Another (legit) nitrox ?

[Steve SDI/TDI HQ]

I thought the OP was talking about no deco dives. 32% and 1.4 ppo2 is 114 feet. The US Navy air tables give a NDL of 15 minutes at that depth. The NDL from the PADI or NAUI tables may be slightly different, but it is definitely not one hour. It would take a lot more than three no deco dives in 24 hours to reach the limit for o2 exposure with 32% nitrox. It might not even be possible.

The OP was talking about sport dives and so am I... Since you mentioned the US Navy tables, let's use them (and I'm doing this from memory and in imperial units so forgive any slop... I am sure you'll get the picture and understand my concerns).

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To return to the OP's question. Here's an example...A basic nitrox user on a liveboard fills a cylinder with EAN39 or 40 and does a mid-morning dive (11:00) to 80 to 85 feet. The EAD is about 60 feet and I think her oxygen depth would be about 1.4 bar... or close to it. She is using the US Navy tables and they give her a 60 minute bottom time within the NDL. But that's cutting it close to the NDL so she stops and sucks back surface supplied oxygen on the hang bar which sits at 15 feet. She remains there for 8 minutes.

She surfaces, has some food, play deck quoits, watches a moving and does an afternoon dive. Same mix same depth but only stays down 30 minutes.

She goes to the salon, has a chat, reads a magazine, and opts into a twilight dive with some buddies who want her to model for them while they take snaps... same reef and a similar dive to her afternoon dive but on this one, she is in the water for 40 minutes... this puts her close and so she hangs on the bar again for 10 minutes.

She has a drink, eats supper, plays guitar, sings, goes to her cabin and sleeps. So far today she has been in the water for about 130 to 140 minutes with an oxygen partial pressure of about 1.4 bar (and that's NOT taking into account her time on the hangbar breathing oxygen at about 1.45 bar). Her bottom time is close to the 24 hour limit but no cigar.

She wakes at 07:00 and decides to do a pre-breakfast dive... Because some Muppet has told her she cannot exceed NOAA's 24-hour CNS limit doing sport dives, her dive plan does not take into consideration her 24 hour CNS loading. She plans a similar dive to the one she did the previous morning except she plans only a 45 minute bottom time.

Now, I'm OK at maths and not so great with arithmetic but hasn't she just done more than 180 minutes bottom time within a 24 hour period? She did four dives... a couple of them close to the NDL but that's common. Hell, unfortunately that's normal.

My intent here is to answer the OP's question as clearly and responsibly as practical in a public forum. I'm kind of compelled to, since I work for a training agency, training divers and instructors is part of my full-time job, I have to write textbooks about this stuff and I have been teaching nitrox programs since TDI first introduced them in 94. Hell, I'm no expert. But I have done a few dives using nitrox, and think that validates my methods.

You may be comfortable ignoring the NOAA limits on your dives, but the OP did not ask about diving in a cavalier fashion. He asked a reasonable question and I think my answers have been equally reasonable.

To restate what I posted already...

I believe that many, many divers using nitrox on multi-day, multi-dive profiles come close to exceeding or in fact exceed NOAA daily limits for CNS toxicity.

I think there are many situations where the total bottom time over a 24-hour period approaches 180 minutes... the NOAA daily limit for an oxygen partial pressure of 1.5 and 1.4 bar. That's only three hours. Sport dives on live-aboards of one hour plus are the norm in my experience. Doing three in a day is not uncommon. In fact, three dives a day is conservative. Add to this the practice of hanging off the back of the boat sucking surface supplied oxygen for a few minutes after every dive and our punter is getting awfully close.

My opinion is based on observation. I am not pulling this out of a hat. High daily doses that approach NOAA's limits are real not imagined.

As an aside... basic nitrox certified divers are not restricted to 32 or 36 mixes, and in any event, it is the oxygen pressure and time of exposure that should concern us. Oxygen pressure is a function of depth and fraction of oxygen. Citing a mix without associating it to a depth and time renders the example meaningless.

I strongly suggest you do not consider half times when calculating daily limits. I can post an article I wrote covering this but it actually deals with technical not sport exposures. The tenets are the same but we will spend the next 487 posts dealing with bull****... PM me your email address.

 

[Michael Schlink]

Steve, nice post and well done, there are a lot of opinions flying on this question. Problem is there are also too many variables. We should all agree that the mix only matters as to the depth and time it is dived. 32 or 36 can be dived to yield the same OTU's. It's the pressure(which is the depth) we have to watch. Then the amount of time at that depth.
That's why we take a class!
And those that post that diving Nitrox on an air table or computer is safer, does anyone have any facts about that statement. I don't need opinions, I'm looking for some properly documented studies that actually have proven the stated increase of safety.Do these exist??

 

[raymond phule]

Steve, nice post and well done, there are a lot of opinions flying on this question.

And people actually looking at the tables and plan dives in V-planner. That is not only opinions and no one have yet answered their results for ean32 in any real way.

Problem is there are also too many variables. We should all agree that the mix only matters as to the depth and time it is dived. 32 or 36 can be dived to yield the same OTU's. It's the pressure(which is the depth) we have to watch. Then the amount of time at that depth.

Dont everyone agree with that OTU's is not a problem for NDL diving? CNS% is a problem. The "problem" with 32 and 36 and your analysis is that the NDL time is lower compared to 40, this makes it harder to get a high CNS% due to limit in bottom times.

Why is it supposed to be so dangerous to try to find out how and with what gases it might be a problem with CNS % Is it a problem with air? No. Is it a problem with ean40? Yes.

That's why we take a class!

The same answer all the time.

And those that post that diving Nitrox on an air table or computer is safer, does anyone have any facts about that statement. I don't need opinions, I'm looking for some properly documented studies that actually have proven the stated increase of safety.Do these exist??

I have seen this before and I believe the answer is no. But is it safer to make your dives shorter than the NDL limit? Is it safer to make your dives so your M values have a max at 50% compared to 100% I would say that all theory I have read say that it is safer to dive conservatively instead of at the NDL limit.

Diving nitrox on air tables or computers give the same result. Shorter dives, lower M values. All theory about diving say that this is safer. There are of course other parameters that also matters for DCS but the tables, M-values, rgmb etc are still the main theory and diving nitrox on air tables are definitely going to be safer according to that theory.

 

[Michael Schlink]

I'm not sure where the NDL comes in, that's time related only, CNS is pressure. 1.6 is the limit on the pressure so I think it' reasonable to assume that we're diving safe, that is, that we're NOT pushing the O2 pressure. So we come back around to the subject of the OP and the question of OTU's getting too high during a week of 4-5 dives a day for a week. Since I have done this more times than I can count my answer is yes you can hit the limit, but you probably won't. If you grab a tank of air every now (call it an air break) than I think most divers won't hit the OTU limits at all.
It's worth mentioing that a lot of this type or style of diving( liveaboard or resort) is also "multi level" which certainly has some influence on how much OTU loading occurs.
If you use 32 and don't spend a lot of time at 90' you'll probably never hit anything BUT deco limits and here is where Nitrox shines

 

[raymond phule]

I'm not sure where the NDL comes in, that's time related only, CNS is pressure.

I dont understand what you are trying to say. CNS % is the dive time at a given ppO2 divided by the maximum available time at that ppO2. A leaner nitrox mix result in shorter NDL times at pp02=1.4 and it is going to be very difficult or even impossible to reach the daily CNS limit depending on the % O2.

I dont know much about OTU but every thing I have seen about it suggest that you need to do very extreme diving to get a problem with it.

 

[The Chairman]

Raymond, you are mixing OEUs with CNS.

CNS (Central Nervous System) toxicity is a function of depth. It's what the MOD is all about.

OEUs (Oxygen Exposure Units) have more to do with pulmonary issues. While these can effect depth tolerance, there is no quantification of this that I know of.

 

[raymond phule]

I might mixing things but I follow the notation and names on this site DIR-diver.com - Nitrox Class Part 2

Most other things I have read seem to be in agreement with this to. V-planner gives CNS % and OTU and I am pretty sure the definition of those follows the above link.

 

[The Chairman]

So, now we know WHERE your misunderstanding comes from. As best I can tell, he is also mixing the two. OEUs and OTUs are the same thing and are expressed in terms of % of exposure.

Perhaps we can get Dr Deco in here to explain it better.

 

[ekewaka]

To return to the OP's question. Here's an example...A basic nitrox user on a liveboard fills a cylinder with EAN39 or 40 and does a mid-morning dive (11:00) to 80 to 85 feet. The EAD is about 60 feet and I think her oxygen depth would be about 1.4 bar... or close to it. She is using the US Navy tables and they give her a 60 minute bottom time within the NDL. But that's cutting it close to the NDL so she stops and sucks back surface supplied oxygen on the hang bar which sits at 15 feet. She remains there for 8 minutes.

Steve, that was a good post, but you forgot one thing. The OP was asking about using the air NDL for conservatism. That means you don't use EAD for dive planning, just the actual depth. The air NDL at 80 feet is 40 minutes, not 60. I don't have the tables with me right now, so I can't calculate the time of the repetitive dives.

 

[Steve SDI/TDI HQ]

Steve, that was a good post, but you forgot one thing. The OP was asking about using the air NDL for conservatism. That means you don't use EAD for dive planning, just the actual depth. The air NDL at 80 feet is 40 minutes, not 60. I don't have the tables with me right now, so I can't calculate the time of the repetitive dives.

Thank you and although the OP qualified his original message (not definitively, but somewhat) there were several later posts that indicated to me a less than comprehensive understanding of what the NOAA daily limits are all about. Hence taking the time and trouble to try to set some parameters... unfortunately, lost in the noise now.