Recreational Trimix Diver course with Techs Mex Divers, Part I

[Kevrumbo]

OOOHH . . .wait a minute Reg:

As a rule, you don't want to do switches to Nitrogen rich/heavy mixes below 21m/70' (I hope your Instructor would have realized that). . .

Did Oliver explain to you anything regarding Isobaric Counter Diffusion?
[i.e. where one inert gas enters a tissue faster (Nitrogen) than another can leave it (Helium), hence creating a super saturation and potential bubble trouble]

Becomes a big issue on deeper Trimix Dives and higher mixes of Helium, but you should be aware of it. . .

 

[TSandM]

I'm not sure switching from a mixture with 48% nitrogen to a mixture with 62% is enough to worry about isobaric counterdiffusion . . .

But I would be a little edgy about planning a dive to 120 feet on an Al80. Did you discuss minimum gas reserves?

 

[Reg Braithwaite]

Did you discuss minimum gas reserves?

We certainly discussed minimum gas reserves. I was instructed to plan the dive such that I was on the boat with 500 psi. I did use a consumption rate that was more aggressive than I would normally use, about 60psig SGC. Also, this was a drift dive where I could breathe "all usable." During the classroom, I also did gas planning exercises featuring half plus 200 and thirds plus two hundred strategies.

According to my notes, for my reserve at 2,000' I calculated 277 psig for problem solving, 204 psig for a single deep stop, 672 psig for travel, 289 for the absolute minimum 3 one minute stops at 30', 20', and 10', and 500 psig to be a reserve reserve. Call it 2,000 psig. I used similar math to derive a 100' reserve of 1,700 psig, an 80' reserve of 1,500 psig, and a reserve of 1,200 for 60'; and shallower.

On the dive itself, I was responsible for signaling the level changes when we hit our time or gas limits, and I hit those reserve numbers before the time limits, so they ended up driving the dive profile.

If a friend with my setup (non-redundant gear) asked me to do that dive today I would use a more conservative consumption factor, probably 75 psig. I would also plan more stops which would use up more gas. As such, it would probably be impossible for me to have enough gas to dive to 120' with a 'normal' buddy on an AL80. I cannot speak for Oliver, but I will speculate that he was relying on a combination of his redundant back gas rig and his ability to bail out to EAN38 at 90' to handle the case where I actually needed to donate gas to him at 120'.

 

[Reg Braithwaite]

As a rule, you don't want to do switches to Nitrogen rich/heavy mixes below 21m/70' (I hope your Instructor would have realized that). . .

I switched from 32/20 to 38/0 at 40'. As for Oliver, we did not discuss his back gas mix. He may not have been diving any He at all in his back gas. I should make it clear that although he told me what he was doing and let me try breathing off the stage in my second dive, gas switches are not any part of the course curriculum, so he was not training me to carry an EAN38 stage.

I can't speak for him, but he's a wily fellow with a lot of experience as an instructor. He probably was "giving me a taste" of tech diving so that I would get excited about it and book something a little more advanced with him in the future

 

[Rick Inman]

(...It qualifies you to dive trimix to your existing depth training or a maximum of 130 feet.[/I])

Hummm... The IANTD standard is:
"This program qualifies divers to dive to 160 fsw (48 msw) and perform decompression stops required dives up to 15 minutes. "

Most of the ART classes I've heard about are taught in doubles. My guess is that the 130' limit is imposed by the specific instructor based on using single tanks.

Did you do the following?:

• Perform gas switches to a decompression gas mixture no greater than 1.5 PO2 on at least two dives
• Perform valve shutdowns on each dive.
• Deploy lift bag 2 times, using it as an ascent platform.

 

[Reg Braithwaite]

Hummm... The IANTD standard is:
"This program qualifies divers to dive to 160 fsw (48 msw) and perform decompression stops required dives up to 15 minutes. "

Most of the ART classes I've heard about are taught in doubles. My guess is that the 130' limit is imposed by the specific instructor based on using single tanks.

You are describing Advanced Recreational Trimix. AFAIK, that course requires either doubles, H, or Y configuration plus a deco bottle. ART definitely goes beyond recreational limits.

I took the much less popular Recreational Trimix (no "advanced" up front), which IANTD allows to be taught in K, Y, or H valves. I deplore IANTD's naming conventions. The difference between "Recreational trimix" and "Advanced Recreational Trimix" is vast, almost exactly the difference between "Nitrox Diver" and "Advanced Nitrox Diver." What I took is the Trimix equivalent of "Nitrox Diver."

 

[Rick Inman]

You are describing Advanced Recreational Trimix.

Oops! Yeah, I just realized that. Sorry, I miss-read.

 

[Blackwood]

I'm not sure switching from a mixture with 48% nitrogen to a mixture with 62% is enough to worry about isobaric counterdiffusion . . .

It's not simply about FN₂1 versus FN₂2. ICD is about how much helium you are dropping compared to how much nitrogen you are adding.

A common rule is the 1/5 rule: don't increase the FN₂ by more than 1/5 of the FHe reduction.

That said, I've also seen CCR divers consider an increase in FN₂ of more than 10% to be unacceptable as a hard and fast rule for selecting bailout gases on trimix dives.



But ICD is typically considered for deep dives. I'd personally have no concern doing the switch Reg indicated, though there is no way in hell I'd lug an extra 80 just to increase my FO₂ by 6%

 

[Reg Braithwaite]

ves. I'd personally have no concern doing the switch Reg indicated, though there is no way in hell I'd lug an extra 80 just to increase my FO₂ by 6%

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.

 

[rjack321]

AL80s with 32/20. The second dive featured a switch to EAN38, then back to back gas for stops.

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

OOOHH . . .wait a minute Reg:

As a rule, you don't want to do switches to Nitrogen rich/heavy mixes below 21m/70' (I hope your Instructor would have realized that). . .

Did Oliver explain to you anything regarding Isobaric Counter Diffusion?
[i.e. where one inert gas enters a tissue faster (Nitrogen) than another can leave it (Helium), hence creating a super saturation and potential bubble trouble]

Becomes a big issue on deeper Trimix Dives and higher mixes of Helium, but you should be aware of it. . .

For 5 or 10mins of BT at 120ft ICD is basically irrelevant. Where's this magic 70ft level come from?

But ICD is typically considered for deep dives. I'd personally have no concern doing the switch Reg indicated, though there is no way in hell I'd lug an extra 80 just to increase my FO₂ by 6%

No kidding