Flying after Diving - the science

[S. starfish]

I've been thinking about flying after diving and was wondering if anybody here could explain, in a fair bit of detail, the science behind it as at the moment I'm in a bit of a rut.

To me it breaks down like this: When we go diving we are surrounded by an additional atmosphere of pressure for every 10 meters we descend. This increased ambient pressure requires the pressure of air delivered to us by our regluator to be higher so that our lungs can inflate. With this increased pressure comes more of all the gasses that are in regular air, our main concern here is the increased partial pressure of nitrogen. With a higher pp of nitrogen in our air than in our tissues a pressure gradient is formed and the tissues begin to abosorb nitrogen on their way to reaching a state of equilibrium between tissues and air. When we ascend the opposite happens, the pp of nitrogen in our air becomes less and the nitrogen in our tissues is expelled. Issues arise when we ascend too fast making the pressure gradient so steep that the nitrogen cannot be effectively removed from our bodies fast enough and we get all bubbly. With my current level of understanding my problem is this: Say you're flying in a boeing 747. It's going to take it at least ten minutes to reach it's optimum cruising altitude, which is about 10km. Now no matter how high you fly in a commercial liner the pressure difference between takeoff and crusing is not going to exceed one atmosphere of pressure, especially since the cabin is pressurised. Supposing that you were to leave the atmosphere and take 10 minutes to do so, it's the equivaent of ascending from a dive at 1m per minute, which is fairly slow.
So what am I missing here? Does it have to do with the various absorbtion rates of different tissues? Is it that, even though after a dive we're fine at ground level the change in pressure during a flight is too much for nitrogen loaded compartments and "pushes them over the edge"? If anybody could explain it in more depth to me or point me in the direction of some literature on the subject?
If it helps with the explanations i was up until recently a cellular biology and genetics major, so i'm quite familiar with scientific terms and such.
thanks

 

[kjundvr]

OK you dive to arround 50 ft when you come up you have to off gass the excess nitrogen you aquired during your dive. If you decide to hop on a plane (pressurised to 8,000-10000ft) before you are finished offgassing from your dive you must off gass the 50ft of nitrogen plus the nitrogen normaly in your system at sea level. Also keep in mind that it is not necessarily the absolute change in pressure but the speed at which the pressure change occures. So in a typical jet plane which has a climb rate of 2000 to 2500 ft/min (or more) you decrease pressure from 1atm to ~.6atm so add in the 50 feet and the pressure gradiant from depth to 8000ft is big which can cause rapid bubble formation. Hope that this may clarify things a bit but just in case, here is a very good article.

http://www.skin-diver.com/departments/diversafety/flyingafterdiving.asp?theID=98

 

[S. starfish]

Interesting, thanks.

 

[scububa]

I'll start off by acknowledging I am no expert and really ought to know better than to jump in here...

Rather than try and answer the question asked directly, I am making an observation to which if it is off-base, I am sure will be corrected swiftly...

While most of your setup seems okay, I would not make this statement

"With this increased pressure comes more of all the gasses that are in regular air"

There is not more of the gas(es). They are just at different pressures (in this case higher).

Also, the statement that

"mind that it is not necessarily the absolute change in pressure but the speed at which the pressure change occures"

Isn't it always both. If the absolute pressure didn't matter, then there would be no such thing as decompression diving. (I said that while acknowledging the line of reasoning that says there is no such thing as a non-deco dive). You would just come up at an appropriate rate. But, deco stops allow for a timed interval to allow changes to absolute pressure changes to equalize. (I also realize that some will say deco stops are averaging the ascent rate to a (s)lower number.) The trade off is staying at depth too long where the absolute differential drops and the off gassing slows to a rate that is not an advantage to the diver or even crosses into on gassing.

Also, it seems that an effective approach for no-fly time determination should factor in for a potential sudden lost of cabin pressure. If that were not a statiscally relevant factor, would airlines put O2 masks at every seat? Then both the rate and the abolute kind of peg the needle.

The smoking lamp is lit

 

[jimclarke]

The way diving used to be done under Haldane rules was pretty close to what we do now but more simple to describe. PLEASE SEE THIS AS A THEORETICAL ANSWER AND DO NOT TAKE IT AS GUIDELINES.

If your body was saturated with gas at any ambient pressure you could safely halve the pressure. This meant you could saturate at 2 bar (10m) and then return to 1 bar at the surface.

Now if you fly you are going to "return" to a surface pessure (cabin pressure) of 0.75 bar. By the same rule you can only saturate your body at 1.5 bar (5m).

Generally dive tables were designed to stop your slow tissues getting to the equivalent saturation at 2bar. Before Flying that needs to be 1.5 Bar (5m)....so its shallower or shorter.

Of coures the cabin pressure acident makes it all even worse. The cabin pressure could drop to 0.26 Bar. You could suffer decompression sickness even if you had aclimatised on a mountain at 0.52 bar (5200m). After such an acident the aircraft dives rapidly and everybody gets oxygen. For a dangerously over saturated diver full of microbubbles it might well prove fatal.

 

[cmalinowski]

Of coures the cabin pressure acident makes it all even worse. The cabin pressure could drop to 0.26 Bar. You could suffer decompression sickness even if you had aclimatised on a mountain at 0.52 bar (5200m). After such an acident the aircraft dives rapidly and everybody gets oxygen. For a dangerously over saturated diver full of microbubbles it might well prove fatal.

So might the accident

 

[Gene_Hobbs]

A good place to start for me is the US Navy Dive Manual.
http://www.coralspringsscuba.com/miscellaneous/usn_manual.htm

Volume II, Section 9-12 Diving at High Altitudes
Volume II, Section 9-13 FAD

Are two basic chapters on the topic. From there, you can pull the papers the Navy references to go more in depth. The FAD chapter is worth note since it has the table we published for surface interval prior to 8,000' altitude exposure.

There have been two workshops on the topic:
Sheffield PJ (ed). Flying after Diving. 39th Undersea and Hyperbaric Medical Society Workshop. UHMS Publication Number 77(FLYDIV)12-1-89. Bethesda: Undersea and Hyperbaric Medical Society; 1989; 222 pages.

The other is available through DAN. (site is not coming through Duke firewall so I can't post the link. Sorry...)

Take care,
Gene

 

[jimclarke]

Volume II, Section 9-12 Diving at High Altitudes
Volume II, Section 9-13 FAD

Gene, thanks thats really a great link.
If the dive computer showed the pressure group currently reached it would enable me to end the dive at the right point to allow a planned drive home through the hills.

So might the accident

At least the 2 risks dont add. If the accident kills you the bubbles wont get a chance.