28% Nitrox

[Jim Lapenta]

I use 1.3 as a max because not long after going thru my NAUI Tech Intro and Helitrox, we had an incident in the caves in Florida where a diver we all knew in my group at the time toxed at less than 1.4. Jeff was an experienced diver with dives on the Doria, many in Lake Erie and an accomplished cave diver. We do not know enough about deco to say for any certainty that 1.4 is safe. From this thread, The Deco Stop
it appears it may not be. One of the posters (#5) was affiliated with the shop I used to dive with and as a result there was a great deal of discussion on this. It was decided that for us (and I still adhere to that) we would mix and plan for less than 1.4. I chose 1.3 because of a few factors. The dives we were doing at the time were stressful - bottom of Erie is 38- 40 degrees, dark yet with good vis, I was pushing 50 and have now made it, and deco is not an exact science. A few extra minutes on deco is worth the peace of mind and extra safety margin. 1.4 is not a rule or a law. It was and is a recommendation. Some recommendations fit most but not everyone. It is not harder to plan for less, v-planner has no issue with it, and my CNS clock likes it as well.

And to touch on one other item. I don't normally use 32 as the bottom is most times deeper than that and that's what I mix for. Round here partial pressure blending is more the rule than the exception and it's as easy to mix 26 or 28 as it is 32. In fact there is only one place I know of that banks 32 and I wouldn't use their shop anyway. If i did I'd likely only get a 2600-2800psi fill on my LP tanks and have to top off with air anyway to my usual fills of 3000-3200. so I'd end up with a different mix anyway. But again that won't happen cause they also insist on those godawful nitrox bumper stickers being on tanks as far as I know.

 

[Blue Sparkle]

Perhaps the OP did not learn how to figure out EAD. I believe that SSI only teaches fixed mixes (I am not sure if SSI still does this or if they have changed recently).

I just took the SSI EANx class in December, and we did cover EAD and how to figure out dives with various mixes. EAD tables were included as one previous poster showed.

One handy thing I found with the SSI dive tables as opposed to my original PADI ones: Because they do not break things down into quite as many pressure groups, they are much easier to read (due to the size of the numbers; not because of simple vs. complicated).

 

[scubadada]

I use a pO2 of 1.4, seems very conservative and I'm a population guy rather and an individual occurrence, anecdotal experience kind of guy. That's just me, perhaps it's because I'm a scientist and clinician

Good diving, Craig

 

[BoltSnap]

What is the PO2 limit for NOAA now?

 

[James228]

1.6 I think

 

[BoltSnap]

1.6 I think

That is what I know too. NAUI is 1.5 unless they have changed it recently.

The question now why would NOAA accept 1.6 where as some others have it much lower at 1.4 and some are privately going to 1.3?

 

[Rainer]

GUE recommends 1.2 average, 1.4 max. That's what I plan and do. A few extra minutes of deco just aren't a big deal to me. Toxing at depth would sort of suck...

 

[BoltSnap]

But how are these numbers arrived at? What scientific (or otherwise) facts have been used (other than anecdotal ones)? We can just stop using Nitrox altogether and we can argue that we are safer.

 

[Rainer]

You find a number where people haven't died, and you set your standard there. I don't know of any reports of divers toxing at 1.2 (or 1.3), but there have been fatalities at 1.4. Also, a lot of GUE (and other "tech") divers are doing bigger dives (with lots of eventual O2 time), so keeping a lower PP02 on the bottom keeps CNS totals in line for longer deco. No one likes dancing underwater or burned lungs. For really big dives (very deep, long), even lower PPO2s are recommended. GUE is extremely conservative. I'm ok with that.

 

[kanonfodr]

But how are these numbers arrived at? What scientific (or otherwise) facts have been used (other than anecdotal ones)? We can just stop using Nitrox altogether and we can argue that we are safer.

Well, my understanding is a lot of this is a bit of empirical evidence with a "healthy" dose of personal risk assessment and experience thrown in. Decompression and Oxygen Toxicity can be variable even within the same person between days. Theoretically you could go out one day, do a dive perfectly within the limits and be fine. The next day you could get a type II hit or tox doing the same dive within the same parameters.

Occasionally you will hear of people getting "undeserved" hits of DCS, ie being within the tables but still catching a bend. Other people are simply more susceptible to DCS than other, and the same goes for Oxygen Toxicity. The RISK of toxing at 1.4 is statistically lower than some of the older conventions of 1.6, 1.8, or even 2.0. That is all, no guarantees, just probability. If you really want to feel bad, consider that you are conducting an experiment on yourself with every dive . That thought certainly keeps me diving conservatively.

Peace,
Greg