Best Mix VS Standard Gasses (split from a GUE fundies course report)

[Storker]

How is He offsetting any amount of pure CO2 narcosis?

It's a known fact that gas density affects CO2 ventilation. I have the impression that poor CO2 ventilation increases CO2 narcosis, but I may be wrong on this. If I'm right, I believe that the mechanism is rather obvious.

 

[ChuckP]

It's a known fact that gas density affects CO2 ventilation. I have the impression that poor CO2 ventilation increases CO2 narcosis, but I may be wrong on this. If I'm right, I believe that the mechanism is rather obvious.

So, lighter the gas mix, easier to breathe, less chance of CO2? Ok, I can see that.... makes sense.

 

[PfcAJ]

How do you use 30/30 for 120 ft dive without violating GUE guidelines for keeping PPO2 under 1.2? At a 120 ft dive you are at a PPO2 of 1.4 which is accepted by mainstream but not by GUE as I understand.

Me personally?

Its a short enough exposure on dives like we're describing that I wouldn't think twice about it.

I think its worth reminding the world that these things are guidelines. A working po2 of 1.2 is a guideline, and a pretty good one at that. A bit of time at a 1.4 isn't the end of the world.

 

[rjack321]

So, lighter the gas mix, easier to breathe, less chance of CO2? Ok, I can see that.... makes sense.

You got it. This is the genesis of the max 6.2g/L density guideline.
Gas density guidelines

At 100ft 30/30 is 3.9 g/L while 32% is 5.2 g/L. That's a big difference in ventilation work if you have to chug along in heavy flow for an hour. And CO2 is way more narcotic than N2. Good fitness can help a little but Little River and Manatee are a ton of work to kick.

 

[rjack321]

Me personally?

Its a short enough exposure on dives like we're describing that I wouldn't think twice about it.

I think its worth reminding the world that these things are guidelines. A working po2 of 1.2 is a guideline, and a pretty good one at that. A bit of time at a 1.4 isn't the end of the world.

I'm sure @johnkendall will be along shortly to give us the hard rules instructors have to follow lest the boss boot them out

 

[Storker]

Me personally?

Its a short enough exposure on dives like we're describing that I wouldn't think twice about it.

I think its worth reminding the world that these things are guidelines. A working po2 of 1.2 is a guideline, and a pretty good one at that. A bit of time at a 1.4 isn't the end of the world.

I guess it also depends on what shape you're in, and a lot of other factors.

I'm not GUE trained, but I've set my personal depth limit at around 30m/100' because I start feeling the nark around that depth. Some days I'm comfortable with pushing that limit somewhat, other days I prefer to stay well away from that limit. It depends on my current shape, the type of dive, the dive profile and a bunch of other parameters I have no control of.

 

[johnkendall]

I'm sure @johnkendall will be along shortly to give us the hard rules instructors have to follow lest the boss boot them out

GUE recommend a maximum PO2 of 1.4 for recreational dives (i.e short exposures)

 

[rjack321]

I guess it also depends on what shape you're in, and a lot of other factors.

I'm not GUE trained, but I've set my personal depth limit at around 30m/100' because I start feeling the nark around that depth. Some days I'm comfortable with pushing that limit somewhat, other days I prefer to stay well away from that limit. It depends on my current shape, the type of dive, the dive profile and a bunch of other parameters I have no control of.​

I'll dip to 110ft on 32% occasionally in OW if the vis is good and the workload is low. There are a few spots in FL caves that dip to 105ft too (Bozell's is one IIRC) and I'll even 'break' Shrek's 100ft END rule diving 32% there.​

GUE recommend a maximum PO2 of 1.4 for recreational dives (i.e short exposures)

I was just teasing you, sometimes threads like these get pretty black and white

 

[Storker]

I'll dip to 110ft on 32% occasionally in OW if the vis is good and the workload is low.

That difference - at least if it's temporary - is small enough that my panties will happily remain untwisted.

 

[stuartv]

It's not the standardization of gasses that is limiting.

The standardization of gases IS limiting if you want to do a dive and the prescribed Standard Gas is not available...

Deco theory was no so far advanced when that list of standard gases was originally made. One reason to have standard gases is to reduce the number of variables. If everyone is using the same gases, the deco plans and algorithms can be changed when necessary and everyone benefits in a broader sweep when variability is more limited. There are fewer edge cases to worry about. When I took trimix classes, my instructor pointed out that WKPP probably used more He more often than anyone else, and he saw some logic in using the same mixes to take advantage of that large body of experience. Some years later, and lots of He bubbles later, I agree that he was absolutely right at the time, and much of that "rightness" persists even though we have learned a lot more.

The other thing to consider is that GUE stresses a team diving approach with its roots in cave diving. Everyone is rigged the same. Everyone uses the same procedures. Everyone uses the same cylinder markings. And, everyone breathes the same gases. There are at least two big advantages to the standard gas approach when diving as part of a team. One is that anyone can donate gas at any time, to anyone, and no deco recalculations are needed. This reduces the stress and complexity of gas donation scenarios. The second is that everyone knows the minimum and maximum operating depths for everyone else's gases, because they're the same across the board. So when double-checking gas switches (part of the protocol), nobody needs to think about the MODs. Likewise, everyone has the same deco schedule, so the whole team can act like a school of fish without leaders and laggards.

I think the second part stands, still, as a good idea in that kind of diving.

Further, if you are a GUE-trained diver (I am not), the urge to dive the same way, all the time, regardless of recreational or technical settings (which I share) is likely a strong one. Why change all those procedures that work? No particular reason in favor, and some against.

To me, none of this should necessarily guide a firm choice between "best*" mixes or standard gases for a non-GUE diver on a recreational dive. They're chalk and cheese in that context anyway, and the meaningful differences are negligible most of the time in my view.

* The very definition of "best mix" has changed since my original nitrox cert.

(Edited to fix typo.)

I get where they came from and why. Ratio Deco depends on an assumption of what gas you are using for a certain depth range. There was a time when all that was needed.

It seems to me that that time has passed.

It also seems like requiring people doing (relatively) shallow, warm, clear water drift dives to use specific gases that made sense for the technical cave divers who came up with the list is simply a case of using the wrong tool for the job.

Your other statements seem to carry an implication that if you are not GUE-trained and not diving with Standard Gases that means you are not adhering to good team diving principles like gas matching. I don't know about other training, but my OC tech training has been all TDI and it most certainly included things like everyone on the team having all the same gases. And the same algorithm and parameters. So, everyone also has the same deco profile. None of that requires use of Standard Gases.

I did a deco dive a couple of years ago with a buddy. The bottom was something like 135'. We used Air for the bottom gas because neither of us felt like helium was needed for the specific conditions and Nitrox was not needed because we were not concerned with NDLs. Using Air was less expensive and the most convenient. We decided it was the best mix for the planned dive. We used a single deco gas of 80%, because we DID want to be prepared for significant surface surge that would make hanging at 20' more difficult and less pleasant than hanging at 30'.

We did just under 50 minutes on the bottom and a total runtime of 102 minutes. it was a fairly long hang at 30'. And I was really glad I wasn't having to hang at 20'. 30' was bouncy enough.

We used the best mix(es) for the plan and I think we got the best dive out of it that we could have. Sticking to Standard Gases for that dive would have cost more, for no benefit, and made the deco even more sucky.

How long time are you planning to spend underwater? And how big is (are) your tank(s)?

With my gear setup (and my bladder capacity) I see very few advantages with 36% over 32%. It's not just about the NDL, it's also about min gas and getting cold (or bored).

On OC, I dive double 120s. At 90', I can easily last as long as the NDL for 36%. The difference in NDL (from 32%) is not much. But, if I'm going to all the trouble to be there (wherever "there" is), why would i not want the extra minutes on the bottom, if I can have them with no downside?

When I get a fill, I take my tanks to the shop, drop them off, and tell them what I want in them. It's exactly the same effort for me whether I say "32%, please" or "36%, please". Same number of syllables, even.