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I can make statements about what appears risky to me. I have experience with nitrogen narcosis. I have experience and education related to the failures in human memory skills and in our inability to correctly perceive things. The human brain tends to see what it expects to see. I haven't found that to be less of a factor underwater.
My statements are tempered by my lack of experience but it doesn't mean they aren't valid. In areas in which I am a recognized expert, those with intelligence but less experience sometimes draw incorrect conclusions but their lack of experience or my belief that they are mistaken doesn't lead me to tell them to stop thinking for themselves or discussing it until they have as much experience and knowledge as I do.
In answer to your direct question, Lacking experience and knowing nothing is not the same thing and I can make statements about my conclusions and reservations because that is why we are here. You are a bad ass scuba diver that has earned a tremendous amount of respect. Which statement that I have made do you feel is wrong? I don't need a response but I would appreciate one if you might take the time. I will not argue with you.
Thanks
My statements are tempered by my lack of experience but it doesn't mean they aren't valid. In areas in which I am a recognized expert, those with intelligence but less experience sometimes draw incorrect conclusions but their lack of experience or my belief that they are mistaken doesn't lead me to tell them to stop thinking for themselves or discussing it until they have as much experience and knowledge as I do.
In answer to your direct question, Lacking experience and knowing nothing is not the same thing and I can make statements about my conclusions and reservations because that is why we are here. You are a bad ass scuba diver that has earned a tremendous amount of respect. Which statement that I have made do you feel is wrong? I don't need a response but I would appreciate one if you might take the time. I will not argue with you.
Thanks
PfcAJ
Contributor
Imo its tough for someone looking to learn about the merits and demerits of RD when there are more and more people chiming in about what it is or is not that have little experience or understanding of the matter at hand.
While I can appreciate an opinion, one formed without complete (or at least complete enough) info isnt worth a whole lot. Not a dig at you personally, of course.
It would be like me opining about something regarding skydiving. I don't do that, I have no training, and no experience. Is my opinion about what is or is not dangerous equal to those with training and experience? Probably not.
Michael Guerrero
Contributor
The lowest stress dive, is the one that is the lowest supersaturation in the dive. That's indisputable. Which means it scores a bigger number on the Simon/Kevin ISS.Kinda works backwards.
But in your Simon/Kevin version of ISS, it overlaps everything, and keeps compounding together, over and over duplicating data... which is why you get such a big useless number.
I'm struggling to understand this post. Doesn't the lowest supersaturation have to mean the lowest aggregate supersaturation? That would mean the profile with the smallest overall number, not the largest one, right?
All the tissues on-gas at the same time, so each is a little store of inert gas. That gas ultimately has to come out of those tissues during supersaturation as you ascend. So I think you're right to say that those gases are compounding, but not duplicative. They are aggregate.
Michael Guerrero
Contributor
@Shearwater : feature request - please implement an option to have a GF lo higher than GF hi, with all necessary and associated warnings and indemnifications. Thanks!
PS, the Teric looks awesome and can't wait to get one!
The Chairman
Chairman of the Board
I'm driving one for the first time here in Cozumel... very easy to read, the AI is awesome, and the compass is devine.
danielmewes
Registered
Hi Simon, I am very confused by this statement. This cannot possibly be true. Otherwise we could get perfect decompression schedules for a given decompression time simply by optimizing the ascent for the lowest integral supersaturation. Decades of decompression research and comparative studies such as the NEDU deep stop study would have been pointless.
I'm happy to throw integral supersaturation into a numerical ascent profile optimizer and see what kind of profiles it will generate. I have a suspicion though that it will simply go straight to the surface, as quickly as possible, as that should lead to the fastest off-gassing and I think lowest integral supersaturation among all possible ascent profiles?
Surely this is not what you mean, but I'm honestly curious what your actual statement here is.
To be honest I have been quite confused in the previous threads over the years about the use of integral supersaturation in explaining deep vs. shallow schedule results. Assuming that integral supersaturation for an otherwise identical dive is connected to DCS risk through a monotonically increasing function (which seems to be what your statement is saying?) would simply make integral supersaturation another model to calculate ideal ascents, with its own set of underlying assumptions. Integral supersaturation does not consider bubble dynamics, so using it to explain why bubble models are not working seems like saying general relativity isn't working because predictions made with Newtonian physics disagree with it (comparison borrowed from another poster here).
I also do understand Ross' point, that ISS heavily depends on the set of model compartments that you use to compute it, as those are the parameters of the "ISS deco model". You will get very different predictions from ISS depending on which compartment parameters you use. ISS with one set of compartments might say that profile A has lower risk of DCS compared to a profile B, while with a different set of compartments it might predict profile B to be lower than A. (again, I haven't verified this, but I can attempt to come up with an example if this doesn't sound right to you)
I'm not trying to discount the evidence against deep stops of the form predicted by some of the current bubble models, I think that evidence is starting to be quite conclusive. I just do not see how ISS is a good measure for the decompression stress / goodness of a profile.
On a side note, I'm a bit concerned about how deep stop profiles are being conflated with bubble models in general. You can model bubble physics with parameters that give you very shallow stops. It's not a property of modelling bubbles per se, simply a property of the specific model parameters commonly used (even though there seems to be a tendency in such models towards deeper stops, if some parameters such as the compartment model are kept unchanged from dissolved gas models). The reason this concerns me is that a future bubble model might be dismissed by the community because they have been hearing bubble model = deep stops = bad, even if said future model might actually predict safer (and likely shallower) profiles.
First, you have to define "perfect decompression schedules". Normally it means the minimum total decompression time without symptoms or tissue damage. Your post leads me to suspect that you are significantly underestimating the human and environmental variability in the equation. People don't ingas or outgas 100% consistently, between individuals or dives.
huwporter
Contributor
I think you have this totally wrong.
1. Direct ascent to the surface would have massive supersaturation over a long time specifically in the slower tissues which will have had no opportunity to off gas. Although you are correct that it would certainly have the fastest off gassing, that would not be much comfort compared to the DCS you'd probably also have.
2. You seem to be missing the in-context point that integral supersaturation is -one- way of comparing two profiles of the same length, to give one possible indication of decompression stress comparison between the two.
3. Nowhere does Simon say or imply that integral supersaturation is the only thing that is important in decompression schedules.
danielmewes
Registered
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, ISS 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.
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