On Sun, Jul 24, 2016 at 2:22 PM, Linus Torvalds < [email protected]> wrote:
> On Sat, Jul 23, 2016 at 11:48 PM, Miika Turkia <[email protected]> > wrote: > > Hi Linus, > > > > what happened with the no-fly feature that you worked with some years > > ago? Did it turn out to be totally bogus, not worth including in > > master? > > It wasn't really totally bogus, but it also wasn't really useful, and > it was something that might be seen as a bit too dangerous. > > Basically, if you just use the same "is it safe to ascend" logic as > for diving, and just extend it to "is it safe to ascend to even lower > pressures". gthings *work*, and logically and physically it should be > equivalent, but there may be other factors. > > The cabin pressure is generally 750mbar (equivalent to ascending to > 8000 feet) or higher, so using that as the "is it safe to ascend" > model is something that I still feel is technically sound. However, it > turns out that if you actually do the calculations, the calculated > no-fly time is generally in the vicinity of half an hour or so. You > don't even get _close_ to the 24 hour PADI suggestion. I think with > repetitive diving, it might have been something like 90 minutes or so. > > So basically, it turns out that the no-fly time has almost nothing to > do with the actual decompression algorithms you use while diving. > Although my Cochran offers very generous amounts of no-deco time the one conservative feature is its no-fly time. At the end of a week of live-aboard diving it often displays no-fly times well over 24 hours. I can only guess that it's offering desaturation time. The fact that in practice a 20 tissue model no-fly time is longer than the 16 tissue model also suggests it's a desautration level and the longer tissues of the 20 tissue model means longer desat times. 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? As an aside the Cochran also gives a deco credit (nitrogen deficit) for flying, which might only help your bottom time if you plan on parachuting out of the air plane with your dive equipment on. > The reason for that might be due to various factors: > > - the usual "let's add some safety padding" taken to extreme measures > > - the fact that in the air, you have other issues than just > decompression sickness - no hospital easily reachable etc. Maybe you > had a slight case of DCS even before flying, and now the lower > pressure trigger more noticeable symptoms: and you have no chamber and > are sitting in a plane for the next six hours.. > > - the fact that maybe the deco algorithms don't work as well, because > you're getting closer to other limits. Some people start to get > altitude sickness at roughly 750 mbar pressures anyway, and that isn't > directly related to any pressure differential, just to lower pO2. > > So there are various reasons that might not be directly about the > usual nitrogen/Helium pressure differential that the deco algorithm is > all about. > > Anyway, the normal deco algorithms definitely don't support long > no-fly times. But there may be other valid reasons for them ("you gas > imbalance is a slight stress factor on your body, flying adds _other_ > stress factors, they may interact"). > -- John Van Ostrand At large on sabbatical
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