Is my approach to diving with Nitrox logical?

[L13]

With Nitrox, everything is a compromise, one factor can be made more conservative at the expense of the other:
NDL/DCS risk <---> MOD/Oxygen toxicity risk
risk of mistakes due to non-standard <---> reduced risk due to optimization
etc.

One risk I like to mitigate is to use a Nitrox with a MOD >= the max depth at my dive site and/or only dive where the hard bottom is <= MOD. I often don't follow this rule especially when doing wall dives in the ocean because other risk/NDL decisions over-ride. When using this rule, I will accept a ppO2 of 1.6 at the hard bottom as long as the planed dive has a ppO2 <= 1.4. But, if a range of Nitrox mixes seem safe, I will often bias to this rule or the dive shop standard mix which often fits this rule.

Don't get over focused on one factor, consider all of them. Pick a good compromise, plan the dive based on that, and then dive the plan.

 

[Nitrox_DiverNY]

This has been a wide-ranging but educational discussion.

I'm going to try to hijack the thread back to the original post, in which @Nitrox_DiverNY asked for opinions on his practice of getting the most enriched blend he can for his anticipated dives.

My response to his query would begin by asking the purpose of Nitrox diving for recreational divers; that is, what can Nitrox do and not do for recreational divers?

The takeaways a student should get from a basic Nitrox course are that:

--Nitrox will extend your NDL for a given dive to that of a depth between a quarter and a third less deep.

--Nitrox will shorten your required surface interval between repetitive dives.

--The cost of obtaining these benefits is that Nitrox caps your maximum depth at a level less than the depth limit for recreational diving. Going deeper than the modified depth limit incurs the risk of oxygen toxicity and death.

Let's apply these effects to the dives @Nitrox_DiverNY discussed.

Sometimes he plans dives with a maximum depth of 30-40 feet. If you look at the recreational dive planners, the NDL for a 40-foot dive is over 200 minutes. I don't know anyone who makes an AL80 last for three hours, so the limiting factor of a 40-foot dive is not NDL, which is a shorthand for the DCS risk the OP is trying to minimize. The limiting factor of these dives is the amount of breathing gas.

Because of that huge NDL, the surface interval before a subsequent dive to the same depth of 40 feet is also immaterial to the risk of DCS. The purpose of the surface interval in this circumstance isn't so much to offload nitrogen as to change tanks, re-hydrate, and rest a bit. By the time those tasks are completed, the NDL for the next 40-foot dive will again be longer than almost every diver can make the tank last.

So for single or repetitive dives to 40-feet, there is no benefit to using Nitrox at all, unless you are among the people who want "geezer gas" because they just feel more fresh and alert after breathing enriched air.

It is for dives between about 70 and 100 feet that Nitrox proves its worth to recreational divers. That's the depth range where the factor limiting the length of the dive shifts from available air to NDL.

The NDL for a 60-foot dive is a little less than an hour: a fair percentage of experienced divers can make a tank last that long at that depth. The NDL for a 100-foot dive is about 20 minutes; even more divers can make a tank last that long at that depth. For these divers, Nitrox flips the tables and again makes available gas the limiting factor.

Maximizing the richness of the Nitrox blend to minimize the risk of DCS for divers who obey the tables or their non-gun-decked computers takes a miniscule risk--and leaves it just as small. Therefore, my opinion is that the strategy does not improve the diver's safety and may distract him from obtaining the primary practical benefit that Nitrox affords the recreational diver.

thanks for the very detailed response! it was very helpful

 

[Nitrox_DiverNY]

I apologize for only reading the first 5 or 6 pages, and I suspect that a few people have already basically said what I will. It's a great intellectual exercise to think about your blend, and O2 toxicity, and NDL risk, but what you are talking about has essentially zero practical application in a recreational dive to 30-40 ft. Honestly if you don't feel safe using air on a dive like that, you probably shouldn't be diving, or at least you are not seeing the larger picture of dive safety. There are all sorts of factors in DCS incidence, and your nitrox blend is only a tiny (and unproven) part of the risk equation. If you are initiating this conversation as a way of learning more, and not so much out of actual concern for your dive safety on a 40 ft recreational dive, then great. Learn everything you can about dive table and computer modeling, compartment theory, clinical cases of DCS and corrollated factors (dehydration, obesity, dive profile control, ascent rates, strenuous exercise immediately after diving, etc....) and then you will become a more informed student of dive safety in general. But if you believe that you will in any real-world way increase your safety on a single tank 40 ft dive by using a custom blend of nitrox, you are really barking up the wrong tree.

If you're talking about multiple daily dives, like for example on a trip to Bonaire where you can basically dive all day, tank after tank, at 30-40 ft, then using nitrox definitely has some real-world effect on your dive physiology, but at that point it becomes a balancing act between N2 and O2 exposure, so using a blend richer than the typical 32% may not have benefits.

As someone mentioned, we use 32% in cave dives in Mexico, which is often 4-5 hours per day of time spent at shallow depths similar to what you mentioned. Very very few people (nobody I know) ever uses custom blends for those dives. For planned deco dives, of course, but that's a different world.

There are just so many other FAR more important factors in avoiding DCS than your nitrox blend.

ver

I apologize for only reading the first 5 or 6 pages, and I suspect that a few people have already basically said what I will. It's a great intellectual exercise to think about your blend, and O2 toxicity, and NDL risk, but what you are talking about has essentially zero practical application in a recreational dive to 30-40 ft. Honestly if you don't feel safe using air on a dive like that, you probably shouldn't be diving, or at least you are not seeing the larger picture of dive safety. There are all sorts of factors in DCS incidence, and your nitrox blend is only a tiny (and unproven) part of the risk equation. If you are initiating this conversation as a way of learning more, and not so much out of actual concern for your dive safety on a 40 ft recreational dive, then great. Learn everything you can about dive table and computer modeling, compartment theory, clinical cases of DCS and corrollated factors (dehydration, obesity, dive profile control, ascent rates, strenuous exercise immediately after diving, etc....) and then you will become a more informed student of dive safety in general. But if you believe that you will in any real-world way increase your safety on a single tank 40 ft dive by using a custom blend of nitrox, you are really barking up the wrong tree.

If you're talking about multiple daily dives, like for example on a trip to Bonaire where you can basically dive all day, tank after tank, at 30-40 ft, then using nitrox definitely has some real-world effect on your dive physiology, but at that point it becomes a balancing act between N2 and O2 exposure, so using a blend richer than the typical 32% may not have benefits.

As someone mentioned, we use 32% in cave dives in Mexico, which is often 4-5 hours per day of time spent at shallow depths similar to what you mentioned. Very very few people (nobody I know) ever uses custom blends for those dives. For planned deco dives, of course, but that's a different world.

There are just so many other FAR more important factors in avoiding DCS than your nitrox blend.

thanks for the response, it was very helpful!

 

[JimO]

Hello all I am looking for some feedback on my personal diving habits with Nitrox, and to see if my view point on it is incorrect?

Apart from extended NDLs I personally use Nitrox for its potential to reduce nitrogen loading thus contributing to DSC. I am certified up to the standard 40%, and I use as rich as a mix as I can regarding the MOD. For example I am doing dive with a max depth of 30-40ft, and had a cylinder filled as close to 40% as possible it came out to 39.8%. the mod of 40% at a pp02 of 1.4 is 82 ft. My point of view is get the mix as rich as possible as long as its not pushing the MOD at 1.4pp02, and spend the extra money for the extra safety margin, but this practice of using such rich fills seems few and far between. looking at the log book after my fill, it was mostly all 32% with one 36%. It seems like most people just go with the standard 32% even if (in my opinion) their mix cold be richer because they are nowhere near their mod.

The MOD of 40% at 1.4pp02 is around 82 ft, wouldn't using a 40% mix under that depth be a more conservative approach? so if your diving to a Max of 60 why not use 40%? I know DCS in rec is rare, but doesn't it make sense to use rich mixes when the dive allows to add in possible safety? why not be as safe as possible?


Does using a mix like 39-40% pose a substantially bigger risk to Low pressure oxygen toxicity (pulmonary oxygen toxicity), the the standard 32% even if the MOD is not pushed? If someone would be able to enlighten me more about this condition and how common it is with rich nitrox, that would be helpful.

when diving How close to 1.4pp02 will you allow yourself to get? I have never pushed it personally, but how close to 1.4pp02 do you find yourself on dives? do you go right up to it?

Is my logic on using rich mixes valid? Does anyone else share the same view point as me and also go as rich as possible? am I missing something? let me know in the comments, thanks !!

Not sure how old you are but I'm coming up on 65 and after 45 years of diving I consider nitrogen the enemy and nitrox my friend. Solo deep dives in the PNW, sidemount, camera and redundancy. For me, I always set my computers for the oxygen concentration of my breathing gas - and switch the value to deco gas when used. I would rather have accurate data on residual N2 and oxygen exposure than a safety margin. Leaving the computer on air is fine for shallow dives but why not set up habits for times you need to track your N2 like boat dives or deeper dives? I'm more reassured looking at my computer and seeing where I'm at than seeing data from a 21% dive. But I'm also very conservative. 3-5 minute safety stop then at least 2-3 minutes working to the surface to maximize the N2 gradient. Keep my reg in to suck up that 32% until gear squared away, fins off and desnotification (in case the ladies are around). Shedding N2 at the 1 atm with lungs exchanging @ insp Fi N2 of 0.68 makes me feel like I'm getting my moneys worth out of my fill.