Spisni study

[tursiops]

He seems to be saying

when taken literally

You might be better off reading what he actually says, rather than reading your interpretation into what he says.

 

[danielmewes]

You might be better off reading what he actually says, rather than reading your interpretation into what he says.

What do you think "when taken literally" means?

I'm asking for clarification, exactly because "what he actually said" here seems wrong, as explained in my post.

Anyway, I'll wait for Simon to respond himself. I'm interested in what he meant, not in debating the use of the words "he seems" in my post or any other meta discussions.

 

[tursiops]

So I ended up implementing the actual calculation. A few notes on the implementation:
* Integration is numeric, in 10s intervals during the dive, followed by 40s intervals for 1 hour after surfacing, and another 23 hours after that with 200s integration steps. The reason I'm having longer steps post dive is merely because I'm using code I had written before, were performance was important. Happy to calculate this again with 10s intervals throughout the dive + 24 hour surface time interval if you want, but it shouldn't make a difference.
* I'm using 9 tissues in this calculation with the following half-times: 2min, 5min, 10min, 20min, 40min, 80min, 120min, 240min, 480min. Again, this is what I had already implemented (I was implementing the RGBM). Happy to change it to ZHL16 tissues or whatever else if you think it's important. Tissues on- and off-gas proportional to the pressure differential in this calculation.

What I found was that which profile has the lowest ISS depends on how oxygen is considered in the calculation.

a) Ignoring oxygen alltogether (i.e. assuming oxygen partial pressure is 0atm in all tissues at all time):
* NEDU shallow profile: 9,684 atm s
* NEDU deep profile: 10,624 atm s
* NEDU bottom time, then ascending within a minute to the surface: 17,681 atm s

-> NEDU shallow best, followed by NEDU deep. No decompression worst by far.

b) Assuming oxygen partial pressures proportional to nitrogen partial pressures (ppO2 = ppN2 / 0.21):
* NEDU shallow profile: 55,022 atm s
* NEDU deep profile: 62,150 atm s
* NEDU bottom time, then ascending within a minute to the surface: 51,302 atm s

-> No decompression best, followed by NEDU shallow, with NEDU deep being the worst

All three have a descent of 3 minutes from 0 to 170 fsw, followed by 30 minutes bottom time at that depth. (I realize this thread is not about the NEDU dive profile or even air diving per se, I just use these as an example here)


So you are right that my intuition was incorrect here. I did not consider the oxygen window when I speculated about the outcome. I believe that this is because the oxygen window adds a non-linear factor to the supersaturation at a given time. The non-linearity comes from the fact that supersaturation is max(0, pressureGradient), and doing a staged decompression can keep more tissues in the 0-case (undersaturation). Hence ISS with oxygen window does not behave the way that I expected.




I'm not missing this point, that is exactly the point I'm making! Thank you for putting it into clearer words than I did. (my argument for this point was by contradiction, so I can see why it was a little obscured)

That being said, SSI by itself seems to be a comparably poor way of providing an indication of decompression stress, given that under the right circumstances it will tell you that going straight to the surface after 30 minutes at 170ft results in less decompression stress than a reasonably safe, staged decompression.



That is literally what he says in the quote I included in my post. Let me quote him here again:


He seems to be saying that for two dive profiles that differ only in their decompression staging, the one that has lower ISS has lower risk of DCS and vice versa. I'm not going to give you a mathematical proof about ordered sets here, but I'm pretty sure that Simon's statement that for any two decompressions A, B
ISS(A) < ISS(B) => P(DCS(A)) < P(DCS(B))
implies that there exists a monotonic function f, such that for any decompression x (from the same dive), we have
DCS(x) = f(ISS(x))

Hence ISS, according to Simon's statement, when taken literally, would indeed be the *only* thing that is important for the risk of DCS when it comes to choosing from a number of alternative decompression schedules for a given dive.

I'm pretty sure that Simon did not mean this. That's why I asked him to clarify how to exactly interpret this statement.

Your logic is flawed. The comparison needs to be with deco strategies that actually avoid DCS....going straight to the surface does not do that. This I'm sure was Simon's point; given two valid deco strategies, the one with lower ISS is "better." You are not comparing valid deco strategies if you go straight to the surface.

 

[huwporter]

That being said, SSI by itself seems to be a comparably poor way of providing an indication of decompression stress, given that under the right circumstances it will tell you that going straight to the surface after 30 minutes at 170ft results in less decompression stress than a reasonably safe, staged decompression.

You must understand that going straight to the surface results in a profile of a different length compared to doing a staged decompression... so dude, you are totally missing the point of using ISS as a tool to compare profiles of the same length.

 

[danielmewes]

Your logic is flawed. The comparison needs to be with deco strategies that actually avoid DCS....

At which % DCS incident rate would you say that a given deco strategy avoids DCS?

My point is that ISS as a criteria leads to wrong conclusions when applied over the whole space of possible decompression profiles. So I'd like to understand better why we should believe that it leads to correct conclusions when applied to subset of the possible profiles. And, most importantly, what exactly that subset is and why.

Please correct me if I misunderstand you, but you seem to be saying that ISS is a good criteria to decide which of two profiles is safer, as long as both of those profiles are set up to avoid DCS (whatever that might mean). I'm curious why you believe that ISS works in those cases, and not in others.

 

[danielmewes]

You must understand that going straight to the surface results in a profile of a different length compared to doing a staged decompression... so dude, you are totally missing the point of using ISS as a tool to compare profiles of the same length.

No, I'm including a subsequent 24 hours at the surface in my calculations. In the case of the "immediately to the surface" profile, I did in fact add the additional minutes to make it equal in length to the profiles with staged decompression. The time that I integrate supersaturation over is equal in all three cases in my computation. No difference in length, only differences in depth at different times.

 

[huwporter]

No, I'm including a subsequent 24 hours at the surface in my calculations. In the case of the "immediately to the surface" profile, I did in fact add the additional minutes to make it equal in length to the profiles with staged decompression. The time that I integrate supersaturation over is equal in all three cases in my computation. No difference in length, only differences in depth at different times.

If you are going to be this deliberately obtuse, then there is no discussing with you. You may as well claim that Simon is saying the best decompression is to go immediately into a vacuum, since the off-gassing will be fastest of all, and don't worry about all that inconvenient suffocating and blood boiling business.

 

[danielmewes]

If you are going to be this obtuse, then there is no discussing with you.

I didn't mean to. I thought you were talking about different time intervals for the integral (which would have been a very valid concern, I almost made that mistake in my calculation). Did you mean the time under water was different?

(fwiw I don't think length of the decompression that happens underwater is a meaningful measure here. If ISS without oxygen window "prefers" going straight to 0 fsw over doing a staged decompression, it will likely also "prefer" going straight to 10ft and staying there throughout the decompression time. Put differently, why would ISS suddenly stop being a meaningful comparator of DCS risk if you go to 1atm during your decompression, instead of staying above 1.3atm? Maybe there is a reason, and I'd be curious to hear it. I can't think of any right now.)

 

[huwporter]

I didn't mean to. I thought you were talking about different time intervals for the integral (which would have been a very valid concern, I almost made that mistake in my calculation). Did you mean the time under water was different?

(fwiw I don't think length of the decompression that happens underwater is a meaningful measure here. If ISS without oxygen window "prefers" going straight to 0 fsw over doing a staged decompression, it will likely also "prefer" going straight to 10ft and staying there throughout the decompression time)

Of course the time under water has to be the same to make any valid comparison... It should be obvious that you can design deco profiles of different lengths (...lengths underwater, sigh...) with different decompression risks. It should also be obvious that nobody is proposing going straight to 10 feet and staying there as a realistic deco strategy.

To spell it out, the entire context for this whole discussion (over many years now) is, given two deco profiles of the same total length, Z minutes, both consisting of X minutes bottom time at depth D plus Y minutes of decompression, both of which have an acceptable risk of DCS, what is the optimal shaping of the Y minutes of deco time to minimise decompression stress.

You are applying reductio ad absurdum to a strawman argument that nobody is making.

 

[danielmewes]

You are applying reductio ad absurdum to a strawman argument that nobody is making.

I'm pointing out issues with an argument that people are in fact making (the argument being that you can use ISS to get an idea of why a deep stop profile is worse than a shallow stop profile), by pointing out extreme examples where the same argument obviously fails. I could make this argument with less extreme examples, but then it would be nowhere as obvious that ISS doesn't generally work to compare profiles.
Extreme example or not, I'm asking for justification for why we think that ISS does work in the cases used in the arguments that are being made, if it clearly doesn't work in general. If you want to use something that doesn't generally work, you should be able to explain why you think it will work in the case where you're using it. I have no doubt that Simon (or David Doolette, or others who have used ISS in this discussion) can make that argument, and learning what it is is what I'm here for.

For what it's worth, ISS (without oxygen window) for ascending after 30 minutes at 170 ft within 1 minute to 10 feet, then staying there for 182 minutes, then surfacing (same total length as the NEDU deep and shallow profiles I used): 51,037 atm s

Interestingly, that's even lower than the going straight to the surface case (51,302 atm s). This indicates that my intuitive understanding of how ISS behaves is still a little off, but in any case my argument stands.