No-fly in Subsurface?

John Van Ostrand john at vanostrand.com
Sun Jul 24 16:33:22 PDT 2016


On Sun, Jul 24, 2016 at 2:22 PM, Linus Torvalds <
torvalds at linux-foundation.org> wrote:

> On Sat, Jul 23, 2016 at 11:48 PM, Miika Turkia <miika.turkia at gmail.com>
> 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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