Nitrox for shallow water artifact diving??

[ginti]

@JimBlay, this entire discussion is about underwater physiology, and its mechanisms are pretty complicated. Today, scientists are not able to describe all the involved phenomena. With no better knowledge, and no cost issues, I would also fill with nitrox, but this forum is a place to understand better, isn't it? So let's try

Even if I'm well within my NDL on a shallow dive, less nitrogen is always better than more nitrogen if all other factors aren't an issue (cost and availability).

Here's what I ask you: why would less nitrogen be better?

Please notice that I am not saying you are wrong - I just do not understand what practical benefits one would have for the kind of dives we are discussing.

 

[Duke Dive Medicine]

So what is it doing @ 25FSW on a 2hour dive? No much it would seem since I have never read of a case of DCS doing a 2hr @25FSW. Or any dive of any length of time at that depth. Have you?

I have a good friend who has treated multiple working divers for DCS from roughly that depth and bottom time. They were extreme sawtooth profiles, but they still defied traditional thinking about the possibility of getting bent from that depth.

I am not arguing against your point.

As far as I understand, there is, in reality, absorption of nitrogen at those shallow depths, but it's minimal and negligible for practical applications. In other words, we shouldn't consider it, as you are doing.

@Duke Dive Medicine, could you clarify? (I hope it doesn't bother you the I am calling you so often in the last periods )

Using nitrox decreases nitrogen uptake, which will increase the no-stop limit. Whether that makes a difference in practice depends on depth and gas consumption, e.g. the NOAA no-stop limit for 36% EAN at 40 fsw is 371 minutes, so there's no possibility of running up against that on a single scuba tank. The benefit there is in decreased nitrogen uptake, which will in turn make a difference on repetitive dives, so it should indeed be considered.

The NOAA no-stop limit for 36% EAN at 90 feet is 48 minutes. By comparison, the NOAA no-stop limit for compressed air at the same depth is 30 minutes. Provided the diver can make a tank last for 48 minutes at 90 feet, the nitrogen load for both dives would be roughly the same, so the benefit of the 36% EAN is in extending the no-stop limit. But, if the diver's gas consumption rate is such that he or she can only make a tank last for, say, 35 minutes at 90 fsw, the benefit of using 36% EAN would be in decreasing nitrogen uptake, because he or she would be ascending well before the no-stop limit was reached.

For recreational compressed air/nitrox divers who use dive computers and dive to the edge of the no-stop limits in the computer's algorithm no matter what gas they're breathing, the practical difference is only on ascent/decompression, as the higher O2 partial pressure in nitrox will provide a small decompression benefit.

Best regards,
DDM

 

[boulderjohn]

Actual 33fsw is 2ATM absolute. For most discussions of this nature 1ATM is used but you can have it your way.

The fact is nitrox is used to extend NDLs. Above 33FSW or 2ATMA there is no NDL so nitrox is no benefit to the OP at those shallow depths. Below 33FSW is where any benefit would be incurred.

I think you are confused by the fact that tables, like PADI's, often have 35 feet as the shallowest depth on the table. That does not mean there is no NDL shallower than that. It means that you are to use the 35 foot limit for dives up to 35 feet in depth.

Right so on a 25FSW dive what NDL is being extended?

On a 25 foot dive, you are supposed to use the 35 foot limit for your dives. I am not near my tables right now, but if you look at air tables and nitrox tables, you will see that the nitrox tables extend those limits quite a bit.

 

[Storker]

Subclinical DCS may be a factor, in which case the nitrox would absolutely help.

Concur.

there is no empirical evidence that nitrox makes you "feel" better.

I have anecdotal evidence to the opposite. Anything that takes you further from the NDL, be it slow ascents, extended safety stops, r-e-a-l-l-y slow ascents after the safety stop or nitrox makes me feel better after the dive.

 

[Storker]

Nitrox is used to lessen nitrogen uptake (compared to air) at ANY depth.

And to increase offgassing compared to breathing air. But that's often overlooked.

If I've gone to the NDL, no matter if that was on air or 32%, I'm offgassing more at my safety stop if I'm on 32%.

 

[ginti]

...

Thanks a lot. I don't know why I wasn't thinking about repetitive dives, which are among the most obvious reasons to use nitrox.

@calabash digger, so here's what I would do if I were you:
- write down the duration of my average and my most extreme dives;
- write down how often I do repetitive dives and the associated surface intervals;

If decompression limits the duration of your dives, or if you want to reduce the surface intervals, you should go for nitrox. Even if you plan to use nitrox only a couple of times per year, you need to take the course.

About the fatigue issue, most evidence is anecdotal, not empirical, so I guess it's up to you what to believe. I tend to trust scientists: if they didn't manage to clarify that nitrox reduces fatigues, I don't see any reasons to believe it.

 

[Divin'Papaw]

@JimBlay, this entire discussion is about underwater physiology, and its mechanisms are pretty complicated. Today, scientists are not able to describe all the involved phenomena. With no better knowledge, and no cost issues, I would also fill with nitrox, but this forum is a place to understand better, isn't it? So let's try



Here's what I ask you: why would less nitrogen be better?

Please notice that I am not saying you are wrong - I just do not understand what practical benefits one would have for the kind of dives we are discussing.

I'm not sure what exactly you want in a response. This is all Decompression 101.

Here is my thinking:

Less nitrogen is always a good thing as a diver. Nitrogen is not metabolized and does NOTHING but saturate various tissues leading to the need for those tissues to then desaturate. This is the whole basis for decompression and NDLs that we learn from the very beginning in our training as divers.

Will the increased nitrogen in air relative to nitrox cause an issue at these shallow depths? As long as you follow the established dive tables/algorithms and ascent rates, no, it shouldn't. However, ANY increased nitrogen causes increased tissue loading which must then be unloaded resulting in "stress" on the various tissues within the body. Shouldn't be enough stress to cause issues as long as all established guidelines are followed, but tissue stress is tissue stress. Why introduce more than necessary on the body?

I will always choose to reduce my nitrogen loading whenever possible even when diving well within NDL.

 

[boulderjohn]

Hopefully this will explain things a bit more.

1. As soon as you begin to descend on a dive, your tissues begin to absorb gases as the pressure increases. If at the end of the dive your tissue pressure is too great in comparison to ambient pressure, especially upon surfacing, you are in danger of getting DCS.

2. Tissues are able to have a certain amount of pressure greater than ambient and still be OK. Otherwise you could not ascend at all. Different tissues have different maximum tissue pressures compared to ambient pressure.

3. It is generally believed that you can surface at sea level (1.0 ATA) with a 1.5 tissue pressure.

4. As you ascend or do a safety stop, tissues that have a pressure higher than ambient pressure will off-gas, and tissues with a pressure lower than ambient will still on-gas. If you stop at a specific depth, they will on-gas or off-gas until they reach that ambient pressure. That is called reaching equilibrium. Once a tissue reaches equilibrium, it stays there until the diver changes depth.

5. The faster tissues will reach that safe level quickly, often during the ascent. Slower tissues off-gas more slowly and need more time.

6. Staying at safety stop depth allows faster, off-gassing tissues to get to 1.5, and the alower, on-gassing tissues cannot get above 1.5. The diver can then safely surface.

7. A diver in the 20-35 foot depth range will be in a range of 1.7-2.0 ATA. If tissues reach equilibrium at that pressure, they are in a gray area of being at risk upon surfacing. The closer to 2.0, the more problematic, so that is why there is a NDL for divers in that range. If a diver stays too long at that depth, the slower tissues will have become saturated enough that they will not have time to off-gas during the ascent.

 

[ginti]

Less nitrogen is always a good thing as a diver.

First of all, I basically always dive nitrox, for two reasons:
- I often dive with GUE divers, and standardization involves a lot of advantages that I am not going to cover here (it's way OT);
- when I dive with people who prefer air, I use nitrox just in case my plans after the dive change, and because it is easier for me to deal always with the same gas (easy to remember NDL, etc.)

But I have good reasons to do it. Even if I believe that nx has so many advantages, I need to be clear with other people who are asking about its use regarding advantages, disadvantages and neutral aspects.

Now, besides the above reasons, I do not see others that are always valid.

Let's assume that one day I have to do a single dive, within recreational limits. After this dive, I do not have to do any kind of activities that involves environmental pressure changes. In this case, having more or less nitrogen after the dive doesn't make any difference to me. What do you think?

EDIT: I understand that you consider that stress something negative; I agree with you, but it's our way to see it. At the end of the day, millions of people have used air without any health issues in the past and are using it even today.

 

[BRT]

So what is it doing @ 25FSW on a 2hour dive? No much it would seem since I have never read of a case of DCS doing a 2hr @25FSW. Or any dive of any length of time at that depth. Have you?

If staying at 25' it is ongassing until saturated. The expectation is that the human body can withstand saturation at that depth and a quick return to the surface. Human bodies may differ.