Deep stops and ascents...

[gcbryan]

Seriously, I know what my body feels like after diving, and I know on some occasions that I've had very far outliers in terms of fatigue and post-dive napping. Those were dives without deep stops and/or on air. I've also got tons of dives where I've gotten cold and that's a different (and more normal) feeling of fatigue.

Are you arguing that deep stops don't work the same on air and with the same results?

They reduce bubble formation period, air, Nitrox, mix, whatever don't you think?

 

[vkalia]

Seriously, I know what my body feels like after diving, and I know on some occasions that I've had very far outliers in terms of fatigue and post-dive napping. Those were dives without deep stops and/or on air. I've also got tons of dives where I've gotten cold and that's a different (and more normal) feeling of fatigue.

That's interesting to know, Lamont, and thanks for pointing it out.

I dont doubt one bit what you are saying - but I do find it is interesting that you notice such a big difference. I do feel a little better when doing deep stops on decompression dives where my bottom mix is air; but I honestly cannot say that I have noticed any difference between deep stops/no deep stops for normal deep recreational dives.

Your point actually reinforces a fundamental question mark I have with that paper posted by Turtle on page 9 of this thread.

That article claims that the rationale for a deep stop is that it prevents bubble formation due to critical supersaturation of the fast tissues. If so, then a slow ascent from depth should achieve more or less the same function. But as per that study itself, a slow ascent does not achieve that. This ties into a couple of papers I read a while back on what triggers nucleation in the human body - and the general gist of those papers was that it wasnt fully clear: the mechanism had been identified, but the triggers hadnt.

Your data point strengthens my suspicion that simple Haldanian-derivative (and that includes Buhlmann) models do not really do a very good job of predicting bubble build-up.

It'll be interesting to see what the developments in this field are over the next few years.

Safe diving,
Vandit

 

[The Chairman]

Are you arguing that deep stops don't work the same on air and with the same results?

They reduce bubble formation period, air, Nitrox, mix, whatever don't you think?

No... air is tiring compared to NitrOx and Trimix.

 

[The Chairman]

Why do deep stops work where slow ascents don't?

Perhaps its a function of the half times. Blood and neural tissues have an incredibly SHORT halftime (about 5 minutes). While we tend to think linearly, the phenomenon is really parabolic. Given a five minute half time, the amount of N2 eliminated in the FIRST minute is head and shoulders more than the last minute. This would really need calculus to resolve (sorry UF, I don't remember my Calc 101-103). To oversimplify it, the first third would probably account for almost 2/3's of the Nitrogen eliminated within that 5 minute half time.

In light of this, Roakey's contention that our deep stop should be 2.5 minutes makes a TON of sense and you can bet that my ascent profile just changed.

 

[joe rock]

I wasn't told this in OW, but we went over it in AOW and did the stop during our deep dive.

 

[The Chairman]

Look at it this way...

If we are on air at 100 feet for 20 minutes we are close to being saturated. Our blood and neural tissues should have 2.99 atms of N2 dissolved.

If we ascend to half of our depth (50 ft) and spend 2.5 minutes. Our initial tension would be just over one atmosphere (1.03). My guess is that we would get rid of 37% (75% of the half time) of the gas during this time. resulting in a reduction of 0.39 atms of nitrogen leaving us with 2.6 atms of n2 in our blood and neural tissues. Now lets continue up to 15/20 ft and do 5 minutes.

The initial tension would be about 1.4 atm. So our 5 minute stay would reduce our dissolved N2 to 1.9 atm. With me? We are now at 2.44 times our normal nitrogen but our bodies can handle this. A slow ascent to the surface would reduce this even further, and we have not begun to calculate the off-gassing during our ascent time: just the stops.

BTW, I have always believed in a quicker ascent from deep and slower as we get shallow.

BTW #2, There has been some discussion as to the definition of "tension". My definition is that tension is the gradient between differing gas pressures. This can be between inspired air and blood, or it can be between blood and another tissue. The greater the difference, the higher the tension. Please feel free to correct me if this is not a correct understanding. I owe my limited understanding of gas kinetics to Mrs. Himes advanced Chemistry class at Colonial High School (Orlando), Dr. John F Baxter's Chemistry Class at UF, and spending 3 years as a student lab assistant/machinist in a cryogenic P-Chem lab operating a Super-con magnet at UF's department of Chemistry. I hear they have almost recovered from my tenure there.

PS: I have finished editing this now! Really, I have.

 

[vkalia]

Why do deep stops work where slow ascents don't? Perhaps its a function of the half times. Blood and neural tissues have an incredibly SHORT halftime (about 5 minutes).

Good point, Pete. Thinking through what you wrote:

2/ If my N2 loading is such that I can go to half my max depth without incurring significant bubbling, then it is simple math to conclude that a slower ascent means I reach that deep stop depth with significantly less N2 loading (given that we are talking about fast tissues) than if we went up directly. So up until this point, a slow ascent is actually better.

So the interesting bit happens from the point of deep stop to the surface. A 5 min deep stop *may* result in the diver leaving D/2 (half his max depth) with less loading than a slow diver who keeps continuing upwards without stopping... but this is a funtion of how fast he came up in either case. Also, whether or not the controlling issue changes also plays a role.

Assuming that the controlling tissue doesnt change, a slow ascent should indeed work better. Taken to a hypothetical extreme, if there was a tissue with a superfast half-time, a slow ascent would result in the tissue pressure equalizing more or less in sync wih the divers ascent. Slow tissues are less likely to be affected. So, depending on the function, either there is an optimal range of controlling tissues where the above doesnt hold; or it doesnt hold at all; or the controlling tissue changes which puts the math far beyond my abilities.

Ok, not only did this NOT reach any conclusion, but I succeeded in confusing the hell out of myself as well. So that takes me to the next 2 points...

2/ Assuming it was simply a matter of half-times, even if slow ascents dont work as well as deep stops, they'd still work better than faster ascents + no deep stops. Which doesnt appear to be the case.

3/ A fundamental assumption in the article under discussion is that bubbles are a result of critical supersaturation. But critical supersaturation = DCS, and we are talking asymptomatic bubbles here, which occur even when critical supersaturation is not reached.

GIT, bless his sweet, stroke-loving self, had a different - and to my mind, a more logical explanation on deep stops help: his idea was that spending time at a give depth allows your blood to circulate through your lungs once, which gives your lungs a chance to control bubble build-up. This makes sense. I've used this when I've done nasty profiles, and it has been ok - I reckon atleast 2 or 3 times, it has saved me from getting bent.

Assuming bubble formation is a simple function of overall saturation (not *critical* supersaturation, where you exceed your tissue M-values), then the above findings would make sense: a slow ascent rate would actually increase overall N2 loading which would lead to greater bubbling; a slow ascent would also, by virtue of circulation, allow the body to control that bubbling. Too slow, and the increased N2 loading becomes dominant; too fast and inability to control the bubbling becomes important - so there is an "ideal" ascent rate, which would be somewhere between 5-15 m/min (18m/min -- too fast; 5m/min -- too slow). My guesstimate this would be somewhere in the 6-9 m/min.

I dont know where I am going with this... just thinking out "loud", I guess. Anyway, F1 is starting, time to pull out a beer and get comfy.

Vandit

 

[The Chairman]

Sometimes thinking out loud helps to expose misunderstandings and might help others make connections. It was Andy's earlier statement that got me to thinking about this.

If our goal is to present ourselves as scholars here, then we have failed. My goal is to learn and to grow in my understanding. I certainly don't have all the answers. Heck I don't even have a quarter of them. I am not sure how many edits my calculation post went through... they were legion.

As for circulating the blood, that actually makes sense. If we merely kept ascending, we might have gotten too shallow too quickly therby increasing nucleation in those neural tissues which I believe is at the heart of these tired feelings after a dive w/no deep stops.

 

[vkalia]

I know I said I was leaving... and I really am.

But one nit:

If we are on air at 100 feet for 20 minutes we are close to being saturated. Our blood and neural tissues should have 2.99 atms of N2 dissolved.

Actually, at 30m/20 min, we have reached a stage where the pressure of N2 in the relevent tissue exceeds the maximum allowed pressure gradient compared to 1 atm.

This is not saturation at depth, but substantially below that. If our tissues were saturated to 4atm (or 3.9999) at that point, then additional time spent at depth would not add any more decompression liability. That is obviously not true in this case.

Ok, I really have to go now.... beer and deco theory dont go too well.

Also, to be honest, while I find the math interesting (I used to be one of those kids that enjoyed math in school - how's that for fitting a stereotype ), it is worth noting that all deco theory is essentially theories and mathematical forumulas built to fit empirical data - so the proof lies in the diving, not in reading white papers.

I'd encourage everyone to try deep stops and see if it makes a difference to you. For some people, it obviously makes a big difference. For me, it does make a small difference on deco dives with air as my bottom mix, and no noticeable difference on recreational profiles, given my typical ascent rate of around 10m/min up to 15m, and 5m/min above that. Dont blindly follow *anybody* - try it out and figure it out for yourself.

Happy diving all, and thanks for an interesting discussion.

Vandit

 

[DandyDon]

I know I said I was leaving... and I really am.

Happy diving all, and thanks for an interesting discussion.

Vandit

Nah, he'll be back. :lol2: But then, that's good news...