No-fly in Subsurface?

[Linus Torvalds]

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.

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").

                      Linus