Tri-Mix Agencies?

[DevonDiver]

The bell curve example is valid IMHO.

For a diver with only 50 dives, is that sufficient 'track record' to offer any assurance that their buoyancy skills are perfected?

Assume some mess-ups in the first 20 dives...as they initially learn..... then that is a measly 30 dives to demonstration 'perfection'. It's quite possible that a fair percentage would get lucky enough during those 30 additional dives and gain a corresponding over-confidence. It is also quite possible that they would still have mistakes to make.... and those would not be unearthed until they were sucking helium into their body and any mistake would be far more catestrophic.

50 Dives? Really? What about having some dedication to the pursuit of skills and showing some patience in your diving development? Why the rush to run before you can walk?

I can knock out 50 dives in a single 2 week diving holiday. Lets get realistic about that figure!

If a diver was serious enough about diving to want to utilise helium, then shouldn't they also be serious enough about diving to accept that a slow, progressive development curve is in their best interest?

There's a multitude of training options that would be far more beneficial to a 50 dive novice, than learning to use helium for recreational diving.

 

[waterpirate]

What sets you up in the position of the all knowing? Agencies set standards for students and train instructors. If the helium student presents to the instructor at the 50 dive mark and is considered qualified by both the agency and the instructor, it is out of your hands. Iantd allows advanced diver status prior to rec trimix on there training tree.
Eric

 

[ScubaFeenD]

 

[lamont]

Helium may be a better gas for decompression, but Trimix is more unforgiving than Nitrox regarding ascent rates. Trimix is not for the messy diver. Recreational divers should be well aware of this. I am not writing that to start some polemics, but just for safety's sake.

I'm not sure that I entirely agree with that for trimix in the recreational realm.

With 30/30 at 100 feet you can basically shoot up to 60 feet without any driving overpressurization gradient at all. While I agree that its best practice to start deco stops at 50 feet, I don't think that you need to be that precise under NDL-ish conditions. Even with tech1 diving, decompression does not need to be particularly precise -- with recreational helium diving, I think if you've got students that are trained to try to hit 1 minute stops from 1/2 max depth and accomplish this on most dives, and are guaranteed to be able to at least pull off 3-5 minutes of deco from 30 to the surface that is going to be sufficient to keep them from being hurt.

While I didn't know what I didn't know back at 100 dives, I think I could have at least pulled that off. It probably would have been too aggressive for me at 50 dives.

I also don't necessarily think that the bar needs to be the GUE RecTriox skills bar. Theoretically every diver out there should be able to do *some* decompression / safety stop without shooting to the surface, and this is really a *fundamentals* level of skill. I don't think you necessarily need to know about all the scenario-driven failures you get in Rec Triox. You need to be able to handle one emergency at depth and then get safely back to the surface without missing a safety stop.

I agree with what you wrote previously about having logged at least 150 dives as a necessary (but not sufficient) condition before getting into Trimix. Here instructors use to say that CMAS *** divers are most dangerous around their 100th logged dive, because that's when they begin to think they are good, experienced divers and can do anything.

Yeah, I was that diver.

Starting with doubles sometime after dive #100 and getting spanked in an introductory technical course around dive 125-150 is a pretty good start to a technical diving career -- aggressive without being too aggressive. That's why I think that roughly dives 1-100 should be single-tank dives, dives 100-150 can be in doubles and it should be 150 minimum for tech1-level decompression. The hypothetical diver that I mentioned earlier able to handle recreational triox after 50 dives might get there a little bit quicker -- but I don't know that there's a *reason* to get there all that quicker though. The difference between tech1 at 150 dives and tech1 at 100 dives should only be about 6 months.

No they don't analyze your gas. But if there is an accident (for example a diver ascends too fast with Trimix 25/25 and gets a vestibular DCS hit) the authorities will.

You're not going to get a vestibular hit with recreational triox.

That requires a switch from mix to nitrox and is a real example of IBCD.

Pretty much have to be doing 200+ foot / 60+ meter dives as well -- fairly unheard of on T1 level dives.

 

[hg frogman]

You're not going to get a vestibular hit with recreational triox.

That requires a switch from mix to nitrox and is a real example of IBCD.

Pretty much have to be doing 200+ foot / 60+ meter dives as well -- fairly unheard of on T1 level dives.

I wouldn't be as adamant. The switch you describe is one cause of vestibular hits (i.e. DCS in the inner ear). But it's not the only one. A fast ascent can trigger this kind of DCS as well. Here is what I have to support my point of view.

1) According to Dr Patrick Grandjean (French specialist of hyperbaric medicine and coordinator of DCS cases studies for the French Federation) an ascent fast enough (even on a few meters) after a saturating dive (whatever the gas is) can generate bubbles inside the inner ear fluids (that are fast tissues) and subsequent DCS. OK, "saturating" means that on-gassing is fairly important, but that's the case for fast tissues after only 10' to 15' at 39 meters/130 feet with Trimix 25/25 (that's the mix I quoted and it's an interesting mix for me, while for my diving the Trimix 30/30 is of absolutely no interest).

2) Ten years of French statistics about DCS show that around 20% of DCS hits are vestibular. That's a lot, and this type of DCS hits many more French divers than the small minority switching from Trimix to Nitrox for deco. Most French divers dive air (also for deco).

3) I personally know someone diagnosed with a vestibular DCS hit after a training dive on air, involving a sawtooth profile and a fast ascent from 18 meters/60 feet to the surface.

4) According to Dr Jean-Louis Méliet (designer of the French Navy tables) an ascent a bit too fast is a major cause of vestibular hits amongst commercial saturation divers.

 

[lamont]

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

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).

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'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.

 

[Rhone Man]

Pretty much have to be doing 200+ foot / 60+ meter dives as well -- fairly unheard of on T1 level dives.

I have always wanted to ask - what exactly do people mean when they say a T1 level dive? Is that a DIR/GUE term?

 

[ScubaFeenD]

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

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).

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'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.

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.

 

[Blackwood]

I have always wanted to ask - what exactly do people mean when they say a T1 level dive? Is that a DIR/GUE term?

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).

 

[Blackwood]

Seems like a switch to 50% for smaller deco dives would be less optimal than just doing a single switch at 6m to 100%.

From a pure decompression standpoint, probably.

There are other considerations. With 50%, you get on the bottle at 70 feet. That means you don't have to carry as much bailout gas (assuming you base your reserve on the switch depth). Also, you start deco sooner, so if you have a bottle failure you can adjust your deeper stops accordingly rather than only extending 20 and shallower.