Nitrox mixes that you use - 'best mix' or 'what is available'

[fdog]

The big split for me is: Deco or NDL diving?


NDL: if I'm at home, I am the guy that blends the gas. So I have lots of flexibility. Still, 99% of the time, it's 32%. (I will very occasionally vary the mix according to the dive's needs. Doing CESA training with students - I'll breathe 40%.)

I stick with a ppO2 of 1.4 for NDL diving, because the surface is always accessible. If it's frickin' freezing, or I will be working hard, I'll drop it to 1.3.

So, even though I easily could, I don't think I ever blend up something leaner than 32% intentionally. That would imply I am going to greater depths, and I might as well stay a while...so, it's...



Deco: I stick with standard gases and a ppO2 of 1.2 for bottom mix. Standard gases are just easier to blend, and I have a "results history" to refer to, instead of lots of different mixes that are rarely the same. (There's other advantages that aren't part of this discussion, too)


In these conversations about best mix, I have found that knowledgeable divers rarely speak about targeting a percentage mix. Instead, they speak towards the selection of ppO2. The mix becomes a means to that end.


All the best, James

 

[manni-yunk]

interesting thread

 

[billgraham]

In the Northeast where I live, the majority of the wrecks we dive are in the 100-130 fsw range. For open circuit diving, I find that if you keep some 28% and 0-2 around, you can do just about any dive that comes up in that range. The fact that the bottom mix is a little leaner than you'd like at 100 feet only adds a few minutes to your deco, it's not worth jackassing around to get a perfect 1.4 bottom mix, I don't think.

 

[letonac]

Usually I use EAN 32 because it is the most available mix. MOD is based on 1,4 ppO2. If you want to gets all benifits which they are coming from EAN there is not doubt - go to the best mix, specially when you will dive some wreck and you want to explore it well. This important for us which are not treined in Tech diving. For me is also important money factors - if the best mix is to expensive I will choose standard mixes (EAN32 or EAN 36). Anyway, I like Nitrox dives...

 

[Doppler]

Most of my diving these days -- unless I'm teaching an OC program -- is on CCR... so I guess i dive best mix, constantly!!

:blinking:

 

[JamesK]

Most of my diving these days -- unless I'm teaching an OC program -- is on CCR... so I guess i dive best mix, constantly!!

:blinking:

Smarty pants.

The dive shop I use banks 30% so that's what most of my dives are done with.

 

[drl]

For Chicago area diving a lot of the best wrecks are in the 100-130' range, and our lds custom blends nitrox, so I ask for the ideal mix for the dive planned. At resorts there is usually 32% banked, so I plan my dives accordingly or use air for deeper dives.
And yes, I'm one of those who feels less fatigue on nitrox. It may be a placebo effect but it works for me.

 

[JS1scuba]

Most nitrox dives can be completed using standard 32% and 36% oxygen mixes. However, there are times when extending the no-stop time or adding in a second or third nitrox gas to extend the no-stop time requires the diver to make specific gas choices.

There may be times however when a dive calls for the most no-stop dive time for a specific depth. This would be achieved by determining the fraction or percentage of oxygen needed for that depth and having a specific mix blended for that dive.

What actually makes a "best mix" truly a best mix? We know that it is inert gas that controls the no-stop time limits and the decompression times should decompression stops be needed. By displacing some of that inert gas with oxygen we can extend no-stop time and shorten decompression time. Ideally; to maintain the absolute "best mix" throughout the dive a constant PO2 rebreather should be used. In that case the user can "dial in" a specific PO2 set point and the unit will maintain that level of inert gas throughout the dive on a constant basis.

But for those divers who are not using a rebreather (which is most divers today) selecting a mixture with the highest oxygen content at the deepest part of the dive (so long as it does not exceed a specific partial pressure ) is appropriate. To further extend the dive time the diver may add in another mix or gas switch to increase the oxygen partial pressure as they ascend. Keep in mind that even though a "best mix" is set for the deepest portion of the dive as the diver ascends the partial pressure decreases and therefore decreases it's effectiveness. Divers who conduct dives that require decompression will normally use one to two "decompression gases" during the ascent to maintain a higher partial pressure of oxygen so as to minimize the decompression obligation.

Ideally a best mix when diving on open circuit scuba should be a 1.4 PO2 level for the swimming part of the dive and a gas switch to another mixture at approximately � the maximum depth of the dive with a PO2 level of 1.6 atm., and a gas switch yet again at � the depth between the MOD of the first switch and the surface.

While this is an ideal gas plan it may not be an ideal practical plan. Consider that the small amount of gas breathed during the gas switches does not affect the no-stop time considerably at all since no-stop limits are just that no-stop.

Selecting a best mix is not only making a choice of breathing mix, it also includes decisions on logistics, gas availability and equipment configurations. Many times one is better off making a shorter dive that utilizes less equipment and bulk than one that requires significantly more equipment when the time benefit advantage is small.

One other critical area to consider is your gas supply. Most no-stop dives conducted with single tanks cannot take advantage of the maximum no-stop time available using "best mix" options. To that end going to the expense of creating a "best mix" is unnecessary. For the most part the standard EAN 32 and EAN 36 mixes work well for the 50-130 fsw dive range.

Cheers

 

[boulderjohn]

This was a topic of discussion when I got my advanced blender certification. I got it at a shop that sells a lot of gas to local divers. It banks 32%, and sells the banked 32% for not much more than plain air. The owner (my instructor) told the story of a guy who came in and wanted a best mix blend that was something like 33-34%. The owner pointed out that he was going to be paying for a custom blend, at a much higher rate than for the banked 32%. And for what benefit? The guy insisted. Oh, well.

The benefits of a percent or two here or there are pretty negligible. It is only significant if you are going to be exceeding the MOD of your gas otherwise. I am happy to go with a standard 32% for anything shallower than its MOD. If there is a better blend ready to go, or if I am blending my own, I will go with it, but otherwise I will take what's there for me.

I have also used a lot of "slightrox." Slightrox is something you use in the field, when you don't have good options for fills. You start with a nitrox fill, and then after your dive you transfill some air into the tank. Then you analyze it to see what you ended up with.

 

[lamont]

partial pressure blended 30-32%, i kind of like the O2 a little lower for dives in the 100-110 foot range. but i generally won't complain if it analyzes out closer to 34% unless it impacts a planned dive, i'll just use that for shallower diving. around here the 130 foot dives that i do are not remotely temping to dive nitrox for -- 45F temps, dark, silty, 2000 foot scooter runs back to your deco stop with direct surfacing not optimal -- and if your buddy hoses the bottom it drops to near zero siltout very quickly. prefer to have helium.