Nitrox for shallow water artifact diving??

[kelemvor]

How do you think nitrox dies this?

Magic

So how does nitrox extend NDLs? We'd all like to know.

I thought you were joking at first. It's a simple matter of reducing your nitrogen uptake by replacing some of the nitrogen with oxygen.

I couldn't say if it's the reduction in nitrogen or increase in oxygen that reduces fatigue. It would be interesting although expensive to do an experiment with 40% nitrox and some 21/19 trimix to discern the answer. I guess it will come down whether the fatigue is being caused by subclinical dcs/nitrogen loading or something else.

 

[Wibble]

Nobody’s mentioned PFO as a possible cause yet…

 

[Divin'Papaw]

I edited my post while you were posting evidently so did they report to DAN?

Good question. This is the instructor who did my open water back in 2005. He frequently ran a large number of students through classes on his own. Breaking standards by the way and was eventually kicked out of PADI. But I digress. He ended up bent after a day of teaching where ALL of the dives were shallow but there were alot of them many in quick succession. This was outside of Indianapolis. He ended up doing a chamber ride in Dayton OH I believe.

I'm not in touch with him any longer and don't recall if he ever reported it to DAN or PADI for that matter. I came to learn that he liked to play a bit loose with the documentation like he did with the standards. So I honestly doubt he told anyone. I learned all of this from other instructors at the shop who had first hand knowledge of the events.

 

[ginti]

This is the part that concerns me. If anything is going to cause you an issue it would be a sawtooth profile with lots of depth changes up and down over the course of a 2 hour dive. For me ... and this is just me ... I would be using nitrox for this reason alone. To minimize tissue loading as much as possible given the sawtooth nature of your profiles.

ADD: I've known instructors who have gotten bent doing repeated CESA dives on air with multiple students from 20 ft.

"CESA dive" is not normal diving and has several other hazards even more likely to happen and with much more serious results than DCS.

By the way did the instructors report their DCS case to DAN? I would think as instructors they would want such unusually hits documented if just for educational purposes

Regarding shallow diving bents and saw-teeth/yo-yo diving, here's something that maybe you are interested in (if you do not already know it).

Look at this picture:

The part of the profile inside the red rectangle can kill you, even if you are within NDL and relatively shallow if there is already sufficient nitrogen in your body.

The reason is simple: the lungs usually filter big bubbles, but in certain scenarios, they don't. Two possible scenarios are:
- PFO, because the bubbles bypass the lungs
- the scenario in the picture; in this case, when the diver goes down so fast, the bubbles become very small. Some bubbles become so small that the lungs cannot filter them: so these microbubbles end in the arterial circulation. When the diver goes up that fast, there can be some of these microbubbles in the arterial circulation: these bubbles will increase their size! So now they are not any more "micro". Since they are in the arterial circulation, they can go directly to important organs and tissues. If they end up in the brain... well, good luck.

This is one reason why saw-tooth/yo-yo profiles are so dangerous, and this effect is damn important at shallow depths (deeper, the pressure gradient would be too low to start this phenomenon).

So this could be the reason why the instructors get bent for aggressive saw-teeth profiles.

However, without better knowledge, I would say that if you maintain a low level of nitrogen in your body, you should avoid this problem (because there are fewer bubbles).

I didn't want to post it because this stuff is a bit too advanced for me, so you better ask some medical moderators for confirmation

 

[boulderjohn]

Magic
View attachment 664496


I thought you were joking at first. It's a simple matter of reducing your nitrogen uptake by replacing some of the nitrogen with oxygen.

You missed his point. He knows that. Everyone except AfterDark knows that. He is trying to get AfterDark to admit that nitrox extends bottom time by lowering the amount of nitrogen in the mix. AfterDark keeps denying it.

 

[boulderjohn]

1.4 is a PP that shouldn't be kept for more than 2.5hrs; I know you know it, but here's a reference for the ones who don't, figure 2:
https://www.shearwater.com/wp-content/uploads/2012/08/Oxygen_Toxicity_Calculations.pdf
NOTE: I used to find online the original NOAA manual from 1991, but I can't anymore. I will be immensely grateful to anyone who can share the link.

I haven't understood yet how long are OP's dives, but at these depths, for artefacts diving, maybe he is diving even more than 150minutes, so the 1.4 standard might be a bit risky... don't you think?

When the time limits for breathing various mixes were created, they were frankly based on guesswork. We are still guessing on this; we have no real data. We teach them to students because they exist and we can are always at risk if we don't.

When I teach technical diving, I go through all of this with my students, as is required. One time one of my trimix students, already an experienced cave diver himself,, asked me how cave divers doing 12 hour dives deal with these limits. The answer is they ignore them.

 

[ginti]

One time one of my trimix students, already an experienced cave diver himself,, asked me how cave divers doing 12 hour dives deal with these limits. The answer is they ignore them.

Thanks for the reply. I know that these numbers come from experiments with navy divers, who dive in conditions very different from us, so in a way, it's really just a guess for us.

However, I am far from these limits now, so I will be conservative and consider them. Maybe one day I will arrive at the point where I must choose between ignoring them or not diving... but it's still a long way

 

[boulderjohn]

I know that these numbers come from experiments with navy divers,

I am not even sure that is true. I think it was mostly guesswork, but, well, I'm guessing.

 

[kelemvor]

1.4 is a PP that shouldn't be kept for more than 2.5hrs; I know you know it, but here's a reference for the ones who don't, figure 2:
https://www.shearwater.com/wp-content/uploads/2012/08/Oxygen_Toxicity_Calculations.pdf
NOTE: I used to find online the original NOAA manual from 1991, but I can't anymore. I will be immensely grateful to anyone who can share the link.

I haven't understood yet how long are OP's dives, but at these depths, for artefacts diving, maybe he is diving even more than 150minutes, so the 1.4 standard might be a bit risky... don't you think?

He may be diving for 150 minutes or longer but he isn't ever getting anywhere near 1.4 with 20' max depths. Even on that one-of-a-kind 40' dive he did he'd need to be above 60% oxygen to hit 1.4. I don't think ppO2 is going to be a concern for @calabash digger based on his description of the dives he does. He should be below 0.89pp02 if he was using 40%

 

[ginti]

I am not even sure that is true. I think it was mostly guesswork, but, well, I'm guessing.

I'll try to find it; if I arrive to some conclusion, I will post them here