rwpatterson357 once bubbled...
I guess I'm looking for some hard evidence that going from 1ATA to 0.80ATA while flying is dangerous. I understand that the problem is due to tissue saturation with nitrogen levels above normal due to diving...and that a larger gradient will exist when I further reduce atmospheric pressure by flying.
I'm just trying to get a feel for whether or not the drop down to 0.80A is really significant. If you compare it to ascending in water its only equivalent to 6-7 feet of rise.
russ
Alas, there is no
hard evidence, that is in the medical sense. DAN is doing such a study as we speak. In the just concluded meeting of May 2003, we were shown preliminary reports for a direct ascent with 1-6 hours after a single NSL dive to 60' on air, a minority of subjects got joint bends.
If you take a decompression program like decoweenie, Vplanner, Decoplanner, GAP etc., and plot a dive that theoretically will surface at 0.8ATA, it provides the necessary added deco stops for what normally is a NSL dive. While this is theoretical, it gives you an idea that the 12,18 or 24h wait time is conservative.
If you review the USN FAD table somewhere linked on this eboard, you'll see reductions on FAD wait time depending on the diver's last dive within 24 hours. WARNING: per the table's author, the table is an extrapolation based on few data points.
To fly, the issue is what decompression for a dive is required to surface below 1 ATA that keep bubbles below a critical radii. This depends entirely on the tissue saturation before flight, which depends on the depth, time, breathing gases used and any accelerated decompression aides, such as 02 or mild exercise, that can enhance offgassing.
In prior threads, Dr. Deco and I discuss the theory behind 02 pre-breathes before flight for non-divers. Such theories are actually in practice by some technical divers on the assumption that their tissues are offgassed enough after a trimix deco dive within hours before flight. THERE is NO data to verify this claim, although you can easily run numbers on theoretical limits using a decompression program, like Vplanner, and see that if they can dive to 200-300' and deco successfully to surface at 1 ATA with 02, what faith can user give such a program to calculate surfacing at 0.8 ATA? You'll find the added deco obligation quite small indeed.
However, to date, there is no 'hard' data to verify such assumptions, but it does make sense. Further the ascent to cruising altitude is not rapid, and provides additional time to offgas.
The theory behind the longer wait in FAD, is that 'slower' tissues continue to on-gas even after a dive. So while some sense can be made of long FAD times without 02 decompression, its known that desaturating pilots with 02 to insure inert gas has been reduced enough to be of use to NASA for EVA procedures and for US Air Force protocols, that the same data can be extrapolated to divers to insure a safe flight to 0.8ATA.
However, since there is no pressing need to dive up to the day of flight, even the smallest risk for joint bends seems unwarranted. In special-ops procedures, the operatives have the benefit of asking for 02 on board to treat the pain, and such is not available in civilian flights.
Take Home Message:
The rules for FAD remain for recreational divers unchanged as of Oct 2002 from DAN
What the future holds:
FAD within hours for slight increase in risk of joint DCI for recreational divers
What technical divers can do:
Formal 02 decompression calculated to included FAD
Formal 02 deco to desaturate tissues after recreational dives
