on very deep dives particularly on fast ascents. Heliox has been used in military diving for a long time, not without problems, although it could be argued that the problems are perhaps more manageable and have less serious implications than using N2 in the mix.

    In 1965 Bennett first described the presence of tremors in divers during experiments conducted at the Royal Naval Physiologic Laboratory (RNPL) using rapid compression to around 183-244 metres. Divers experienced dizziness, nausea and vomiting in addition to tremors. In 1968 during experiments as deep as 363 metres, Brauer noted periods of somnolence termed micro sleep and changes in the brains electrical activity as recorded on EEG, he later coined the term HPNS to describe the combination of tremor, dizziness nausea vomiting microsleep and eeg changes. In 1970, using compression rates significantly slower than those used in the above experiments, Comex a French diving firm successfully exceeded the proposed 367 metre barrier. Divers in this study experienced the effects of HPNS with prominent tremors and EEG changes but were able to function a t depth although at a la lower level of performance. In 1974, the first experiments using trimix were conducted in an attempt to reduce the effects of HPNS and permit more rapid compression rates. Although these dives allowed divers to reach record depths using faster compression rates than when using Heliox, high gas density led to shortness of breath and limited exercise tolerance.

    HPNS is a syndrome primarily of neurological dysfunction, its causes are complex, it was originally termed helium tremors although it is now clear that hydrostatic pressure rather than helium is the cause of HPNS.

    Helium has a solubility in the lipid (fat) or 0.015 whilst nitrogen has a solubility in the lipid of 0.019, on helium mixtures symptoms began to improve after about one hour, this does not occur with compressed air, also on helium mixes, motor skills are more impaired than intellectual tasks, helium excites the brain whereas nitrogen decreases the excitement of the brain.

    Nitrogen can still be used to provide a buffer for HPNS - although a diver must never switch from heliox to any gas containing N2.

    Both Hasenmayer and Exley's deep cave explorations on heliox caused mild symptoms of HPNS manifested as a shaking fit, later dives using trimix were symptomless.


    That is basically the trade off - however Helium on gasses and off gasses faster than nitrogen. A helium bend can still be nasty but Nitrogen is a VERY bad gas to dive deep on with it in the mix. I have been thinking about switching over for a long time but not without doing a lot of research first.

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    Very intresting post, AnneMarie. Thanks.

    Quote Originally Posted by AnneMarie

    Nitrogen can still be used to provide a buffer for HPNS - although a diver must never switch from heliox to any gas containing N2.
    That raises the question of going about the opposite way, using trimix during decent and the bottom section of the dive, and switching to Heliox at one point during ascent.

    That should keep HPNS at bay yet reduce exposure to N2. N2 would still be off gassed into the loop to 'smoothen' the transition if needed (or flushed if not).

    Will different gas densities have another effect, and in what way?

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    Hi, no that totally negates the point of using one or the other - basically heliox is still "uncharted territory" in civilian sport diving, BUT and it's a big BUT it IS known that *some* N2 is beneficial in very deep heliox diving to slow down the effects of HPNS. It would not be good to go on Heliox first and then introduce N2 later - the first depth you breathe the N2 at "sets" the narcotic depth for the dive. If you switch at depth you can get a narcotic shock or also a bad bend.

    Using the trimix and switching to Heliox is again a no no because removal of the N2 from the mix will *increase* the helium tremors past 150m. Also you have already then ongassed N2, which is a slower uptake and downtake gas, you have to still offgas that then the switch to heliox creates a massive He gradient (which will ongas fast) and the decompression profile would be a nightmare.

    If I can put a profile up I would (need to download from VR3) of two consecutive 100m+ dives in a cave, long time ongassing to 100m + section then ascent back up to -10m then back down to deeper than 100m then back up, if I switched onto N2 any later than the start of the dive, it would mean ongassing big time all the way even on ascent phase of middle bit of dive

    Regards

    AnneMarie

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    Thanks again AnneMarie

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    HPNS isn't necessarily due to heliox diving, it presents itself in liquid breathing experiments too where there is no inert gas which has led a lot of people to suspect it is related to many other factors. It may well be that there is a physical pressure limit that the human body just cannot work against.

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    Quote Originally Posted by lizardland
    It may well be that there is a physical pressure limit that the human body just cannot work against.
    Gotta keep trying though LOL!!

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