Tri-Mix Agencies?

[PfcAJ]

To add on to what Blackwood said, another benefit to 50% (over just 100%) on a dive in the 150' range is that in the event of an out of gas diver, you're tethered via the long hose a relatively short amount of time.

Sure, we can all shoot a bag and ascent while on the long hose, is that what you want to do? I sure don't. Getting each diver on a separate gas source (relative to the depth) is a good move.

 

[lamont]

While I am not a tech 1 diver, a tech diver, or helium qualified, I am still curious about this post. So, does that mean that decompressing with 50% with a backgas of 30/30, or 21/35, etc is more dangerous if doing less aggressive dives? Seems like a switch to 50% for smaller deco dives would be less optimal than just doing a single switch at 6m to 100%. Im not trying to provoke anyone I am just interested in the theory.

well, my understanding is that you only need to start worrying about this kind of thing for bigger dives - 200ft or deeper - and i've got my own questions about practices that would make these dives safer and reduce the risk of vestibular DCS, but its more along the lines of if 50/25 wouldn't be a better 70 foot gas than 50%...

for 21/35-bottom mix my understanding prior to this thread was that this didn't matter much and that vestibular DCS wasn't much of a concern...

 

[aquaregia]

Regarding the divecount requirement: As a potential student it's nice to get a ballpark idea of the minimum experience I need before I potentially waste an instructor's time talking about whether I'm ready for a class. Divecount is pretty much the only metric that I feel I can adequately assess on my own before bothering someone else.

 

[hg frogman]

That's at odd with the recent US literature on inner ear DCS.

Maybe in the USA there is not a high rate of vestibular hits related to fast ascents, so there is not much about it in US literature. Most North American deep divers are likely to be more careful and specifically trained than many French deep divers IMO.

It is possible to see inner ear DCS in conjunction with other type 2 symptoms, but my understand of DCS when the major presenting complaint is only vestibular is that it was associated only with helium diving with a gas switch to nitrogen (and had been shows in a dry chamber to occur truly isobarically in a fully saturated diver, with no ascent and only a gas switch).

Yes what you describe do happen. Twenty years ago, Dr Jean-Louis Méliet took rabbits and brought them to full saturation in Air at 4 ATA of N2 pressure (ppN2 = 4 ATA). Then he had them switch to Heliox 21/79 at the same pressure (isobaric switch). Then he analyzed the gas in the blood of the rabbits. He found that the sum of the partial pressures of the two inert gases after the switch (ppN2 + ppHE) was significantly more than 4 ATA for a significant amount of time. Of course, the switch from Air to Heliox is not realistic, and was probably done like this for financial reasons; one can expect less rise of ppN2 + ppHe in the other direction, yet there will still be a rise (this experiment is described in Dr Méliet's postface of Alain Foret recent translation in French of Haldane pioneer work on decompression).

But as I said such a switch is not the only possible cause of vestibular DCS hits.

Also, 15 minutes at 39m/130ft on 25/25 i would almost not consider a recreational triox dive, and I'd expect to see bad results from a direct blowup at those depths for that time.

I entirely agree.

Anyway for me "recreational" is quite meaningless, partly because even PADI tables have deco stops within their so-called NDL (e.g. for 20' on Air at 30 meters/100 feet which is the PADI NDL. PADI recreational table stipulates that the safety stop is mandatory then, which is the exact definition of a deco stop. Same rule for dives getting close to the NDL). As you wrote as well, and I agree, even "Rec" Trimix implies deco stops, but that shouldn't detract people. Most dives are actually deco dives.

I believe that "mild deco" (e.g. less than 15 minutes of total deco time on backgas) is a clearer and less confusing term than "recreational".

A reason for different appreciations among divers about the virtues of Helium for decompression (ascent rates put aside) may come from different usual bottom times. For my diving, typical bottom time on a square profile dive with Trimix 25/25 is around 20 to 25 minutes at 39 meters/130 feet, while I bet your bottom time might be much longer.

I'd also be careful with what your expert is talking about with "saturating dive" they may be talking about a dive to 1,000 ft and brought to saturation for a day, which you can't really extrapolate to the saturation of your very fast tissues after 15 minutes.

Yes that's what he was talking about (and that's what saturation diving is): a commercial diver brought to full saturation on Heliox at 10 ATA or more for days or weeks.

I don't extrapolate from that without a lot of care. But a decompression model or protocol that claims to be comprehensive has to predict adequate decompressions for this kind of diving as well. And IMO keeping this type of diving in a corner of one's mind gives useful insights.

What my expert (Dr Méliet) said was that, for this kind of diving, "Haldanian" ascent protocols (i.e. an ascent quite fast at the start to create a pressure gradient important enough for off-gassing) that were tried at first didn't work, and created lots of vestibular hits. I understand it as a hint that too fast ascents trigger vestibular hits as soon as a diver is saturated enough.

The ascent protocols for saturation diving now imply a very slow (a few feet per hour) ascent rate, that is constant by intervals, and getting progressively slower while approaching the surface. Apart from the rate itself, that's doesn't look very different from your "min deco" scheme (but as you recommend, I won't extrapolate).

 

[MXGratefulDiver]

Regarding the divecount requirement: As a potential student it's nice to get a ballpark idea of the minimum experience I need before I potentially waste an instructor's time talking about whether I'm ready for a class. Divecount is pretty much the only metric that I feel I can adequately assess on my own before bothering someone else.

Dive count only gives you a relative notion of someone's in-water experience. It does not account for their natural ability, the amount of effort they put into improving their skills, or how much importance they place on basic diving safety protocols.

All of those are important considerations when determining whether or not someone is ready to progress into higher-level diving classes.

I've known people with 50 dives who could take and pass a Fundies or Intro to Tech class. I've also known people with thousands of dives who never will ...

... Bob (Grateful Diver)

 

[Rhone Man]

Multiple agencies have classes called 'Tech 1'. I generally understand it as ~150-160 feet with <=30 minutes deco (common catchall limits from first "tech" level courses).

Thanks. I guess there must naturally be a T2 - is there such a thing as a T3 level dive? If so, what are the basic parameters?

(I'll end the hijack after this - promise).

 

[Blackwood]

Thanks. I guess there must naturally be a T2 - is there such a thing as a T3 level dive? If so, what are the basic parameters?

(I'll end the hijack after this - promise).

I don't know what the standard views are on that. To me, T2 level dives require multiple bottles whereas T1 level dives can be done with one.



T3 is, I believe, only a GUE thing (could be wrong, but I've never heard of any other agency offer a course called Tech 3). In keeping with their system, it's exploration-level diving.

 

[boulderjohn]

Thanks. I guess there must naturally be a T2 - is there such a thing as a T3 level dive? If so, what are the basic parameters?

(I'll end the hijack after this - promise).

T3 is, I believe, only a GUE thing (could be wrong, but I've never heard of any other agency offer a course called Tech 3). In keeping with their system, it's exploration-level diving.

A UTD Level 2 diver will use a stage bottle, back gas, and one deco gas. After T2 you can get a T2 Gold certification, which allows you to add a second deco gas, or you can go right to Trimix.To get Trimix certification, you have to complete 25 staged decompression dives after T1. Trimix certification gets you into even more bottles, greater depths, more deco gases, etc.

 

[ucfdiver]

A UTD Level 2 diver will use a stage bottle, back gas, and one deco gas. After T2 you can get a T2 Gold certification, which allows you to add a second deco gas, or you can go right to Trimix.To get Trimix certification, you have to complete 25 staged decompression dives after T1. Trimix certification gets you into even more bottles, greater depths, more deco gases, etc.

To be fair to the population at large, I hope no one uses UTD as an example for anything other than an example of clever marketing.

 

[boulderjohn]

To be fair to the population at large, I hope no one uses UTD as an example for anything other than an example of clever marketing.

Well, then.

Is this a shot across the bow?