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

[Colliam7]

A series of posts in another thread (Devil's Throat question) caused me to wonder - for diving within recreational (NDL) limits, how often do people actually use any kind of 'best mix'? The question is difficult to articulate, so here is the background.

1. In teaching nitrox, I go through a number of calculations with students, to determine what the maximum percentage O2 would be for a given depth and MOD. I generally use a MOD of 1.4 for these exercises, and point out that problems with oxygen toxicity are quite unlikely to present if you stay at or below a PO2 of 1.4 - not impossible, just very improbable. I go on to say that, while you can use any mix that you determine to be best, you will seldom have a problem with the common banked mixes (32% probably being the most common, with 30% also available in our area) in recreational diving. If the depth you want to reach doesn't work with the banked mix available, either stay shallower or go to air. (Or, ask the fill operation if they can give you a more precise, depth-adjusted mix).

2. Personally, I can think of only one time in the past 10 years when I intentionally blended a mix other than a banked nitrox mix for recreational diving. I was going on a wreck at 136 feet, and diluted some 30% to get a slightly lighter mix. (This may contradict my emphasis on 'recreational' diving, but that is not the point.) And, I have used lighter mixes resulting from topping off partially used nitrox bottles with air. But, that was also a matter of convenience. While I have 'clean for oxygen service' bottles available, and the ability to partial pressure blend whatever I want, I also don't blend richer mixes for shallower depths for recreational dives. The other divers I am usually in the water with do the same.

My question: how frequently do experienced divers tweak their mixes, beyond banked mixes, for recreational diving? And, where you do 'tweak' the oxygen concentration - between 21 and 40% - why? Or, perhaps, the better question is, on what quantitative basis do you do so?

I don't intend the question as a criticism of 'best mix' approaches. (And, I am really not interested in resurrecting another recreational trimix discussion, although that may happen.) Rather, I am now intrigued by what the usual and customary practice is outside of my own little world. Maybe, I just opt for convenience (or am lazy, or cheap), and 30%, 32%, or 21% seem to work for what I want to do.

 

[Peter_C]

I continuous blend 32% into the banks (But could blend most anything up to 40%), so that is what I use as a recreational diver. Plus Peter Steinhoff's tables are based on 32%, and since I dive tables it works best for me. Our dives tend to be no deeper than 90 ft and more often than not around 65 feet.

 

[waterpirate]

28% is dove frequently in the mid atlantic region. I think that depending on the specifics of the dive ( work load ) the push is to lean it up amoung frequent divers, below what the book says for NDL dives. The argument I would make for semi- standardised gasses is that for the average punter, they will see repetition in times and depths that will be committed to their memory, that they may find helpfull given a puter hick-up or failure.

I employ that same strategy for my deeper dives to a certain extent. The differance being once you commit to a overhead, it is more about exposure and time, within reason.
Eric

 

[sea_ledford]

I will genearlly use what is available, and aside from the occational 50% mix for deco, I don't have any tanks that I can have partial pressure filled.

The majority of my dives these days are off a livaboard with air and 32%. For our non-deco dives to 130 we'll mix them half and half to get 27-28%, and top them off for later dives. That ends up meaning that the % goes up for each dive and starts over at 27-28% the next day. I suppose we could empty tanks for the second dive and get straigt to 32%, but with the diving we are doing, it doesn't really matter much... a couple minutes of no-deco time maybe.

I should also say that we are doing scientific diving, so when the job is done, the dive is done. No need or desire to streach out every dive for as long as possible. And there is nothing shallower than 85 feet to look at anyway.

I can't think of anytime that I calculated a best mix for nitrox beyond keeping my MOD deeper than my planned max depth.

- Chris

 

[rhwestfall]

Around here, there is no banking. You ask for what you want. For me, my most "common" blend is 28% as the wrecks we go to are at 130' to the mud (there are deeper ones, and folks are tri-mixing). For me, 28% becomes about as universal as air (just for conservatism - not stretching NDL), though I dive air (21%) for most of the shallow stuff (shallower than 100'). Two dives a day and cold water with lots of folks diving wet, so they are short-ish in duration. Very little opportunity to get to dive #3 in a day, so not much need for voodoo gas.

 

[knfmn]

When I dive nitrox, I usually use 32%. It is what most people who blend nitrox up here do and is the easiest to get. I have changed that a couple of times when diving deeper wrecks, where I got 28% blended for me. It's a matter of convenience.

Kristopher

 

[nimoh]

I am a firm believer in standard mixes. 32% up to 100', after that, add helium. I think it just makes things easier.

 

[DivemasterDennis]

I am big fan of 36 when it is available and dive profiles permit. 32 is a distant second. With EANX 36 I believe I actually feel better- less fatigued, and also I believe it "taste's" good. Just a few subjective comments that Debbie thinks are all in my mind.
DivemasterDennis

 

[exposure]

It is a very interesting point that you raise, but either you can do your own blend and then you should go for the best mix or you need to convince the dive center and then you tend to stick to the 32%. As far as I am concern, I tend to choose a dive center that is doing technical dives so I am pretty sure that they can blend what I need for a dive. Therefore, in liveboard and most of the dive center where I have been, they tend to pre fill banks or use membrane set up for 32%. So in this case, like you said, it is either 32% or air. I don't like to dive on air as most of the time while travelling I do 3 or 4 dives per day.

When I can choose my blend, I increase the O2 during the day as I decrease the depth. Let's say if I plan a 40m dive as the first dive I will ask for a 28% (or maybe something like 25% to don't be at 1.4), the second dive will be on 32% and for the third of fourth that can be a night dive no deeper than 18m for instance I will ask something like 40%. Unfortunatelly, it seems simpler to get 28% than 40%

For the max PPO2, even if I will never exceed 1.4 in recreationnal dives, I will choose one depending on the dive profil. If it is a squared profile like on a wreck where your are at the bottom most of the time, I will choose something like 1.2 to 1.3 as a maximum. For a dive along a reef where with time passing you decrease your depth, I will use 1.4 at the bottom part. Finally, for a 40% mix, I will tend to choose a max PPO2 of 1.3 because a small variation in depth has a big variation in your PPO2.

 

[soltari675]

If I am just doing a couple dives that are not that deep, I use plain air. If I am doing a lot of repetitive dives it depends. If the area uses mostly banked air, I will use the highest percentage I can that they have banked and let's me go to the depth I want. Usually ends up being 32% or 36%. I have been on wrecks where I asked specifically for 28% just because of the depth and I am not certified for any trimixes or anything (yet ) . I have used up to 38% because I was doing so many dives and the local LDS did partial fills, so it worked out. I felt great after too! But if the shop has banked I try not to be difficult. I just use what they have.