Last Deco Stop Depth, Oxygen Window, and Deco Efficacy

[LiteHedded]

The s curve has been more or less replaced with a linear deco. So there is more time at the gas switch stops than what buhlmann or vpm prescribe, but it's not as pronounced as the older a curve ascent strategy.

yea. and it seems to be just to make the maths easier. in my mind it mostly doesn't matter where you spend your time on those intermediate stops

 

[lefrogster]

Coming back to the OP, breathing O2 at 10ft or 20ft does not have an effect the rate of off gassing of inert gases, but Deco for divers says pretty clearly that breathing high ppO2 helps in reducing bubble formation:

Table 26: "Increased from in partial pressure between arterial and venous side as a result of breathing high partial pressure of oxygen" -> "Reduces bubbling to a greater degree".

So the 20ft stop vs the 10 ft stop doesn't have an effect on decompression efficiency (if we mean this to be increasing the rate of off gassing of inert gasses), but it is safer decompression to do the 20ft stop to reduce bubble formation.

 

[stevensamler]

The 20' stop...easy....o2 is most efficient in aiding offgassing at that depth(higher ppo2 with zero ppn2 or ppHe).

In relation to the previous point, light finning and moving wouldn't be enough exertion to worry about during o2 deco, but fighting current, or swimming around excessively could cause increased Carbon Dioxide levels....there are many, many, did I say many? Cases of oxtox during increased exercise while breathing high ppo2's. The only explanation I have seen for this is carbon dioxide.

I believe the ambient pressure at 20' vs 10' will drive the nitrogen out of solution better and will make for a cleaner deco.

 

[LiteHedded]

I believe the ambient pressure at 20' vs 10' will drive the nitrogen out of solution better and will make for a cleaner deco.

some of the NEDU guys are advocating this as well. i've done it both ways and haven't seen any difference

 

[TSandM]

The only rationale I can come up with to support the idea that a higher ambient pressure would result in better offgassing is if you are trying to control bubble size. Gas in solution will be more likely to offgas with a lower ambient pressure on backgas, but should have no different behavior in the face of O2. Gas diffuses poorly out of bubbles, though, so reducing bubble formation and growth could have some benefits.

 

[Michael Guerrero]

I think the difficulty is quantifying the benefit of either approach. Our bodies can endure a certain amount of abuse (individually determined) from bubbles, and high ppO2 exposure. To me, the unanswered questions are: how much more bubbling is going on at 10' vs. 20'; how much more likely am I to incur an oxtox event based on my past diving (think repetitive diving, potentially over multiple days) and current physiological and environmental conditions. Since we don't really have a way to quantify either of these, from any study or book I've ready anyway, it comes down to a personal preference that should NOT be a vehement stance.

We know what we don't know here. And we know that either way, we're pretty safe when we stay within the accepted margins of technical diving.

---------- Post added September 5th, 2014 at 03:26 PM ----------

I believe the ambient pressure at 20' vs 10' will drive the nitrogen out of solution better and will make for a cleaner deco.

Can you describe how this works? I haven't heard of this.

 

[rjack321]

The only rationale I can come up with to support the idea that a higher ambient pressure would result in better offgassing is if you are trying to control bubble size. Gas in solution will be more likely to offgas with a lower ambient pressure on backgas, but should have no different behavior in the face of O2. Gas diffuses poorly out of bubbles, though, so reducing bubble formation and growth could have some benefits.

Anyone who has been bent during a dive (or who has done IWR) absolutely knows this to be true. <raises hand>

Once formed, its a bitch get bubbles to re-dissolve so the blood can re-perfuse the tissues and move that nitrogen to the lungs. This is why this irrational fear of toxing is shortsighted. Yes you can move up at a certain point, no it doesn't influence the actual offgassing rate - ASSUMING that by moving shallower you don't have any overpressure gradients large enough to create bubbles. But people have embolized and died moving up from 20ft to 10ft on O2, Denton Myers was one of them. You can search for the thread about his death on TDS. Whether the emboli were from inadequate deco using the 85% GF high (which is now known to create significant post dive bubbling), the aneurysm, or a momentary breathhold, or all three we will never know for sure.

One thing is for sure, people are not toxing even from long decos at 20ft with backgas breaks. All the O2 toxicity events over the past 5 or more years have been due to breathing the wrong gas at depth. That suggests that the fear of a 20ft 1.6 ppO2 \ and desire to lower than ppO2 by doing (a greater portion) of the O2 deco at 10ft is ill-founded.

 

[Kevrumbo]

Anyone who has been bent during a dive (or who has done IWR) absolutely knows this to be true. <raises hand>


Once formed, its a bitch get bubbles to re-dissolve so the blood can re-perfuse the tissues and move that nitrogen to the lungs. This is why this irrational fear of toxing is shortsighted. Yes you can move up at a certain point, no it doesn't influence the actual offgassing rate - ASSUMING that by moving shallower you don't have any overpressure gradients large enough to create bubbles. But people have embolized and died moving up from 20ft to 10ft on O2, Denton Myers was one of them. You can search for the thread about his death on TDS. Whether the emboli were from inadequate deco using the 85% GF high (which is now known to create significant post dive bubbling), the aneurysm, or a momentary breathhold, or all three we will never know for sure.


One thing is for sure, people are not toxing even from long decos at 20ft with backgas breaks. All the O2 toxicity events over the past 5 or more years have been due to breathing the wrong gas at depth. That suggests that the fear of a 20ft 1.6 ppO2 \ and desire to lower than ppO2 by doing (a greater portion) of the O2 deco at 10ft is ill-founded.

No Richard (and yet again you miss this additional perspective) . . .the risk is not "ill-founded" when considering the strategy of doing repetitive staged mandatory deco dives per day especially with short surface intervals. . .

I think the difficulty is quantifying the benefit of either approach. Our bodies can endure a certain amount of abuse (individually determined) from bubbles, and high ppO2 exposure. To me, the unanswered questions are: how much more bubbling is going on at 10' vs. 20'; how much more likely am I to incur an oxtox event based on my past diving (think repetitive diving, potentially over multiple days) and current physiological and environmental conditions. Since we don't really have a way to quantify either of these, from any study or book I've ready anyway, it comes down to a personal preference that should NOT be a vehement stance.

We know what we don't know here. And we know that either way, we're pretty safe when we stay within the accepted margins . . .

From George Irvine's BAUE Lecture in 2002 with regards to mitigating oxygen toxicity in repetitive diving with mandatory staged decompression:

Audience: I was hoping you could walk us through an example of how you would do something that we commonly do here like a boat dive. For a dive on the Cypress Sea charter boat, we'll go on what they call an advanced trip. You do three dives in one day, maybe between 90 to 130ft on the bottom. It's not terribly deep but you're doing three dives. You bring one set of doubles and maybe one or two small stage tanks. You have maybe an hour and a half between dives. Could you go through how you would plan which gas to bring and how you'd decompress from 30 &#8211; 35 minute bottom times or maybe a little longer on the shallower dives?

George: OK, well one of the things that seems to determine ability to tolerate decompression is your degree of vascularity on a capillary level. Not like big veins sticking out on your arms &#8212; I'm talking about what you can't see. What you can't see is the vessels that are created by working out. Working out, depriving the tissues of oxygen and nutrients causes angiogenesis. It causes vessels to grow and gives better perfusion. It will be harder to bend those people or hurt those people or injure those people. There are certain tissues that just aren't perfused, like fat. Fat just might have one capillary per cubic millimeter. Nothing. It's like trying to take a stick of butter and decompress it. It's not going to work. On the other hand you don't care if you damage it, you'd like it to get damaged and get rid of it. So you have everything in between, every kind of variation in between.

You have to determine what exactly is going to work for you personally. The biggest thing is the shape of the decompression. If you do the right shape of decompression you're going to be on the right track. Then you just need to fine tune it for the person. You don't want to change the decompression by doing a ton of oxygen at 20ft and altering the gradual ascent shape. That's not what I mean by change. But maybe just be a little more meticulous about certain stops and steps &#8212; make sure you do everything by the book. Whereas a guy that's like really well vascularized can probably get away with screwing up and not getting hit. That's how you fine tune your own deal by being meticulous.

As far as these multi-dive days, the biggest risk is really not getting bent, the biggest risk is oxygen exposure, oxygen damage and of course tox. You want to plan your dives so that you're not loaded up with oxygen too early in the game. If you do, when you get to your last dive you can't use it if you need it, so to speak. What I tend to do is to use the deeper deco gasses on the first dive and add the shallower deeper gasses after we do more dives. So let's say it's a two-dive day &#8212; two 250ft dives. I'll breathe my 70ft bottle and deco out without oxygen on the first one and I'll add the oxygen to the second one. I generally won't do a full-blown all-gasses-deal each time because I don't want to build up the exposure. The other thing, when you're diving repeatedly, if you approximate the correct deco on the way up &#8212; just approximates it &#8212; you're in pretty good shape.

You don't really have all this residual stuff during the surface interval. It doesn't really count. It doesn't really work that way. You can go back in the water and do it again as long as you're not bouncing. Remember what happens when you bounce. Once you get out of the water you're still cascading bubbles. You've started that process. You get back in the water, go down and come back up. When you relieve the pressure that process is going to resume and you're going to cause yourself problems. It's actually better to do deeper dives. But it's especially important if you're doing a second or third or whatever dive is that you stay down and ascend properly. You don't bounce it. You stay down and when you do come up that you do it meticulously. You really don't have to do a whole lot more deco on the second or third dive because you've pretty much taken care of it by the first dive. And if you're in real good shape, you're going to be fairly clear after 30 minutes or so out of the water. And even if you're not in really good shape you're going to be, for the most part, clear. And you're not going to do any better if you're not in good shape. You get into that four-hour bit. If you go back in the water and recompress you're probably better off than if you're just sitting out of the water not diving. So you're not really going to have to, let's say, do twice as much or three times as much deco. I've done that for years, repeated dives and back-to-back dives, and it's never been an issue. But I also ascend real carefully. I think the bigger issue is tox. . .

. . .What the Navy found was that in multi-day diving and long exposures, that a sudden spike in the oxygen would trigger a tox regardless of the level. So they tend to back off. What we did was back off the bottom partial pressures. We backed off the deep decompression-gas partial pressures. So we don't go beyond a certain point, at depth, in partial pressure. All of our other deco gasses may start at 1.6 partial pressure and decline from there. For repetitive diving, you merely move your bottle to the next stop up. In other words a 70 bottle would start at 60ft if on a repetitive dive. Two dives or multi-day diving you would generally move the bottles up higher because again you don't want to spike your PPO2 on a multi-day or a long exposure. . .

We did a whole series of body recoveries in Palm Beach where we didn't have enough divers and I had to dive repeatedly. I did a lot of back to back to back 250s with not even an hour in between them, maybe 30 minutes. I did the same deco on each one and it didn't make any difference at all. I started with 50% and then I do 50% and oxygen, some of them we just did 120ft deco [35/25 intermediate mix] and 50% depending on what it was. We had to stay right on the bottom because that's the only way you can see &#8212; so it was all rectangular. You can't see down. and it's too dark, so you have to look sideways. On these offshore trips I'd always carry extra oxygen just in case there was a problem and I had to get back in the water.

 

[rjack321]

You don't really have all this residual stuff during the surface interval. It doesn't really count. It doesn't really work that way. You can go back in the water and do it again as long as you're not bouncing. Remember what happens when you bounce. Once you get out of the water you're still cascading bubbles. You've started that process. You get back in the water, go down and come back up. When you relieve the pressure that process is going to resume and you're going to cause yourself problems. It's actually better to do deeper dives. But it's especially important if you're doing a second or third or whatever dive is that you stay down and ascend properly. You don't bounce it. You stay down and when you do come up that you do it meticulously. You really don't have to do a whole lot more deco on the second or third dive because you've pretty much taken care of it by the first dive. And if you're in real good shape, you're going to be fairly clear after 30 minutes or so out of the water. And even if you're not in really good shape you're going to be, for the most part, clear. And you're not going to do any better if you're not in good shape. You get into that four-hour bit. If you go back in the water and recompress you're probably better off than if you're just sitting out of the water not diving. So you're not really going to have to, let's say, do twice as much or three times as much deco. I've done that for years, repeated dives and back-to-back dives, and it's never been an issue. But I also ascend real carefully.

George is just wrong, this 12yo "information" is just plain wrong. There have been copious actual scientific studies on bubbling rates and residual gas loading and just redoing the same deco is BS. Yeah I believed George once too, and I got bent on the second dive even though the 2nd dive's deco was perfect and more conservative than the first. I was bent by the 30ft stop on 50% before I even got to the O2 stop. I don't recall if I bought O2 on that dive, I don't think I had it along or I would have fixed the problem (or at least mitigated it). We did get back in the water and do a 3rd dive that day. That fixed my minor bends and I have not had any issues treating George's expertise with a very broad skepticism since...

 

[Kevrumbo]

George is just wrong, this 12yo "information" is just plain wrong. There have been copious actual scientific studies on bubbling rates and residual gas loading and just redoing the same deco is BS. Yeah I believed George once too, and I got bent on the second dive even though the 2nd dive's deco was perfect and more conservative than the first. I was bent by the 30ft stop on 50% before I even got to the O2 stop. I don't recall if I bought O2 on that dive, I don't think I had it along or I would have fixed the problem (or at least mitigated it). We did get back in the water and do a 3rd dive that day. That fixed my minor bends and I have not had any issues treating George's expertise with a very broad skepticism since...

Richard, the empirical concept that George is discussing is not "wrong" per se --it also depends how you tactically AND CORRECTLY apply it. Sometimes even within a prescribed general deco profile, you have to tailor it to your own individual physiology (and George clearly points that out in the lecture above).

For instance some years ago, the GUE rule-of-thumb of diving 30/30 triox like NDL Eanx32 utilizing MinDeco/ascent rules resulted in the "chokes" for me (residual Helium microbubbles overwhelming my lungs' alveolar bed), after two repetitive dives to ave depth 24m with a 90min SIT. I now always use Oxygen for deco on the last repetitive dive when using recreational triox or any leftover Trimix, even on a "no decompression limit" dive above 30m. . .

btw, That other incident of yours Richard in 2008: something about --

. . . I did a recreational dive on leftover 19/50 (or something approximating this) about 4 years ago. Felt like poo afterwards and I did the (~10min) ascent from 70fsw on EAN50 too. I don't recall the bottom time for that dive. It was on the artificial wreck of the HMS Chaudiere so average depth would have been about 90-95fsw. . .

If you're coming off a bottom mix with an FN2 of 31% ["leftover 19/50 mix"], why did you choose an inefficient deco mix with a higher FN2 of 50% [EANX 50] for the entire decompression profile strategy (or for that matter -even if you did imply adding an Oxygen switch later in the profile)? Do you finally understand the additional dynamic factors of greater Nitrogen Solubility vs faster Helium Diffusivity along with mitigating Oxtox concerns in repetitive decompression diving?