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

[rjack321]

I can only speak to the training I have personally had. Which STARTED with "figure out what FO2 gives you a ppO2 of 1.4 at the max depth you are possibly expecting. And figure out what FHe gives you an END of 100'."

It then went on to (in short), "adjust those numbers as appropriate for the other factors (factors other than depth) that may be relevant to the particular dive you are doing."

One example: Planning a dive that is the nth in a series of deco dives with a lot of O2 exposure? Consider adjusting the mix for a max ppO2 of 1.2, instead of 1.4.

The insistence in equating Best Mix to a very simplistic approach to dive planning that means it is always "ppO2 = 1.4 and END = 100" is not what I have learned. That is just where I would start the process of selecting 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.

Air for a 135ft dive is not best mix by any conventional definition of the term. It has nothing to do with helium, its purely a calculation of fO2. Narcotic depth or the decision to include helium is not part of the best mix definition or calculation.
Best mix is: target ppO2 at depth / ATA = fO2

or in words
“Best mix” is the nitrox mixture with highest fraction or percentage of oxygen that can be used at the target depth.

Nitrox: Selecting Your Best Mix - SDI | TDI | ERDI
Nitrox Blending Calculator
http://subseaservices.com/pdf/Color/Unit 4 Choosing the Best Nitrox Mix.pdf
Nitrox - Wikipedia

 

[RainPilot]

it seems like that is a time when you would LEAST want to take shortcuts (like not discussing the details of the gas plan)?

Gas plan in terms of quantity, of course. IF everyone on a team knows that everyone dives standard gases exclusively, then there would be no reason to even mention it except cursorily, unless someone has an "odd" gas for some reason. This opinion of mine is not based on my (zero practical, SB-theoretical) knowledge of cave diving but I have spent many years operating with teams from various locations with standardised procedures where the briefings are almost 100% task based and very little procedural discussion since it is "just known".

 

[PfcAJ]

As dives get more and more complex, reducing variables where possible becomes more and more attractive.

 

[rjack321]

Gas plan in terms of quantity, of course. IF everyone on a team knows that everyone dives standard gases exclusively, then there would be no reason to even mention it except cursorily, unless someone has an "odd" gas for some reason. This opinion of mine is not based on my (zero practical, SB-theoretical) knowledge of cave diving but I have spent many years operating with teams from various locations with standardised procedures where the briefings are almost 100% task based and very little procedural discussion since it is "just known".

Standard gases absolutely avoid us both having different understandings about what the max depth might be, or the max we planned on (which is especially problematic when a dive gets blow out and you want to use a full set of tanks someplace else) or what "best" is etc.

Take the Vandenberg wreck, if I want to do that (max 140ft) I'll take a GUE buddy we don't have to worry about one of us showing up with 21/35 (the GUE choice), and the other with 26/17 (a NAUI gas), or 25/25 (a UTD gas), or with EAN27 ("AN/DP nitrox "best mix"), possibly 27/12 (best mix with an END of 100ft), or possibly air. I mean holy schmoly there are 5+ different possibilities all with different deco schedules?!?

No thanks, if I'm diving that OC I'll go with a GUE buddy who I know will bring 21/35. They may have a slightly tweaked deco schedule or GFs - but at least we start with the same gas. Does it meet the criteria for best mix with fO2, END, or price? No its not a perfect match for 140ft on any of those metrics. Does it work and not require the whole plan to be customized? Yes it works.

 

[stuartv]

It just seems odd to me that, in an endeavor where we go to such great pains to make SURE of pretty much everything, so many people would be so comfortable just ASSUMING what gas their teammates have, simply because they (presumably) have established what the max depth is.

I don't know anybody that I would dive with where I would buddy/team with them and just assume what gas they are going to be using. As I mentioned way up thread, I have at least one GUE Tech (something) and Cave (something) friend that I have asked before about some specific dive and his response was "I might use 21/35 for that, or I might use 18/45". And this guy spouts the GUE Kool-Aid all the time, He's not a diver with GUE training. He is a GUE diver. (from what I can tell, anyway)

 

[ChuckP]

Standard gases absolutely avoid us both having different understandings about what the max depth might be, or the max we planned on (which is especially problematic when a dive gets blow out and you want to use a full set of tanks someplace else) or what "best" is etc.

Take the Vandenberg wreck, if I want to do that (max 140ft) I'll take a GUE buddy we don't have to worry about one of us showing up with 21/35 (the GUE choice), and the other with 26/17 (a NAUI gas), or 25/25 (a UTD gas), or with EAN27 ("AN/DP nitrox "best mix"), possibly 27/12 (best mix with an END of 100ft), or possibly air. I mean holy schmoly there are 5+ different possibilities all with different deco schedules?!?

No thanks, if I'm diving that OC I'll go with a GUE buddy who I know will bring 21/35. They may have a slightly tweaked deco schedule or GFs - but at least we start with the same gas. Does it meet the criteria for best mix with fO2, END, or price? No its not a perfect match for 140ft on any of those metrics. Does it work and not require the whole plan to be customized? Yes it works.

Stuart and I are gonna dive the 27% cause we're old farts, need the extra O2. With the money we save on He, we're gonna dive it twice just in case we missed something the first time. With the other extra money we saved, we'll be eating steak and lobster while you all chomp on some burgers.

Absolutely just joking.

Seriously folks - 14 pages? Both plans work right? Let's quite bitching and go dive, find a buddy that does what you want to do and find some water....

Folks get twisted outa shape when someone stands up and says my way or no way - it's hard to step away from those "arguments", myself included but I try really hard.

 

[stuartv]

@ChuckP

Not even. With all the money I saved on helium, I bought a CCR, so I'm gonna go dive it one LONG time and have a ppO2 of 1.3 ALL the time.

 

[NAM001]

Air for a 135ft dive is not best mix by any conventional definition of the term. It has nothing to do with helium, its purely a calculation of fO2. Narcotic depth or the decision to include helium is not part of the best mix definition or calculation.
Best mix is: target ppO2 at depth / ATA = fO2

or in words
“Best mix” is the nitrox mixture with highest fraction or percentage of oxygen that can be used at the target depth.

Nitrox: Selecting Your Best Mix - SDI | TDI | ERDI
Nitrox Blending Calculator
http://subseaservices.com/pdf/Color/Unit 4 Choosing the Best Nitrox Mix.pdf
Nitrox - Wikipedia

I have never liked the term best mix because it only applies to a minimal test as to what is best. Best and perfect/ultimate are 2 different things. I hear this word best on so many posts like what is the best reg to buy, or wet suit to buy or mask to buy. To give an example of that what is the best mix for a 50 ft dive. You answer is x% but that % is unavailable to the nitrox diver so as such he has no "best" mix. The battle between hypothetical and reality is always a hard one. The nitrox course teaches best as it applies to a very limited test of ox tox. I would submit that instead of enriching O2 to get there one should dilute the N2 by using trimix. that however is not best for the pocket book given the cost of He and the course. Somewhere there is a definition of what the best is but like most things some of the components of that mix is not available for one reason or another.

 

[NAM001]

Standard gases absolutely avoid us both having different understandings about what the max depth might be, or the max we planned on (which is especially problematic when a dive gets blow out and you want to use a full set of tanks someplace else) or what "best" is etc.

Take the Vandenberg wreck, if I want to do that (max 140ft) I'll take a GUE buddy we don't have to worry about one of us showing up with 21/35 (the GUE choice), and the other with 26/17 (a NAUI gas), or 25/25 (a UTD gas), or with EAN27 ("AN/DP nitrox "best mix"), possibly 27/12 (best mix with an END of 100ft), or possibly air. I mean holy schmoly there are 5+ different possibilities all with different deco schedules?!?

No thanks, if I'm diving that OC I'll go with a GUE buddy who I know will bring 21/35. They may have a slightly tweaked deco schedule or GFs - but at least we start with the same gas. Does it meet the criteria for best mix with fO2, END, or price? No its not a perfect match for 140ft on any of those metrics. Does it work and not require the whole plan to be customized? Yes it works.

Your point goes to prove that many gasses will work for the dive, and that the real difference in the end is the deco. There for the use of standard gasses are to allign the deco's of all involved. Even if the gasses were slightly different the mix you will be using is good for a depth window (IE 120-200 ft) for the actual dive portion.

 

[rjack321]

It just seems odd to me that, in an endeavor where we go to such great pains to make SURE of pretty much everything, so many people would be so comfortable just ASSUMING what gas their teammates have, simply because they (presumably) have established what the max depth is.

Who said anything about assuming?
More like...

Me: "We're doing the Vandenberg right? 140ft max so 21/35 & 50%?"
Buddy: "yea man, got you covered"
Me: "thanks see you next week"

We both long ago agreed to keep ENDs <100ft so we are bringing helium of some sort. We both know that we don't push ppO2 on the working part of the dive for the sake of a few minutes off the deco schedule. 21/35 is readily available and if we get blown out we don't have doubles full of a custom mix that will have too much O2 or too little helium for a 150ft site or to just sit and wait from some other 100~150ft dive.​

Your point goes to prove that many gasses will work for the dive, and that the real difference in the end is the deco. There for the use of standard gasses are to allign the deco's of all involved. Even if the gasses were slightly different the mix you will be using is good for a depth window (IE 120-200 ft) for the actual dive portion.

21/35 is not a 200ft gas. Most GUE divers would plan to use 21/35 up to about 150ft but if the site was inadvertently a little deeper not balk at doing a 160ft dive on it. 160ft+ plans are for 18/45.