Apologies for the top-post. The most informative resource on this I've seen so far comes from DAN's 2002 workshop on flying after recreational diving: https://www.diversalertnetwork.org/files/FADWkshpBook_web.pdf
R On 25 July 2016 at 13:10, Rick Walsh <[email protected]> wrote: > > > On 25 July 2016 at 10:33, Linus Torvalds <[email protected]> > wrote: > >> On Sun, Jul 24, 2016 at 4:44 PM, Linus Torvalds >> <[email protected]> wrote: >> > On Sun, Jul 24, 2016 at 4:33 PM, John Van Ostrand <[email protected]> >> wrote: >> >> >> >> So rather than calculate the deco stop at 0 ft for a safe ascent to >> 8000 ft, >> >> maybe it should do a desaturation to an arbitrary percentage, like 1.5% >> >> above ambient at 0 ft. Isn't 6 half-times considered desaturated? >> > >> > That might be a useful thing to do regardless of no-fly times. And >> > yes, many dive computers seem to consider the two things the same >> > (sometimes with a "no-fly is desat time with a minimum 24-hour") >> >> Side note: I'm not going to have time to look at it, since the merge >> window for 4.8 just started, but I'm assuming that it should be fairly >> easy to just hook into calculate_deco_information() at the end, where >> we have the tissue saturation data set up for the after-the-dive >> situation. >> >> I *think* you could just solve it analytically by looking at each >> compartment, calculating how long to desat for that compartment >> (exponential decay towards surface pressure with the factors for that >> compartment), and just taking the max time. >> >> Sounds like something Robert could do in his sleep in five minutes. >> Robert? >> >> Or perhaps Rick knows what the VPM-B model considers desaturated. >> >> >> The VPM-B model uses the same method (ok there's a different assumption > on effective water vapour partial pressure but it's nearly negligible) and > same theoretical tissues as Buhlmann to track gas saturation, so the > desaturation time will be the same. The difference between the models is > in how the allowable gradient (difference between current ambient pressure > and tolerable ambient pressure) is calculated. > > Something similar to how Linus calculated the no-fly time could be done > with VPM-B, but I'm pretty certain it would also end up with a very low > no-fly time compared to dive computer calculations and guidelines by DAN > and others. I don't think we should implement a feature that produces less > than commonly accepted no-fly times, lest someone relies on it and ends up > bent. > > I wouldn't be surprised if it could be shown that the Buhlmann method > could be applied to determining no-fly times, but with a different > theoretical 'governing tissue' for lower than atmospheric pressure (e.g. > flying), which is slower than the slowest of the 16 Buhlmann tissues. I > have no idea what half-life the governing tissue would have, or what the > corresponding 'a' and 'b' factors would be. > > Being proprietary, I don't think we will ever know how Suunto calculates > no-fly times. Maybe we could look at data from the dive computer and > back-analyse something. > > The OSTC method is apparently just a portion (default is 60%) of the > desaturation time: > http://forum.heinrichsweikamp.com/read.php?2,9870,9878 > > A no-fly feature could be implemented, but unless an analytical method is > published and accepted, I don't know what the calculations should be. I > think options for discussion could be: > 1) Implement DAN's guidelines without complicated calculations: 12hr for > single no-deco dive, 18hr for repetitive no-deco dive, >18hr (maybe 24hr?) > for deco dives > 2) Use the method Linus did previously, along with a really conservative > factor (ignore gradient factor preference) to come up with something > similar to DAN or dive computer calculations > 3) Use the OSTC method: 60% of desaturation time (we have to determine how > many half-lives there are in effective desaturation) > 4) Use a Buhlmann based method, but with a really slow "tissue", fudged > until we get the answer we expect > 5) Something completely different - are there guidelines from someone's > NAVY? NASA? > > The problem with all these methods is that they need to be calibrated to > get the expected value (or risk being more aggressive than any other > recommendation). Unless it's backed by published research, I think I'll > personally stick to DAN's advice. > > Cheers, > > Rick > > >
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