Dive safe! A short story from a chamber operator

[ReadyDiverOne]

@rsingler: Many thanks awesome post super helpful that will be the plan going forward. Thanks again.

 

[rsingler]

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.

There's absolutely nothing wrong with doing 4, 5 or even six dives a day for ten days straight!
But here's the catch: you need to use what you've learned in class. I don't mean this condescendingly. Since this is the Basic Forum, I'm really writing for others that are reading.
Once you've acquired a nitrogen load from that 100' first dive of the day, don't keep diving 100' if you have another week of diving ahead of you. Don't agree to doing a deep dive after a bunch of earlier dives without a nice long surface interval. Why? The N2 is still packed away in your "slower tissues" from your first dive, and will be for hours, if not days.
- If you're diving Nitrox for safety (e.g., less nitrogen accumulation over time), then don't dive to the NDL limit on your computer every dive. If you do, you're no different than when diving air, from a safety standpoint, even if you did get longer dives.
- If you're diving Nitrox, watch your MOD.
- For your fourth, fifth and sixth dives of the day, how about being conservative with your depth this time?
- Use sensible Surface Intervals. 30 minutes might work between dives 1 and 2, but take a lunch break. Take a dinner break.
- Enjoy the shallows! There's plenty there.
- Leave plenty of time before you climb in an airplane with an even lower ambient pressure (and thus an even larger pressure gradient between your slow tissues and ambient air).
- Perhaps most important after many repetitive dives with a lot of gas loading, when you finish your (5 minute?) safety stop, don't pop to the surface as though all your offgassing is done. Take that last fifteen feet nice and slow - it's when your offgassing gradient increases the fastest. Then sit down and relax for a bit, and DON'T jump right in the hot tub! That's known to induce skin bubbles. Wait a half hour while you have a drink, instead.

Just back from Grand Cayman, with 25 dives in a week. One dive the first afternoon to check equipment. Five to six dives a day/night for four days. A medium shallow "good bye" dive the morning of the second to last day. Sightseeing only, on the last day before boarding the plane. Always Nitrox. Well-maintained equipment. I've read and understood my computer's manual.

You can dive as much as you want to, if you use your head. Been doing that for 30+ years, and I've never been bent. Artificial dive number limits don't make you safer. Smart limits make you safer. Go have fun! There's nothing wrong with trying to make your once-in-a-blue-moon $$$ trip worth every penny with as many dives as you can fit in. They just need to be smart dives.

 

[Kimela]

So the idea is to be calm and zen when at depth. Don't work hard. On your safety stop, do some light exercise. As in, swim around a bit. Don't do a cross-fit routine.

I get to the gym 3 or 4 times a week for a 45 minute date with the elliptical - I won't be tempted to do a cross-fit routine EVER! But this does make sense. I'll use my safety stop to get a little light exercise in. Thanks for the info!

 

[Ayisha]

I set my computer to the actual gas that I'm using (usually around 32%) with a PPO2 of 1.4, and I don't dive right to the MOD or NDL. I try to keep my depth at least 10 feet above the MOD and begin my ascent when there is at least 5 minutes remaining on the NDL. In other words, not "riding the computer". I find that this is the best of both worlds.

 

[ChuckP]

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

Numbers produced from a re breather where your PO2 stays at the set point your whole dive - much more O2 exposure verses diving open circuit.

I know this is the basic forum but everyone starts somewhere and if you're interested in "things" that affect you while diving, there's a few great people that have given presentations, done studies etc where it concerns diving.

Some questions / comments were asked about DCS/DCI - this video is a hour long but is well presented in that you don't have to have a PHD to understand it - Dr Neal Pollock talking about decompression stress. I feel it's really well worth the hour.


Dive safe and enjoy it!

 

[Superlyte27]

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.

It's virtually impossible to run out of oxygen clock diving recreationally. This had been addressed a dozen times on this board.

 

[Marie13]

It's virtually impossible to run out of oxygen clock diving recreationally. This had been addressed a dozen times on this board.

Thank you for the reminder that “stupid” questions are not tolerated. I’ll try to remember that. After all, I’m just an idiot cold water diver, not one of the enlightened warm water gods.

 

[Superlyte27]

No one stated you were stupid or an idiot or that questions would t be tolerated. Don't over react for nothing.

Just do the math. It's really really simple. I've yet to see a real life scenario where a recreational diver approached 100%. Then we can talk about how we regularly blow past 100% as technical divers by double, triple, even 10x without consequence.

 

[BlueTrin]

No one stated you were stupid or an idiot or that questions would t be tolerated. Don't over react for nothing.

Just do the math. It's really really simple. I've yet to see a real life scenario where a recreational diver approached 100%. Then we can talk about how we regularly blow past 100% as technical divers by double, triple, even 10x without consequence.

Can you clarify the second one ?
Why don’t you have any consequences ?

 

[chillyinCanada]

@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).
View attachment 529514
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!
View attachment 529527
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.

Do you dive 1.2?