Dive safe! A short story from a chamber operator

[ReadyDiverOne]

Can someone--moderator, Dr. Deco, medical person--pls clarify this important point!! It seems like there are two views:

View A--hard if not impossible to get oxygen tox from recreational depths (assuming no MOD violations), so ok to set computer to Air settings while diving EAN so long as know and follow MOD limits

View B--absolutely possible to get toxicity, so dive EAN on EAN settings but use Air tables​

Which is correct? I have heard from other very experienced divers/shop owners that View B is correct and that it is dangerous to dive EAN with computer on Air settings.

Thanks.

 

[ljwillia]

I know people who actually dive Nitrox without setting their Nitrox enabled computer for Nitrox. Multiple day trips with 4-5 dives a day for at least 3-4 days. They admitted they don’t keep track of their O2 exposure, when I specifically asked them. That’s why I posted this question.

I had skin bends several years ago and was advised to dive nitrox on air tables. My computer is a ProPlus 2. When selecting a nitrox mix of 21% (air) it ignores O2 and assumes your breathing air. I am uncomfortable not being able to track O2 but I'm still diving far more conservatively. So far, this has worked like a charm. No further occurances.

 

[Divetech Cayman]

Just to provide some context on the CNS clock discussion.... I went back to my last Lionfish cull tournament, where I did 10 rebreather dives over 2 days and looked at my Shearwater cloud download. Here are the dives, durations, and CNS clock at the end of each dive.

Day 1
Dive 1 - In at 6:47, 1:05 dive, 28% CNS
Dive 2 - In at 9:17, 51 min dive, 33% CNS
Dive 3 - In at 11:36, 52 min dive, 38% CNS
Dive 4 - In at 13:33, 54 min dive, 47% CNS
Dive 5 - In at 15:19, 48 min dive, 57% CNS

Day 2
Dive 1 - In at 6:57, 39 min dive, 9% CNS
Dive 2 - In at 9:08, 57 min dive, 19% CNS
Dive 3 - In at 11:09, 1:02 dive, 38% CNS
Dive 4 - In at 13:24, 1:01 dive, 52% CNS
Dive 5 - In at 15:25, 37 min dive, 55% CNS

All of these dives were using a rebreather at a PO2 of 1.2, except for day 2, dive 1 & 2, I dropped to 1.0.

Tony

 

[Marie13]

I had skin bends several years ago and was advised to dive nitrox on air tables. My computer is a ProPlus 2. When selecting a nitrox mix of 21% (air) it ignores O2 and assumes your breathing air. I am uncomfortable not being able to track O2 but I'm still diving far more conservatively. So far, this has worked like a charm. No further occurances.

Why can’t you do the manual calculations to track O2 exposure?

 

[ReadyDiverOne]

@Divetech Cayman : Tony, thanks very much. Are the CNS numbers you had related at all to your PO2 settings? On my Suunto D41, the default is 1.4. Thanks.

 

[ReadyDiverOne]

I had skin bends several years ago and was advised to dive nitrox on air tables. My computer is a ProPlus 2. When selecting a nitrox mix of 21% (air) it ignores O2 and assumes your breathing air. I am uncomfortable not being able to track O2 but I'm still diving far more conservatively. So far, this has worked like a charm. No further occurances.

So I'm confused, do you dive with your computer in Air or Nitrox mode? If Nitrox mode, do you calculate NDL using Air tables? Thanks.

 

[caydiver]

During the past few weeks some of our patients have been doing 4/5 dives a day on air in some cases doing free diving as well. They were all of the mindset that they have been doing it for years when they go on a dive holiday. I know what I won’t be doing. I would rather live to dive another day than cram it all into one week. Don’t know that it is a factor but the unscientific observation is leaning that way.

 

[rsingler]

Can someone--moderator, Dr. Deco, medical person--pls clarify this important point!! It seems like there are two views:

View A--hard if not impossible to get oxygen tox from recreational depths (assuming no MOD violations), so ok to set computer to Air settings while diving EAN so long as know and follow MOD limits

View B--absolutely possible to get toxicity, so dive EAN on EAN settings but use Air tables​

Which is correct? I have heard from other very experienced divers/shop owners that View B is correct and that it is dangerous to dive EAN with computer on Air settings.

Thanks.

@ReadyDiverOne , the answer is probably A, but not necessarily. And I wouldn't choose A even if it's technically correct. It's complicated, and I'll explain why. But before I do, the easiest solution is to set your computer for your mix, and then adjust your diving using your brain, rather than the toy. In other words, "sort of View B", but dive your computer set to your mix and trim your NDL rather than use Air Tables. The computer is just an estimate anyway, and acute cumulative ox tox is such a rare event (as opposed to a seizure from violating your MOD) that the best way to proceed is to collect the data using your best tools, and adjust your dive. For example, take a 4 hour break if you hit 50% O2, or always ascend to a shallower depth when your NDL reaches "X" minutes instead of 1 minute. Don't dive deep four times a day. Etc., etc.

But the reason the answer to your question is "maybe this, maybe that" but not Air Tables, lies here:
Here's a dive profile for the Hilma Hooker in Bonaire from a few years back. 96 feet on EAN32 for 45 minutes. Wait! What? The PADI EAN32 tables limit you to 30 minutes at 100 feet. And Air Tables only allow you 20 minutes at 100 feet (rounding up, as we were taught).

Well, computers were an improvement over tables because we don't dive square profiles. If you dive using Air Tables for your NDL, you will really shorten your dive. It's way more conservative than necessary.
As you can see in this dive, the average depth was only 53.6 feet. That's why the computer gave us such a long NDL. And after 45 minutes, the ending O2% was only 9%, for the same reason. The actual integral sum of ppO2 was lower than the tables would suggest from the maximum depth. @Marie13 that's why using the O2 tables will really crimp your style - they assume a square profile dive. Using tables to compute O2 toxicity will give you scary numbers that just aren't true, because your average depth was (probably) much less than your maximum depth. Why not just set your gas mix and come out with better numbers to ponder?

So probably, for the average recreational diver even doing multiple dives over multiple days on EANx, you don't have to worry about your O2%. Probably. It's been commented on above with numbers from a rebreather and predictions from Multi-Deco. Getting to 100% on Nitrox is hard to do! So setting your computer on Air to make it "more conservative" makes some sense. That way (the thinking goes), "I can dive to the [shorter Air NDL] limits and still have a margin of safety". BUT...

If you ask me (diving doc, former USAF Hyperbaric doc, scuba instructor, old guy), the method of setting your computer on air to give you a safety margin (by shortening NDL) increases your risk. Why? Because set on Air, your computer won't beep for an MOD violation. And following a pretty fish down to a ppO2 of 2.0 because you forgot you were on EAN40 may have immediate and fatal consequences. If you want a margin of safety, make your own rules and stick to them. Hey, even after 30 years of diving, it could still happen to me. How do I know? Because I was on EAN32 in Grand Cayman just a few months ago diving Magic Mountain. This huge rock mushroom comes out of the blue topping at maybe 60 feet, but has beautiful vertical walls going straight down to well past 250 feet. It's awesome!

So I'm head down, mesmerized by the walls, just following it down. The DM says the limit for the dive was 100 feet. Blah, blah, blah, I'm just looking around. Visibility is 120 feet! Except, per habit, I glance down at my Perdix again (which doesn't beep, but WAS set for the correct gas), and the MOD flag has turned yellow! I'm at 110' and heading downward, and that's a ppO2 of 1.39. Oops! It can happen to anybody. If my computer had been set to Air, there would have been no reminder of an impending MOD violation. I'd completely forgotten about it amidst the beauty.

Can you accumulate more than 100% O2 (or 100% CNS, or whatever your computer signals) using repetitive dives, EAN, deep depths and long dive times? It's possible, but still hard to do. As I've just suggested, the real problem is MOD. Set your computer for your mix. Period.

NOTE: Since this is the Basic Forum, I'm presuming we have some Open Water divers not yet Nitrox certified lurking in this thread. The short version of this issue revolves around the benefits versus the risks of Nitrox.
Nitrox lets you dive LONGER (with a big tank) because you accumulate less nitrogen at depth.
Nitrox does NOT let you dive deeper, because the increasing number of molecules per unit volume under pressure at depth creates a situation in which oxygen becomes toxic to the brain. The Nitrox depth limit for recreational scuba (which has a huge built-in pad for safety, naturally) is called the MOD, or Maximum Operating Depth, and varies inversely with how much oxygen is in your mixture: 32%, 36% or custom mixes from 22% to 40% for recreational diving.
Nitrox is a great way to give you longer dives, a little extra safety, or a little of both, at the cost of strict attention to MOD.
Go get your EAN card! Some home study and a one or two night class is common nowadays, and you don't even have to do a class dive with it! It's a great addition to your diving skill set.

 

[Graeme Fraser]

Thanks Marie13 and yle. I was actually just thinking about this issue last night, before seeing this discussion, as I'm booked on a LOB to Turks and Caicos this coming winter. I'll be diving Nitrox; I got certified on it last winter but haven't used it since, as my local dives tend to be limited by my air consumption and cold tolerance (and more recently my buddies' air consumption, as mine has improved but now I've gotten my friends who got certified with me but hadn't dived since to come out diving with me.) On this trip, though, there are 5 dives per day available. The water should be close to 80F and I'm leaning toward bringing my 7mm just because I have it and have my weight dialed in, so the cold won't stop me. I rented a 100cf tank, so my (now pretty decent, likely to improve further by then) SAC rate won't stop me. And it's wall diving, so I can easily go right up to (and right past if I'm not careful) the MOD. I was looking over the chart for exposure over a 24-hour period, and while I don't have it in front of me, I think I remember it being 150 minutes at 1.4 partial pressure over 24 hours. With 5 dives in one day, that's only half an hour per dive at depth--easy enough to do. Please correct me if I'm misremembering the numbers or getting the math wrong, but I'm taking from this that 1) I should set my computer accurately, and 2) I should definitely not do all 5 available dives per day anywhere near the MOD.

Also, I have definitely heard people advocating for diving Nitrox on air settings for safety without any mention of keeping track of O2, including one seminar presenter at the Long Beach Scuba Show this year. I'll see if I can remember who it was, but if anyone else was there, I was the blonde in the back who asked about doing that vs. choosing a more conservative setting on your computer. His response was along the lines of "that works too, but this is easier because you don't have to futz around with the settings," which is very much NOT in keeping with what Divetech Cayman is advocating (using Nitrox setting but planning dive to stay within air table limits.) Much of his talk was also geared toward older divers and how they're more in need of this safety margin--but aren't older divers the majority of LOB customers?

Hi EM

A few observations:

Personally I think 5 dives a day on a multi-day LOB is a lot. I'd probably be limiting myself to 4 and sometimes even 3.

180 minutes is maximum exposure for a 24hr period at PPo2 1.4. 150 mins is the maximum single exposure. That said, NOAA guidelines recommend not exceeding 80% of these values. Remember, these limits assume you are staying at a given PPo2 for the duration. On a typical multi-level dive you may go to MOD for 10mins, then gradually ascend throughout the dive, during which time your PPo2 is dropping. For example, at 21m on 32% the PPo2 is 1.0 which has a daily exposure of 300 minutes.

If it's a recreational charter I would think that the LOB's daily itinerary is a mixture of dive profiles, some of which will be quite shallow. Very unlikely all dives will push to anywhere near MOD for your gas.

To summarise my ramblings, yes you should track and stay within your CNS%, but it is unlikely to become an issue within recreational diving limits.

 

[BlueTrin]

Only way to answer that question is to track it, right?

It's interesting how some "experienced" people are responding to the O2 exposure issue with "oh it's not an issue with the kind of diving being described here." And you know that... how? Again, only way to know definitively whether or not it's an issue is to actually measure it.

We do new divers a grave disservice when we give them the impression that "when you become experienced, there are some things you just 'know' and don't have to keep track of any more." Much better to ensure that new divers understand how to use their computers properly, that they can see how their computer tracks their O2 exposure, and they can then verify precisely whether or not that O2 exposure is an issue for a particular day of diving. Unfortunately there are plenty of "nitrox certified" divers who have no clue about O2 exposure... likely because they were taught by "experienced" instructors who decided that their students would never do the kind of diving that would require them to keep track of O2 exposure.

My experience has been that four dives in one day on nitrox does have the potential to approach O2 exposure limits. Your experience might be different.

I was about to get my tables to do it but I was commuting when posting this and couldn’t download them easily. This is why I asked to see if anyone worked out a simple example.

Rest assured that I would check for o2 exposure if I had any concerns for myself and do any kind of different diving, this includes more dives or longer dives than usual.

I am usually fairly conservative in all aspects in life. My question was a real question, not a passive aggressive affirmation that it wouldn’t be an issue