Nitrox and repetitive dives, what mix first?

[Blackwood]

I can only see O₂ being an ideal mix down to 6mts, so anytime I'd like to go deeper some physiologically inert gas is required.

Right. To be clear, I wasn't advocating diving pure oxygen to 110 feet (or whatever was said). Rather, I was attempting to post a response which made it clear that nitrogen content isn't the only concern in selecting a gas.

My general point was that, often times, practical considerations come into play in addition to gas properties.

[edit]grammar police[/edit]

 

[Kern]

While Mr. Neuman isn't throwing reverse profiles under a bus, it doesn't sound like he plans to dive them either.

Richard

I saw this paper about a decade ago. From memory there are several other rationales & a bit of input from the bubble model exponents like Yount & Wienke (sp) decrying the removal of the reverse profile dive limitation.

So that's some rationales & mathematical models against revers profiles but still no scientific evidence that I'v seen to date that says revers profiles cause DCS. However, I no longer have access to a database with peer reviewed articles, so things may have changed in the more recent past. Does anyone have a source that says reverse profiles can cause DCS.

 

[John_B]

John, while it may be the preferred mix for you're diving philosophy, it is it's not the ideal mix for all depths down to 111' if minimising N₂ absorption is you goal.

I think you are reading this backward. I haven't fit my diving philosophy around a methodology, I've picked a methodology that fits well with my overall philosophy. This has nothing to do with DIR, its about practicality.

111' is as deep as most rec divers go (its at the point when many get pretty narc'ed). 32% is commonly banked -- in part because tech divers use it for blending -- and banked 32% means you don't need O2-cleaned tanks, so the OPs doesn't need to dive one tank with air or wonder which tank to use first. Two dives with 32% will have an NDL as long or maybe longer than one tank of 36% and one tank of air. Since 32% has an EAD credit of 20% (note: that credit is in feet not minutes!) the maths to convert from your air table is dirt simple and memorizing one table for 32% is even easier. From a logistical standpoint, with 32% you can get your fills anytime and just grab them whenever you get a chance to go diving.

Sure, you can add a few percent more O2 for one "best mix" dive but, unless that's a dive you do all the time, you're restricted to what dives you can use that tank on or what depth you can go to. All that for what amounts to possibly only a couple of additional minutes (and maybe even fewer minutes considering that second tank of air). On balance, 32% gives you the most options for the most bottom times for the most dives. MIB dive gear not required.

John_B

 

[Kern]

John, I don't remember mentioning DIR or any other agency. I simply said that you have a philosophy that you dive by, which you have expounded quite nicely above. While I don't subscribe to it, I have no problem with others doing so. I was just entering into the discussion. I too have my own philosophy that I dive by. For instance, I don't believe in NDL/non stop diving as espoused by the agencies & used by most divers. It's based on the work done by an Australian researcher into pearl divers in the 70's, bubble models that grew out of that research & Doppler studies.

I take back what I said about the maths being very straightforward. I forgot that Americans persist with the imperial system of weights & measures & so make things a little more difficult than they are for the rest of us.

 

[Blackwood]

I take back what I said about the maths being very straightforward. I forgot that Americans persist with the imperial system of weights & measures & so make things a little more difficult than they are for the rest of us.

The math is simple regardless of unit system. But not having to do it at all is even easier

 

[rjack321]

Since the benefits of 32% are useful down to 110ft or so... And EAN36 only gives you slightly improved EAD of -25% I don't really see the benefits of trying to match a mix e.g. 28 through 40% to a dive. Below 110ft I am starting to look to trimixes and for long shallow dives I would keep my ppO2s lower on the bottom and bring O2 for any deco obligations (beyond the OPs cert I know).

I prefer zero math UW. Just skipping the 36% fill would have avoided the OP's whole problem in the first place with a trival N2 penalty for shallower diving. Just look at the NDL for EAN36 on a 50-60ft dive, its way beyond reality for a single tank. EAN32 would have been just as good but not hosed him on deeper dives.

That said if I was forced to dive these mixes/tanks/depths, I would have chosen the deeper nitrogen depth first and the shallower second.

 

[black1]

I dont go past 100ft usually and most of the time between 60 and 95ft so i always choose this mix and have no probs and plenty of time underwater.

1.6ata EAN36 (M.O.D.)105ft

The 105 is what i set my dive comp to for safety and i still have a few feet to go to hit maz mod, which i never do.

 

[John_B]

I recently returned from Roatan (easy for me to say, LOL!)

I did 28 dives in 5-1/2 days; all with EAN32 which made the maths dirt simple. I couldn't have done those same profiles on air.

 

[Reg Braithwaite]

I'm late to the party and I haven't the experience to back up my opinion, but FWIW I would dive the richer mix on the deeper dive (up to appropriate PO2 limits et cetera). My reasoning has nothing to do with the accumulation of nitrogen in the tissues during the dive itself, rather it's a question of having a slightly better gas gradient when ascending from depth. I'm going to do more ascending and reducing more ambient pressure on the deeper dive, so adding in an improved gradient is going to give me more than using nitrox on a dive with very little deco.

Assuming I stay within minimum deco limits (and I have no business doing anything else), I am going to be doing stops from 30' on a shallow dive and from 50% of average depth on a deeper dive. Perhaps from 50'. So on the deeper dive I'm going to be spending at least 66% more time decompressing, so I'm going to pull a figure out of my a-- and say I'll get 66% more benefit diving Nitrox on the deeper dive.

Okay, I know this is just conjecture, but this is my thinking.

 

[treywilly]

I had a similar question when diving here in NJ on the Boats. I asked what was the right mix to bring, cause some boats go out and tell you to have the right mix for the dive. I was told to bring 28% for the deep dives you know around the 120+ range and 32 for the 100 and above range. 36% you may want if your downing swallow dives or in a stage bottle on you 15 ft hang.