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

[leadduck]

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.

Now you confuse me. You don't call a gas "best mix" because there's helium in it?

These best mix calculations lead to the easy mixing of banked EAN32 and He instead of O2 blending for dives below 30m:
At MOD with pPO2=1.3bar and END=30m (pPN2+pPO2=4.0bar), you get pPO2/(pPO2+pPN2) = 1.3/4.0 = 32.5%, i.e. EAN32.
Same story less conservative with pPO2=1.4bar and END=33m, again 1.4/4.3 = 32.5%

Hence best mix for pPO2=1.3 & END=30m, as well as pPO2=1.4 & END=33m, at any depth, is always a mix of EAN32+He, such as 21/35 or 18/45 but also 28/15 or 26/20.

I think standard gas sequence (21/35, 18/45, ...) is fine enough so why would you bother with 19/40 instead of 18/45, the advantage in deco and cost by 5% less He and 5% more EAN32 is small. Only for the range of 35m-45m I can see why one could save time and money with 26/20 instead of 21/35, but that's also the depth range where most still dive air anyway.

 

[stuartv]

Who said anything about assuming?​

All the previous comments that said something along the lines of "we don't even have to discuss it. I just know they'll show up with the same gas as me."​

 

[huwporter]

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.

But isn't that one of the major points of standard gases in the first place? We know what the maximum depth is going to be, I know what the standard gas is for that depth, you know what the standard gas is for that depth, I know that you know what the standard gas is and you know that I know. Therefore I know that if you have something different, then you'll mention it, and you know that likewise I will. So if you don't mention it, then why shouldn't I assume?

If you do have something (significantly) different and you don't mention it, and I find out afterwards, then we are going to have words because that was my spare gas you were dicking with... (Corollary of this; if I'm not diving with you I don't care what gas you use. You do you.)

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)

If I have a chance to do a 45m/150 ft dive and I only have an 18/45 fill available, then of course I'll do that dive, why not? It satisfies END and PO2 standards, and it isn't hypoxic. Sure I'd prefer to save my He for something deeper, but a dive is a dive right? I will mention it to the team, but not as a big deal, probably something like "oh, by the way, I have 18/45 so I'll add 5 minutes to the deco." on the way out.

If I only have 32% available then I'm definitely not doing that dive.
If I only have 12/65 or 10/70 available, then I'm not diving.
If I only have 15/55 available (and we are assuming I can't get a squirt of 32% from a dive shop to bring that PPO2 up a bit) then I'm probably not diving unless all the team are hypoxic trained, I can also bring along a second deco gas, and all the team are cool with it - and you can be sure that this would be well discussed in advance. (I'd probably also have to be a bit desperate to get a dive in to blow that He so shallow...)

 

[rjack321]

Now you confuse me. You don't call a gas "best mix" because there's helium in it?

These best mix calculations lead to the easy mixing of banked EAN32 and He instead of O2 blending for dives below 30m:
At MOD with pPO2=1.3bar and END=30m (pPN2+pPO2=4.0bar), you get pPO2/(pPO2+pPN2) = 1.3/4.0 = 32.5%, i.e. EAN32.
Same story less conservative with pPO2=1.4bar and END=33m, again 1.4/4.3 = 32.5%

Hence best mix for pPO2=1.3 & END=30m, as well as pPO2=1.4 & END=33m, at any depth, is always a mix of EAN32+He, such as 21/35 or 18/45 but also 28/15 or 26/20.

I think standard gas sequence (21/35, 18/45, ...) is fine enough so why would you bother with 19/40 instead of 18/45, the advantage in deco and cost by 5% less He and 5% more EAN32 is small. Only for the range of 35m-45m I can see why one could save time and money with 26/20 instead of 21/35, but that's also the depth range where most still dive air anyway.

Those are not best mix calculations.
Best mix is defined in nitrox class, I gave you 4 links to that definition already but there are more with a little googling.

Best mix is: target ppO2 at depth / ATA = fO2
Target ppO2 was historically 1.6 but got revised to 1.4 over a decade ago.

The ratio of N2 and Helium (even 0% helium) and your chosen END has nothing to do with how best mix is defined. GUE standard gases rarely align with best mix - the whole point is that you can use them across a range of depths and they are "good enough" even if they do not necessarily have the optimal fO2. Best mix is about picking the "ideal" fO2 that gives you the least deco or maximum NDL for every dive site. Usually customized for that exact max depth like the EAN30 for the square profile 120ft dives in North Carolina mentioned earlier in this thread.

 

[EireDiver606]

Then how does anyone know what the GUE Standard Gases are? Apparently, UTD has Standard Gases that are different. It's starting to sound like "Standard Gases" really are not standard at all.

And yes, I want to know 100% by the GUE book. I want to know what a GUE diver would do if they were strictly following their training.

I can see how "TX30/30 and stay 5' off the bottom" would possibly be the answer (presuming that TX30/30 is an official GUE Standard Gas). That is the answer I came up with, anyway.

Whether I would want to pay for 30% helium on every NDL dive (including the ones to 70') down in NC is a whole nother question. My LDS is the cheapest shop I know for helium and they charge $1/foot. $30 of helium in one HP100, for one dive to 70'.... Hmmmm... $60 gas bill for a 2-tank dive day.... $72 in gas to do 2 dives with my double 120s... The other LDS charges $1.50/ft. $108 in gas for 2 dives....?

Or, more applicable to most folks. $1.50/ft and 2 x single HP100s. $90 in helium for 2 NDL dives.

Standard Gases may sound nice on paper, but does the concept really survive contact with the real world? I mean, the real world of places that don't conveniently have their depths lined up with one row on the Standard Gas chart? If all your possible sites are all fine on EAN32, that is great, but it seems like there are a lot of places where that doesn't work.

Just take fundies already.

You know you want to.

 

[EireDiver606]

Their rec curriculum is a mess. Maybe? I think “rec 2” does? Rec 3 is stupid so I try not to think about it.

Why is the recreational programme stupid? I’m not disagreeing with you. Have you taken it? Apparently, it’s navigation, 30m depth, triox, rescue and more advanced skills all in one course. What’s not to like?

 

[leadduck]

Those are not best mix calculations.
Best mix is defined in nitrox class, I gave you 4 links to that definition already but there are more with a little googling.

Best mix is: target ppO2 at depth / ATA = fO2
Target ppO2 was historically 1.6 but got revised to 1.4 over a decade ago.

The ratio of N2 and Helium (even 0% helium) and your chosen END has nothing to do with how best mix is defined. GUE standard gases rarely align with best mix - the whole point is that you can use them across a range of depths and they are "good enough" even if they do not necessarily have the optimal fO2. Best mix is about picking the "ideal" fO2 that gives you the least deco or maximum NDL for every dive site. Usually customized for that exact max depth like the EAN30 for the square profile 120ft dives in North Carolina mentioned earlier in this thread.

The link you gave were for SDI Nitrox class, of course without helium. See for example:
Custom Mix vs. Standard Mix: Best Mix is a question of balance - SDI | TDI | ERDI
they don't make that distinction for deeper dives with He, best mix / custom mix / optimal gas all the same, END and MOD are equal parts of the calculation. Anyway, let's call it custom mix instead. If you define your custom mix by pPO2=1.4 at target depth for O2 calculation and END=33m for He calculation, then automatically for any depth it always results in a mix of EAN32+He, and the GUE standard gases fall in line.

 

[NAM001]

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.​

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.

rjack why do yo keep doing this to my posts.

I never said the thing you are twisting. You again proved just what I said,,,,, that the standard gases are used in a depth window ie 120 - 200 not that that preticular gas was used in that preticular window. Or that that preticular example of a depth window was an actual window. The point is that you can use standard gasses because they are sutable for any depth with in a depth band / window and are not designed to be ultimate for the exact depth you are diving at that day. They are not based on best gas concepts........ They are designed for the simplicity of deco process as first priority of selection to support Ratio Deco.

 

[CptTightPants21]

All the previous comments that said something along the lines of "we don't even have to discuss it. I just know they'll show up with the same gas as me."​

The mixes are always known. It's not a discussion about this blend of He vs this blend or this amount of O2. Depending on the dive, it is really a quick check about picking 1 out of 2 known gases. In the Vandy example, there is a ton of wreck from 100-60ft. Question would be are we staying shallower and dive 32% or do you want to do some serious penetration, go deep and bring 21/35?

To sort of highlight an example, I was discussing a dive plan with a non-GUE diver about a dive in the 140-160 range. Talking about gas--21/35. Nope. Was going to dive 24%. What about any helium--maybe, maybe 15-20% he says...Talk about deco-- Was just going to use extra back gas. Me (a little confused about dive times and min gas now)--no you should have at least a 50% bottle. Him: Ok-50%
A little more back and forth about plan and I discover he was talking about putting 50% in an al40....

Most of regular dive buddies are either GUE or are trained by instructors very close to GUE. The conversation above really brings home what it is like diving in the 'wild'. There is so much that you can take for granted with GUE. Things are either implied or it is a questions of is it a or b, cool, let's go.

Why is the recreational programme stupid? I’m not disagreeing with you. Have you taken it? Apparently, it’s navigation, 30m depth, triox, rescue and more advanced skills all in one course. What’s not to like?

Rec 1 and Rec 2 are good. A little expensive and maybe a little bit condensed for a someone with no scuba knowledge, but great courses that encompass most if not all the specialty classes a person would need to take. A great value if you can convince someone to make the larger initial investment. AJ just REALLY hates Rec 3--doubles, helium, 50% deco for 15 min, 130ft limit AND it DOESN'T qualify you for a tech pass in fundies. In Rec 3, you drop money for the instructor, money for helium, money for boat dives, and money for deco gas and you are certified for what???. Rec 3 is the blue balls course of GUE training, all the time and money of a tech class without any climax.

The link you gave were for SDI Nitrox class, of course without helium. See for example:
Custom Mix vs. Standard Mix: Best Mix is a question of balance - SDI | TDI | ERDI
they don't make that distinction for deeper dives with He, best mix / custom mix / optimal gas all the same, END and MOD are equal parts of the calculation. Anyway, let's call it custom mix instead. If you define your custom mix by pPO2=1.4 at target depth for O2 calculation and END=33m for He calculation, then automatically for any depth it always results in a mix of EAN32+He, and the GUE standard gases fall in line.

best mix/ custom mix/ optimal gas LOGICALLY can't be all the same. That would mean that breathing air at 120' and air at 80' would both be "best mix".

There are three types of gas mix techniques
1. Standard Gases---GUE and UTD have the same standard gas list expect UTD replaces 30/30 with 25/25 instead
2. Best Mix - This was the highest percentage of O2 you could get in your back gas and not violate MOD for a particular dive. The calculation is 1.4 PP02 / ATA at max depth. Helium could be a part of the equation as well, but as far as I am aware 100' END is a guideline, I have seen trimix divers use 130' END. Best Mix has always been more about driving the percentage of O2, END dictated the helium percentage, but that was a personal preference. You could dive using END less than 100 and it would still be a best mix dive if you are maxing out your PPO2.
3.. Custom Mix - This is what ever you want the mix to be. Rando gas. Diving air at 135', diving 21/20 at 150'. Any random arbitrary mix that doesn't fall into the standard gas or best mix category for that particular dive. And yes, a best mix dive that gets blown could then become a custom mix gas blend if you don't end up doing a dive with a similar depth profile as the blown out best mix dive.

 

[Ayisha]

...Best mix is: target ppO2 at depth / ATA = fO2
Target ppO2 was historically 1.6 but got revised to 1.4 over a decade ago.

The ratio of N2 and Helium (even 0% helium) and your chosen END has nothing to do with how best mix is defined...

I am very surprised that anyone thinks that Best Mix has traditionally involved any consideration of a "recreational" level END (Equivalent Narcotic Depth) such as 100 feet or even 130 feet. Best Mix as far as I recall was always simply the highest fO2 for your chosen PPO2 at your planned max depth.

Interesting that it took that many posts to question that definition even though END consideration in Standard Gases was mentioned early on with the preference for helium mixes for dives over 100 feet to keep any depth dive (ex 300 feet) to an END of no more than 100 feet.