Recreational Trimix Diver course with Techs Mex Divers, Part I

[rjack321]

I want to make it clear that I did not analyze Oliver's back gas, nor was this practice part of my instruction, so I am not qualified to endorse it or debate it. I believe he handed me the EAN38 stage to extend my dive purely for my own enjoyment: We had already fulfilled the requirements for the course, so if he had simply wanted to do the "minimum" as an instructor we would have ended the dive right there. But instead, he handed the stage off to me and as a result I got some more dive time and enjoyed some more of the reef.

A recent post here on SB gives one possible rationale for carrying a rich nitrox stage on a multi-level dive: http://www.scubaboard.com/forums/4311575-post28.html. Again, I have no training in gas switching dives of any description, so I mention it only for interest. I am not qualified to know when or whether this is an appropriate strategy.

I think you'll find that the general consensus is that gas switches have no place in recreational diving. And that very very few people maximize their O2 %ages throughout the dive by carrying extra tanks and using gas switches to try and stay within NDLs. That would be the loose equivalent of a poor man's CCR.

 

[Reg Braithwaite]

There is some speculation running about right now (speculation - no hard data anywhere)... That high N2 levels (acting as a CNS depressant) allowed deep air divers of yore to get away with ppO2's that are really high. And that once you start adding He into the mix the lack of CNS depression creates a greater chance of O2 toxicity. Since your 32/20 mix had a ppO2 of 1.48 at 120ft you might want to back off on the O2 or the depth as you go forth on your own. For instance, I limit my bottom ppO2 to 1.2-1.3

Thanks for the tip. Given that the game up here is wrecks in cold water and limited visibility, I'm thinking that if I dive Trimix, it will be 25/25 for dives in the 100 - 130 foot range and EAN32 for everything shallower. I basically took this class for one dive. I did it last year on air, I'd like to try it again on 25/25 this August.

I think you'll find that the general consensus is that gas switches have no place in recreational diving. And that very very few people maximize their O2 %ages throughout the dive by carrying extra tanks and using gas switches to try and stay within NDLs. That would be the loose equivalent of a poor man's CCR.

I agree, gas switches have no place in recreational diving, so you won't see me slinging a bottle any time soon. FWIW, Oliver is a CCR instructor so I think he probably agrees with your assessment. Maybe he was slinging the bottle because IANTD standards require him to dive OC when teaching OC? Or perhaps he anticipated handing me a stage and as long as he was handing me a stage it might as well be the "best mix" for the depth? I really don't know, and I can't speak for him.

 

[Blackwood]

I want to make it clear that I did not analyze Oliver's back gas

Yah, I'm just responding to the thread as it has played out

That said, I believe IANTD standards require the instructor to also be on mix, but I'm not completely certain.

 

[rjack321]

Thanks for the tip. Given that the game up here is wrecks in cold water and limited visibility, I'm thinking that if I dive Trimix, it will be 25/25 for dives in the 100 - 130 foot range and EAN32 for everything shallower. I basically took this class for one dive. I did it last year on air, I'd like to try it again on 25/25 this August.



I agree, gas switches have no place in recreational diving, so you won't see me slinging a bottle any time soon. FWIW, Oliver is a CCR instructor so I think he probably agrees with your assessment. Maybe he was slinging the bottle because IANTD standards require him to dive OC when teaching OC? Or perhaps he anticipated handing me a stage and as long as he was handing me a stage it might as well be the "best mix" for the depth? I really don't know, and I can't speak for him.

I use 25/25 on occasion (few times a year) - for me its a -10% EAD gas. So 15 mins is the "minimum deco" limit for 120ft.

I'm guessing the extra bottle was just along so you could get a decent amount of time UW. If you'd used doubles you'd have had more reserve on the bottom and cut into it once you've moved shallower to extend out your dive time (most of us do this recreationally). I'm guessing he didn't have any doubles or was trying to minimize your He costs by not "wasting" it on the <~50ft portions of the reef.

 

[Reg Braithwaite]

you'd used doubles you'd have had more reserve on the bottom and cut into it once you've moved shallower to extend out your dive time (most of us do this recreationally). I'm guessing he didn't have any doubles or was trying to minimize your He costs by not "wasting" it on the <~50ft portions of the reef.

Perfectly reasonable conjecture. Oliver and I discussed training in doubles, H-valves, or K-valve, and I went with K-valve because that's what I'll be diving this season up here. Oliver could easily have been thinking about He costs: He has doubles, stage bottles, and plenty of steel tanks for his clients and could easily have put me into a 100 or 120 if he wanted to, but of course the fill would have been that much more out of my pocket.

 

[Blackwood]

A recent post here on SB gives one possible rationale for carrying a rich nitrox stage on a multi-level dive: http://www.scubaboard.com/forums/4311575-post28.html.

What is stage decompression? Diving in which you switch to a richer gas to accelerate decompression?

What is does breathing a richer gas along "to extend" NDL" do? Accelerate decompression. That suggestion, while valid, is stage decompression. If you didn't have that hot mix, you'd have to hang for a while on backgas.

I think that when you start with a weak foundation, i.e. calling something a "no decompression limit," you invite ways to skirt meaningless terminology. In my opinion, if stage decompression and "NDL diving" are incompatible terms, then bringing richer gases for the purpose described in that post has no place in "NDL diving." It's exceedingly risky behavior for any diver who doesn't know how to handle/hasn't prepared for a loss of that gas.

 

[rjack321]

What is stage decompression? Diving in which you switch to a richer gas to accelerate decompression?

What is does breathing a richer gas along "to extend" NDL" do? Accelerate decompression. That suggestion, while valid, is stage decompression. If you didn't have that hot mix, you'd have to hang for a while on backgas.

I think that when you start with a weak foundation, i.e. calling something a "no decompression limit," you invite ways to skirt meaningless terminology. In my opinion, if stage decompression and "NDL diving" are incompatible terms, then bringing richer gases for the purpose described in that post has no place in "NDL diving." It's exceedingly risky behavior for any diver who doesn't know how to handle/hasn't prepared for a loss of that gas.

Close, more like "deccelerate compression" tho

 

[Reg Braithwaite]

I think that when you start with a weak foundation, i.e. calling something a "no decompression limit," you invite ways to skirt meaningless terminology.

I have to agree, the more I learn the less I like expressions like "no decompression." So far I like using the term "minimal decompression" to describe a profile where you believe that the gas absorbed by the slow tissues is insignificant.

In my opinion, if stage decompression and "NDL diving" are incompatible terms, then bringing richer gases for the purpose described in that post has no place in "NDL diving." It's exceedingly risky behavior for any diver who doesn't know how to handle/hasn't prepared for a loss of that gas.

There we agree again. Even though the dive ends up being so-called no-deco, the practice outlined in that post is a non-recreational or technical practice requiring additional training. Taking myself as an example, although I have Nitrox certification, I haven't been trained to:

• Carry a bottle
• Make switches under water
• Perform gas planning with contingencies for loss of back and/or stage gas
• Calculate minimal deco limits for dives involving gas switches

To circle back to my course, part of the problem here is the cultural bias that associates "no decompression" diving with "recreational diving." (Another imprecise term, but easier to agree on). The "Recreational Trimix" course is terribly named. It's really a technical course with an extremely weak in-water skills component, and one that teaches using trimix on minimal decompression dives that would otherwise be considered "recreational" if conducted on air or Nitrox.

But the course really does introduce the concept of a virtual overhead, and although there is no requirement to dive with redundant gas, the course standards emphasize planning and in-water skills to make an ESA an option of last resort. To my mind such dives are mildly technical, it's just that their profiles happen to mix well with divers conducting advanced recreational dives.

 

[rjack321]

So what mix(s) are you certified to use and are your buddies in Ontario going to be diving similarly?

 

[Blackwood]

I have to agree, the more I learn the less I like expressions like "no decompression." So far I like using the term "minimal decompression" to describe a profile where you believe that the gas absorbed by the slow tissues is insignificant.

I like "min deco" too.

I much prefer the term "No Stop Diving" to "No Decompression Diving." And really that's what NDLs are: depth/time Limits for which a No Stop ascent profile is sufficient (in which the ascent has a capped ascent rate). Similarly, MDLs are depth/time Limits for which a "Min Deco" ascent profile is sufficient (in which "Min Deco" means something specific).

There we agree again. Even though the dive ends up being so-called no-deco...

Better yet: the dive becomes a No Stop dive (assuming the diver is competent enough to make the switch while maintaining a controlled ascent ).

The "Recreational Trimix" course is terribly named. It's really a technical course with an extremely weak in-water skills component

I have no particular qualms about IANTD's Rec Trimix program. It seems great for people who aren't comforatable with narcosis evident in the depth ranges for which the training is designed. Hell, the book is called Tek Lite or something, right?