Geezer Gas

[KenGordon]

This is starting to get abovce my head but are you speeking about a single tissue or all tissues when you cal the pont of saturation? I dont believe you ar saturated when yo hit NDL. By the definitions you are citing,,, to come out of saturation you just go deeper brvsudr your tissues can now take on more. Something for me is missing from your position on this.

If you go deeper you are no longer saturated. Henry's law.

I don't like 'saturated' as a term here. I think 'in equilibrium' is better.

Different tissues get saturated at different times, the rate at which the gas is transported varies. Tissues are modelled with half times down to about 5 minutes so it is likely that on any typical dive some tissues are saturated. For example blood gets a pretty good exposure to the nitrogen in the lungs pretty fast.

Whatever tissue you consider, given a lower ppN2 due to nitrox in the lungs it will end up with less nitrogen at any given time into the dive than had air been used.

 

[Storker]

This is starting to get abovce my head but are you speeking about a single tissue or all tissues when you cal the pont of saturation? I dont believe you ar saturated when yo hit NDL.

If you go deeper you are no longer saturated. Henry's law.

I don't like 'saturated' as a term here. I think 'in equilibrium' is better.

The problem here is the sloppy use of terms which in other contexts have a very specific meaning. In chemistry "saturated" means "at the upper limit of concentration". So by that use of the term KWS' confusion is quite understandable and KenGordon's arguments that I've quoted are correct.

The point is that recreational diving (in the meaning "dives for fun", including "technical" recreational diving) hardly ever - if at all - leads to saturation. That's the realm of saturation diving, which IIRC, Akimbo has extensive experience in. We recreational divers don't reach true saturation, in the meaning that our deco times don't increase further with increasing bottom time. However, we use (or mis-use) the term "saturation" when we mean nitrogen loading. I've done that many times, at least. It seems to me that the term "saturation" has been (mis)used in this thread to mean "reach your no-stop limit".

So maybe we should agree not to use the term "saturation" when we in reality mean "nitrogen loading" or "nitrogen loading above NDL"? Maybe we would have less confusion that way?

 

[GrumpyOldGuy]

Both depth and fraction of nitrogen matter.

Saturation is when the partial pressure of a gas (ppN2) in the tissue reaches the partial pressure of the gas in the lungs (inspired ppN2). So at the surface on air that is 0.79 bar and at 30m it is 3.16 bar. After a time your tissues will stop taking on more nitrogen because it reaches 3.16 bar and the gas going in and coming out is in equilibrium. On 32% at 30m saturation is 2.72. Hence there is less mass of nitrogen dissolved and less to be off gassed on the ascent.

Safer means a lower probability of injury. I can't give a number but I can influence the contributing factors in the right direction. Less dissolved nitrogen is better than more nitrogen.

I think you may want to read up on decompression models. Nitrox is not as simple as EAD which is an overly simplistic model. Various tissues allow nitrogen to move in/out at various rates, this rates are very different. At different times, different types of tissue are the limiting factor. In decompression models the tissues are lumped into various groups (compartments) to make calculation reasonable. Bottom line is it does not come down to a simple amount set against a limit. Its analyzing a bunch of different compartments and finding the the worst one (for NDL) or finding an ascent / stop profile that satisfies all the offenders for decompression dives.

This is why a blanket statement of Nitrox is safer is dangerous. Too many variables are in play that are dive profile dependent. If you lock in the same dive profile, it may be true (diving nitrox on air tables). As soon as you start taking advantage of the extra bottom time and shorter SI, you need to recalculate. You may want to play with VPlanner or some of the other software tools to prove it to yourself.

 

[tursiops]

As I read it, KenGordon is correct but takes it too far. The only thing that drives N2 uptake is PPN2, and that is less (at the same depth) if the N2 percentage is lower (i.e., Nitrox). Yes, less N2 uptake is good....but ANY N2 uptake allows for DCS if you ascend too quickly, and ANY breathing of a compressed gas allows for AGE if you ascend too quickly.....and LESS N2 means (for Nitrox) more O2 and that has its dangers...so Nitrox gives you nothing more than apparently diving at a lesser depth....which is what gives you the additional BT.

There is nothing wrong with the EAD concept; it completely takes into account the PPN2 issue.

Safer? As PADI puts it, how can you be safer than safe? If you are already safe on air, how does Nitrox help more? The BIG dangers with breathing a compressed gas do NOT go away with Nitrox and you have one additional big danger (O2 tox)....all you get is more bottom time, and if you use that extra time then you've gained nothing.

 

[Storker]

Safer? As PADI puts it, how can you be safer than safe? If you are already safe on air, how does Nitrox help more? The BIG dangers with breathing a compressed gas do NOT go away with Nitrox and you have one additional big danger (O2 tox)....all you get is more bottom time, and if you use that extra time then you've gained nothing.

Safer from clinical DCS is one thing. More comfortable and less exposed to sub-clinical DCS, that's another thing.

I know that if I skip on the safety stop or if I ascend too quickly from SS depth, I want to hit the bunk pretty early in the evening (or even in the afternoon). And that's not in the "I'll be in my bunk" meaning. I know that my son, who's some 30-odd years younger, doesn't get it that hard. I also know that if I dive two tanks fairly close to my NDL, I'm ready for my bunk even in the afternoon. I also know that my son, who's some 30-odd years younger, don't get it that hard.

Now, to move from facts to hypotheses, I believe this is caused by sub-clinical bubbles and my middle-aged geezer body being less capable to deal with that than my son's sub-20 year old body. If that hypothesis is true, anything (like nitrox) that reduces nitrogen loading should help. If that's correct, nitrox is a pretty good geezer gas.


--
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[tursiops]

Safer from clinical DCS is one thing. More comfortable and less exposed to sub-clinical DCS, that's another thing.

I know that if I skip on the safety stop or if I ascend too quickly from SS depth, I want to hit the bunk pretty early in the evening (or even in the afternoon). And that's not in the "I'll be in my bunk" meaning. I know that my son, who's some 30-odd years younger, doesn't get it that hard. I also know that if I dive two tanks fairly close to my NDL, I'm ready for my bunk even in the afternoon. I also know that my son, who's some 30-odd years younger, don't get it that hard.

Now, to move from facts to hypotheses, I believe this is caused by sub-clinical bubbles and my middle-aged geezer body being less capable to deal with that than my son's sub-20 year old body. If that hypothesis is true, anything (like nitrox) that reduces nitrogen loading should help. If that's correct, nitrox is a pretty good geezer gas.

Me too. I'm older than you are, and agree completely on the tiredness and on its mitigation with Nitrox. On Nitrox I can do 4-5 dives a day....on air, two and I'm done. What I don't understand is that it also happens if I do air dives at pretty much any depth....it is the duration that tires me out....even 4m! So there is something about the N2 I'm breathing to which I am very sensitive...and reducing it is very helpful. After almost two decades using Nitrox (and about 1200 Nitrox dives in that time) I'm pretty convinced of my body's response.

 

[NAM001]

Equalibrium i woudl agree with but not saturation.

If you go deeper you are no longer saturated. Henry's law.

I don't like 'saturated' as a term here. I think 'in equilibrium' is better.

Different tissues get saturated at different times, the rate at which the gas is transported varies. Tissues are modelled with half times down to about 5 minutes so it is likely that on any typical dive some tissues are saturated. For example blood gets a pretty good exposure to the nitrogen in the lungs pretty fast.

Whatever tissue you consider, given a lower ppN2 due to nitrox in the lungs it will end up with less nitrogen at any given time into the dive than had air been used.

---------- Post added February 10th, 2015 at 06:07 AM ----------

Thanks all , Cause i am very weak on deco theory other than the basic concepts. I have considered NDL as the point where your fastest compartment hits saturation or a certain percentage from saturation. That may or may not be correct but it has always worked for me. It makes it simpler to explain to to new divers that way. The more complartments you saturate the longer (exponentially ) you have to deco. I tell them they dont have enough backgas to off gas the slower compartments and that is why we dont push ourselves into deco. They except it that way, so it serves its purpose I guess.

 

[Akimbo]

Equilibrium was used to describe saturation in my training as a saturation diver. What’s the difference?

It is a common misconception that sat divers are only concerned with the slowest tissues. That is correct on final decompression but we also have excursion limits above and below our holding depth where the faster tissues rule.

 

[Water Boy76]

What I've learned on this thread. It's all in my head.....

Silly me. I thought I felt better diving nitrox based on real world experience until the keyboard commando's told me that I don't. Who am I to argue - they have equations.
In the future, I will remember to ignore my first hand experiences and ask that the SB scientist's how I feel after my dives. They can get out their slide rules and derive my feeling based on their pie charts and such.

 

[GrumpyOldGuy]

What I've learned on this thread. It's all in my head.....

Silly me. I thought I felt better diving nitrox based on real world experience until the keyboard commando's told me that I don't. Who am I to argue - they have equations.
In the future, I will remember to ignore my first hand experiences and ask that the SB scientist's how I feel after my dives. They can get out their slide rules and derive my feeling based on their pie charts and such.

Wow, must be nice to be immune from the placebo effect......

When did you do a blind trial to validate your feelings?