Can you do too much deco?

[justinthedeeps]

So, if you use deep stops, you have the less useful bottom time for the same total run time with a higher risk of DCS according to the NEDU data.

This "same runtime" thing always comes up. If you are trying to match runtimes, absolutely the plan with the lower GF_low will have the diver leaving the bottom sooner, because they will have to complete more relative deco to stay within their chosen GF limits. The higher risk of DCS occurs if they don't do sufficient deco for the chosen runtime and GF choices. But that is not what people are doing. They are not violating their computers.

The Powell video someone posted above actually includes a study where different relevant GFs were compared more properly. What did they find? Everybody was fine and there were not many differences. Much more relevant to the diving we actually do.

Nobody dives like the NEDU (unless ordered to)

Actually no. With 10/90 you spend more time at elevated GFs than with 90/90, and have a higher risk of DCS. Buhlmann GF theory has two components to risk Tissue Tension(GF) and Time. Extended times at elevated Tissue Tension is worse than short times at the same Tissue Tension.

This "time at tissue tension" theory is interesting, are you saying that the Buhlmann deco algorithms in our computers are missing something important and not accounting for that? Do we need our computers to add a penalty for "time at tissue tension?" Or is this just another fancy way of saying "more deco time is needed for lower GFs," which is already factored in when we follow our computers?

It is a reasonable idea that a larger overall amount of on- and off-gassing, over a longer period of time, could increase risks. But that would mean that our current Buhlmann and GFs models inadequately capture what is really happening.

Actually, they are easily distinguishable.

I can all but guarantee that if you plotted someone's real dive log data, following either plan, it would differ from those two tissue profiles much more than they do between each other. GF deco talk is cool and all, but in the context of the original post (OP), we are truly splitting GF hairs.

 

[L13]

This "same runtime" thing always comes up. If you are trying to match runtimes, absolutely the plan with the lower GF_low will have the diver leaving the bottom sooner, because they will have to complete more relative deco to stay within their chosen GF limits. The higher risk of DCS occurs if they don't do sufficient deco for the chosen runtime and GF choices. But that is not what people are doing. They are not violating their computers.

Even if they complete their deco per their GF limits without violating their computers when GF-low is "deep stop" low, they are at greater risk of DCS than someone with the same GF-High that uses a higher GF-Low, and they pay for that greater risk with greater time as well.

Do you honestly believe that all GF-Low's are equal? With no good reason to pick one over the other? Even the deep stop proponents don't make that inane argument.

I should have given up a dozen posts ago.

 

[justinthedeeps]

Even if they complete their deco per their GF limits without violating their computers when GF-low is "deep stop" low, they are at greater risk of DCS than someone with the same GF-High that uses a higher GF-Low, and they pay for that greater risk with greater time as well.

The study in the Powell presentation (it's near the end), which tested more typical GF profiles, did not mention higher DCS incidence in the lower GF_low group. The divers all did their deco and they were fine.

I don't count myself among the "low GF_low" group, but I am also not assuming that they are all going to get DCS (they don't...) Thousands of people are still diving 30/85 or whatever (even though we don't), and they are not getting DCS simply because of that GF if they follow their computers and have good dive fitness and ascent control, etc. They'd probably be fine at 60/85 too, and get out of the water sooner. But it's their choice. Are they all getting much higher rates of DCS? I don't think so.

Do you honestly believe that all GF-Low's are equal? With no good reason to pick one over the other?

I don't personally think a very low GF is helpful, but I also am not sure where in the real data or in the Buhlmann model it says this is actually a higher DCS risk, if one does in fact manage to complete all of the deco it suggests. The deco/profile suggested with a GF_low of 10 is of course silly to look at, and you won't see me doing it. It's a poor choice for several additional reasons, such as longer runtimes, exposure, drifting, getting left behind by the boat etc. But the whole point of the Buhlmann and GF math is that if you follow the deco properly, then you don't go into DCS territory. Is it not?

Back to the OP, "can you do too much deco?"
Is 30/60 "extra cautious about DCS" with a "pretty good safety margin?"
#TriggerAlert

We are going to be doing a lot of archaeological dives on CCR on a wreck in 65 metres/213 feet - daily diving for weeks on end. We are fortunate that the water is warm but the location means that we need to be extra cautious about DCS.

We are looking at various profiles in MultiDeco, and the question has become - is there such a thing as too much deco? (provided that you are comfortable, warm, and stay within OTU and CNS limits). Is too much deco bad?

Most profiles/run times give us a working time on the bottom of around 23 minutes which we are fine with, something that has been considered is using a GF of 30/60 which should build in a pretty good safety margin.

Thoughts?

I'll take it as some indication that the very first answer, from a prominent advocate of ditching excessive "deep stops," basically said you're fine mate, enjoy your dives.

 

[justinthedeeps]

I should have given up a dozen posts ago.

I don't want to sound too personal, but are you some kind of lurking expert?
How does someone with "100-199 dives" from the inland US become a leading expert on gradient factors and DCS studies?
I'll take my indications from Simon Mitchell and Mark Powell, who evidently also treat these questions with quite a bit more open-mindedness.

 

[leadduck]

I have never seen any kind of estimate for increased gas loading due to exertion, but 20% seems awfully high. Divers are almost always at least swimming gently at depth, so some exercise has always been part of the algorithm. I don;t know the current situation, but at least a decade ago ScubaPro had computers that tried to use heart rate is an indication of increased on-gassing (I guess that is what it was for), but I don't think it made anything close to that much difference.

It's not clear to me either what is the best factor to use. The rules of the Deco 2000 tables created by Bühlmann's coworker Max Hahn and used by German/Austrian/Swiss sports divers even ask for a factor 1,5x on the bottom time (+50%) for dives with longer phases of hard work, which has been taught in OWD classes using tables here for a long time.
Bühlmann's book give's the perfusion of various tissues in l/min/kg at rest and at work. For joints & bones, perfusion at work is 2x, for skin and fatty tissue it's 2.5x (depending on temperature, will be less in cold water), for skeletal musculature it's 10x, for brain and spinal cord there's no change, and the intestines' perfusion is even reduced by -25% at work.
The Scubapro Galileo Luna has the optional heart rate monitor discussed here:

Heart Rate monitor for Precise Decompression

You would use your heart rate monitor to determine how fast you can swim without hitting your lactate acid threshold and bonking. Very important for scuba divers because we are such highly trained and conditioned athletes. Whats next, EPO injections.

scubaboard.com

The NEDU 2011 study had divers exercise a lot on the bottom in order to cause more decompression stress. There are studies that say that light exercise at depth has no influence on bubbles, whereas light exercise during decompression is beneficial.

 

[EFX]

Actually, based on modern evidence, that statement is completely wrong. It reduces conservatism, while producing all the side effects you proceed to mention.

The Bubble models suggest that deeper stops add conservatism, but the Buhlmann model suggests that deeper stops are worse because of increased time at elevated Tissue Tensions. And the NEDU data suggests that the Buhlman model is a better predictor of DCS than the Bubble model, except when GF-Low is chosen to make it emulate a bubble model (GF-Low << GF-High).

What evidence? You cite the NEDU study which purportedly compared deeper stops to shallower stops but those deeper stops did not resemble any profiles divers use today or back when the study was done. Also, in the NEDU study there was no attempt to measure bubble size and their quantity and their mitigation in reducing the risk of DCS.

 

[Nick_Radov]

What evidence? You cite the NEDU study which purportedly compared deeper stops to shallower stops but those deeper stops did not resemble any profiles divers use today or back when the study was done. Also, in the NEDU study there was no attempt to measure bubble size and their quantity and their mitigation in reducing the risk of DCS.

What sort of bubble size and quantity measurements are you looking for? Doppler bubble grade observations have been reported in some other deco studies, but they have only a loose correlation with clinical DCS symptoms. Sometimes divers in a high bubble grading category have no clinical DCS symptoms, or vice versa.

 

[Nick_Radov]

As far as not having a big difference between GF_low and GF_high, I happen to agree that a smaller difference (or just a single GF) is probably good, but I'm not sure there is much empirical evidence to actually prove that, for divers who actually complete their deco obligations. It may well be that it doesn't really matter either way, as long as people can do the deco within reasonable runtimes.

GF Low and GF High are separate, independent variables. They're not something you can usefully combine into a "single GF". I think that naming both variables as "Gradient Factors" might be confusing some people. Maybe we should rename GF Low as "Deepest Stop Factor" and GF High as "Conservatism Factor", or something like that?

For the OP's plan, having a lower GF_low has about as much effect as adding a couple minutes to your bottom time, or conversely, it's like ending a longer bottom time "early" to do a "deep stop" instead. So what? If it's in the plan and you do the deco, you will not explode.

Yes, OP's plan has a low absolute risk of DCS. It would take thousands of dives to detect any statistically significant difference between a GF Low of 50 or 70 (or whatever specific number you want to pick). There is a lot of noise in the data and many other variables which are impossible to really control for.

If you believe in GF theory at all, then even a 10/90 plan should work out, as long as all of the deco is done. Suboptimal, maybe. You won't see me doing it, but this person should be safe from DCS if GF theory and current models are correct. If not, then you would have to accept that GF and deco models are wrong.

We do accept that the deco models are wrong. That part isn't even in question. The issue under discussion is whether the model is close enough to be useful, and how to apply it in practice.

 

[RedSeaDiver2]

I've been off internet for nearly two days and this thread has grown by about 5 pages - will try to catch up in about another day's time...

 

[RedSeaDiver2]

You may be interested in knowing that research a couple years ago indicated that helium does not need any more time to off-gas than nitrogen does. In fact, in light of the research, some people argue there is no need to include helium in dive calculations. The argument against it is that the same research that said helium off-gases at the same rate as nitrogen also suggested that we should be doing longer deco than standard algorithms anyway, so it does not hurt to plan for that extra time. There is no benefit, however, to doing that extra time during deep stops.

Finally am getting a quiet day to catch up.
That sounds like some very interesting and thought provoking research and conclusions. If they are saying that helium and nitrogen off-gas at the same rate but that we need to be doing longer deco stops then would it be fair to say that what they have concluded is that nitrogen off-gases at the same rate as helium and therefore we need to be doing deco more in line with algorithms for helium than what we currently do? I don't know - I haven't seen the research paper to know how it should be interpreted.