Air strategy for 4-5 tank dive days?

[ChuckP]

I would like to hear SB experts opinion on using Air tables when diving nitrox. In my class i was specifically told to use Nitrox tables and make sure that i do not exceed MOD. I am currently reading a very good book "The last dive" by Bernie Chowdhury. There it was a very good story about guy who ignored Nitrox tables (he said that they are nonsense) and he died. I would recommend "experts" to read this book.

@ChuckP , yes I know aboput ppo 1.6 but i have a question whether you know what it is oxygen toxicity

This is my understanding: The max recommended ppoO used to be 1.6 for working portions of a dive, at some point in time, it was switched to 1.4 for added conservatism as a small percentage of divers still toxed at 1.6 - working portions of dives. When nitrox blends were being tested, there was a lot of doubt and push back to using these new gasses. Today, varying Nitrox blends is extremely common and well accepted as an added margin of safety and/or also allowing longer bottom times.

Breathing a Nitrox gas decreases the amount of nitrogen breathed but can also increases the gradient pressure, allowing for less decompression time. What you'll learn in your advanced nitrox class is that breathing 50, 80 or 100% O2 increases the gradient pressure and is used at deco stops - at a ppo of 1.6 - during a non working portion of a dive - yes I know what toxicity is.

It is taught, in SSI basic nitrox, that you can dive nitrox with a computer set to air or using air tables as an added safety margin but you must still monitor your O2 units and maintain your MOD - see attached page print from the SSI class showing this.

No matter what - a diver will go where a diver wants to - some people set there computers for no audible alarms, some people just ignore them, some people just flat out don't understand/care. Just because you set the gas as 32% and the MOD at 111', you can still go to 125' - alarms may sound, lights may turn red but that computer will not drag you back up to 111' - and believe it or not, most will live to dive another day.

I've seen dive masters leave their computers set pretty conservative and still set to air while breathing 36% - their reason was that they felt they could monitor their divers better that way by keeping them away from the NDL - it makes sense. Many vacation divers using rented computers can tell you what the air pressure is but may not be clear on the NDL - I don't personally do this but his reasoning makes sense, maybe he's had an issue in the past.

Oxygen Toxicity, just like DCI, is a gray science - no two divers react the same. As long as you stay above the MOD, DCI is the monster you should educate yourself about. There's a guy sitting in the chamber in Cozumel right this moment, 65' dive no nothing profile from what I hear, healthy active man with possible kidney failure.

 

[Efka76]

Yes, DCI is a monster which even nowadays is not 100% understood. Actually I am planning to take TDI Advanced nitro classes in short term future and yes, I know about ppo 1.6 on decompression. Also, when I read through PADI EAN book once again I found a small chapter which says that in case your computer is Air only and you diving on Nitrox you can do that but need observe MOD. I think that this book was released when not all dive computers had nitrox dive mode. Nowadays almost all or all modern computers have this nitrox mode. Still, I am keeping my opinion that it is better not to mess with computer and set it on correct mode. Also, computer is only a guide and unfortunately does not give 100% that you will not get DCI even if you follow all current rules.

 

[boulderjohn]

@boulderjohn , it would be very interesting to hear your opinion about actually diving on nitrox but having diving computer settings as "air". In my subjective opinion this is absolute nonsense and significantly increases a risk that diver might forget that he is diving on Nitrox and accidentally violate MOD and have serious issues (even drowning). However, the opinions above state that their instructors told them about such "tricks" and they are acceptable What is your view about such instructors teaching? Do they violate PADI standards?

I just reviewed PADI Ean materials and did not find anything that could support opinions above. It seems that some people are considered themselves "smarter" than others, PADI, TDI or other organisations, and like to add additional complications in places where fatal accidents can happen.

@ChuckP, according to PADI, when in recreational diving with Nitrox MAXIMUM ppo is 1.4. Contingency ppo is 1.6, however this is for emergency.

I have written about this extensively in the past. This post may have more information than you want, so I will put it in bite-sized chunks.

1. Diving EANx with the computer set to air is extremely common and often recommended. I don't do it, and I don't recommend it. (Reason to follow.)
2. The concern about safety because of MOD is not a big deal. It is not a major effort to remember your MOD, and if you accidentally stray below it a little for awhile, nothing bad is going to happen. There is significant room for error in that MOD, and it takes time for it to happen, more than you would probably expect.
3. People looking for that extra margin of safety by diving on air settings are indeed getting an added safety factor in terms of decompression stress, but it is a tiny, tiny advantage for a hefty price. The percentage of recreational dives resulting in DCS is something like 0.002%. Is adding another thousandth of a percent to you odds worth the price? I don't think so. That is why the nitrox courses I have taught discount that advantage in the written curriculum.
4. I dive nitrox for its intended benefit of longer bottom times. That is what I am paying for, and that is what I want.
5. The common belief that you can either have greater safety by diving nitrox in air mode or having greater bottom times, but you can't have both, is hogwash. No matter how many times Alex Brylske writes that in Dive Training magazine, it is still hogwash. That argument assumes that you will go to the NDLs on the nitrox dives, which is rarely true. I have written many posts in the past in which I showed that a diver using nitrox mixes on a typical 2-tank dive will get significantly longer bottom times than the person diving air (or nitrox in air mode) and still end up much farther from the total NDLs for the last dive than a person diving air.

 

[Manatee Diver]

I don't know why PADI and other organizations don't include it in their training materials, but the NOAA CNS oxygen exposure limits chart is available in their diving manual, along with some people publishing it online.

 

[Diver0001]

It seems that we live in different worlds. I still think that IT IS A BIG MISTAKE TO SET DIVING COMPUTER ON AIR WHEN DIVING NITROX. In this case diver adds unnecessary additionally risks and task loading. By reading your comments I still have opinion that you need to read or re-read materials on Nitrox diving as you still do not have understanding of risks involved. Also, I do not know and do not care how many dives you have under your belt but I would never dive with such buddy as you.

I guess it depends on how accustomed you are to diving Nitrox. You're talking about a hazard of the diver "forgetting" that he was diving on Nitrox and/or not having the wherewithal to follow a plan.

None of the people I dive with have things like Mod alarms turned on, exactly for the reason you mention, namely that you don't want to be relying on the computer to wake you up when it's time to pay attention. You want to have your brain engaged from the start.

As for whether or not I would do this today, no I wouldn't. However, that's not because I think it has somehow become risky to do so, it's because we have much better computers now that allow you to manage your tissue loading during ascent with much more precision than the first generation of computers allowed.

As for needing to re-read the materials because we don't understand the risks, all I can say is that we're talking about divers here with decades of diving experience and many hundreds if not thousands of Nitrox dives, both in recreational and in technical contexts. So, even if you have your reservations about the usefulness of this idea, which is, of course, your right, your assumption that the divers don't know what they are doing is not accurate.

Clearly if you don't think it's a good idea then obviously nobody is going to force this down your throat. It was not an uncommon practice in the past but may, indeed, have outlived its usefulness. I mentioned it because it's truly something that some divers used, not because it's a best practice in 2018.

R..

 

[The Chairman]

In no way, did I say it was acceptable.

Why wouldn't you? It's completely acceptable for anyone to do this. It used to be if I were doing one or two dives over the course of a day, I simply did not set my PDC to the NitrOx I was diving. Two dive sites immediately come to mind: Blue Heron Bridge and Venice Beach Florida. There is absolutely no benefit in setting this, so I don't. If I'm making multiple dives on multiple days, setting it becomes a bit more important as I don't want to run into the NDL. Diving is all about limits. Time/depth/gas are the big three, but there are times where some of those become moot. In that vein, I find that there are more important "limits" that will add more to your safety than diving NitrOx as air. It's my opinion that decreasing your ascent rate, lengthening your safety stop and avoiding yo-yo dives make a bigger impact.

It seems that some people are considered themselves "smarter" than others, PADI, TDI or other organisations, and like to add additional complications in places where fatal accidents can happen.

Really? Many of us have been diving NitrOx twenty years or more and that's how almost every agency taught it. Here's a surprise: the emphasis in these learning modules have changed over the years. You have to know the beginnings of "voodoo gas" to fully understand why this was taught early on. NitrOx was pretty controversial at one time. Many agencies thought it was evil and would kill off many divers and perhaps the entire sport. It was so bad that a few people and even agencies were banned from attending DEMA because they embraced NitrOx. Consequently, the early manuals were written in a way to salve the egos and fears of the anti-NitrOxers. Those of us who jumped on the bandwagon remember those early lessons well. No, it's not that this has become "unacceptable", only very rarely done. Just like tables are very rarely used. Diving NitrOx as air is just as acceptable as making a dive using tables.

 

[Colliam7]

I still think that IT IS A BIG MISTAKE TO SET DIVING COMPUTER ON AIR WHEN DIVING NITROX.

Good for you. IT IS! There is no good reason to do it, even though more than a few divers seem to believe (and boast) that they do so, apparently believing they are doing themselves a favor by somehow being 'more conservative'.

In fairness, it was perhaps more common in past years than it is today (just as cigarette smoking was more common, and equally unwise). But, that was then. This is now. And, there is no good reason to do this NOW.

 

[Strategist.]

Why wouldn't you? It's completely acceptable for anyone to do this. It used to be if I were doing one or two dives over the course of a day, I simply did not set my PDC to the NitrOx I was diving. Two dive sites immediately come to mind: Blue Heron Bridge and Venice Beach Florida. There is absolutely no benefit in setting this, so I don't. If I'm making multiple dives on multiple days, setting it becomes a bit more important as I don't want to run into the NDL. Diving is all about limits. Time/depth/gas are the big three, but there are times where some of those become moot. In that vein, I find that there are more important "limits" that will add more to your safety than diving NitrOx as air. It's my opinion that decreasing your ascent rate, lengthening your safety stop and avoiding yo-yo dives make a bigger impact.

I prefer to let divers in the forum read and be able to discern the pros and cons of adopting certain approaches to diving, hence the aversion make a clear stance. My initial thought was to provide a balanced perspective as opposed to people making blanket statements and using threats to ward people off. My position is clear, so long as a diver is able to recognise the limitations of diving EAN with the PDC on AIR, I think it's fine to do so.

In my previous post, I would like to clarify what I meant. I wanted to provide an example of how it was possible to increase NDL conservatism. My statement saying, "In no way, did I say it was acceptable." was to clarify someone's allegation. Again, I went on to clarify that it is up to the individual diver to discern for themselves, if it was acceptable or not. I apologise for any misunderstanding.

 

[The Chairman]

My statement saying, "In no way, did I say it was acceptable."

Is it acceptable or not?

If yes, then why rail against someone suggesting that's your opinion? It's much ado about nothing.

If no, then reread my post. Of course it's acceptable. It might not be preferable to you or most, but that's not the issue. It's not dangerous. It doesn't increase any risks. Just because you don't dive that way doesn't make it "unacceptable". Until I switched to diving Shearwater all the time, it used to happen often.

 

[kelemvor]

I have.

It was many years ago, perhaps 15. I was diving with Dive Paradise. I wanted to dive with EANx 32 on the first dive and air on the second. I got tanks for that plan. Once on the boat, the DM would not allow it. Having never met me, he declared to everyone on the boat that I must be so totally incompetent that I could not be trusted to stay above MOD on the first dive. (Okay, his wording was a little different.) I would therefore not be allowed to use nitrox on the first dive, and I would only (obviously grudgingly) be allowed to use it on the second. You never know what incompetent divers like me are going to do.

Wasn't nitrox also relatively poorly accepted 15 years ago? I dove nitrox with dive paradise on two separate trips and had no problems either trip...well other than one time when they ordered two few bottles of nitrox one day.