Driving to altitude after diving...

[DepartureDiver]

Hi Steve. I'll try to answer one step at a time.

Going from the surface to 33ft down, you'll increase pressure from 1bar to 2bar (33ft equals 1bar).

correct of course

Going from 0 to 1200ft up, reduces the pressure from 1bar to 0.970bar (in a standard atmosphere, 59°F at sea level and 29.92in pressure), which is equal to just about one foot difference in depth under water.

I get it at .96 ... but correct again and the 1 foot analogy is a good one.

I know that I go up and down more than one foot while diving "level" and that hasn't ever been a problem for me.

This is where it starts getting a little off. While this is not entirely correct, in simplistic terms altitude diving is based upon ratios. So a 2:1 ratio at sea level is a change of 33', The same 2:1 ratio at 6000' is 26'. Using your initial thought, you could just shave 8 feet off a dive and then fly (commercial cabin pressure being 8000') ... but we know this doesn't work. What needs to be done instead is to make calculations as though the dive occured at the elevation that is going to be ascended to ... which is also more complicated than it sounds due to pressure differences, etc. Back to your initial example of the 2:1 ratio, what must not be forgotten is that the greatest ratio changes occur in the first few feet. For example a 2:1 ratio is reached at 33', but it is not doubled again until 99'. So seemingly small altitude ascents post dive can really be more significant since the greatest ratio change is upon the initial ascent. But a 1000' ascent should easily be handled by just being more conservative with proper dive ascent procedures and a surface interval. But they still should be taken into account since there have been stories of people getting hits after only slight ascents. However these may also have been after repetitive dives that might have been marginal. Hope that helps.

 

[H2Andy]

sean:

as you know, a minimum "dive to fly" time is usually 12 hours after a single day of
diving (per DAN). In that case, you would be going to 8,000 feet (the pressure at
which airliners are pressurized).

In this case, you will be ascending to 1200 feet, so while not as drastic, you still
have a "dive to fly" time concern here.

to be on the completely safe side, you should treat this as flying after diving and
wait 12 hours (impractical as this may sound).

that said, here's a chart i've found on the net. make sure you check Dr. Kay's
credentials before using, but he appears legit:

http://faculty.washington.edu/ekay/altitude.html

it seems to require 2 hours after low stress, multiple dives, and 8 hours after moderate stress,
mulitple dives, to get you to 2000 feet (erring on the side of caution there).

 

[fan(t)a(s)tic]

Brian,

thank you for taking the time to explain this to me!

Steve

 

[DepartureDiver]

Brian,

thank you for taking the time to explain this to me!

Steve

you're welcome ... it's always great discussion.

 

[BRW]

Steve,

There's a book called High Altitude Diving that explains
how to compute wait time for altitude excursions (FAD
is a subcase to 9000 ft) using the repetitive group approach.
We developed it here at Los Alamos National Laboratory,
and it has also been sanctioned by/for USN Operations.

It's real straightforward, correlated with USN and LANL
data, and available from Best Publishing Company. Has
tables, graphs, and worksheets.

Answers your questions, plus others.

Experiments at DAN suggest many of the FAD rules
are too conservative so far, and things like the "wait
til D group to fly" might be OK.

BW

Brian,

thank you for taking the time to explain this to me!

Steve

 

[pch90265]

Wow,

Thanks all for the detailed responses. This weekend I'll be twelve hours from my last dive to my drive home, so I feel relatively safe... That said, I'll pick up High Altitude Diving before I make my next Catalina trip.

--SM--