Flying After Diving --- A Theoretical Question

[Kstnbike]

OK, now lets look at doing the same series of dives at sea level and pretending that we are at altitude. The big difference is that unlike that series of dives at 8,000' altitude, during your 1 hour SI you are NOT at 0.75ata. You are at 1.0ata. At 8,000' you only reach 1.0ata at somewhere between 8' and 9' of depth. So to accurately simulate your sea level series of dives on an altitude table, you need to treat the whole series of dives as a multilevel of dive that bounces back and forth --- 1 hour at 40', 1 hour at 8.7', 1 hour at 40', 1 hour at 8.7'. Clearly, this leaves a different loading in your slow compartments.

That's why "Just plan the last day as if you were diving at altitude" isn't a good way to plan.

I know that plan isn't a good idea and I completely understand where you get the 8.7ft from (I understood that before you had to explain it). But what you are explaining is that I am going to have less residual Nitrogen leaving my body once I surface from the dive at sea level. This should be a good thing. By surfacing at a dive at sea level and planning it for altitude I am actually diving a more conservative profile. This should have lessened the overall load of residual nitrogen in all compartments making the flight after diving possible. Once you get to altitude in the aircraft the .75ata of 8000ft. altitude is taken into consideration. so you will off gas w/ greater intensity at 8000ft. That is why you plan the original dive as if you were diving at 8000ft. From my understanding 66ft. at 8000ft. and 66ft at sea level at both 3ata you just off gas w/ greater intensity when you surface. The reason I can't understand why the ppN2 would be any different is because the ppO2 stays the same. 1.4pp w/ 32% EANx is still 110ft whether it is 8000ft. or sea level so ppN2 must be the same.

I know there is the argument about the need to reduce the pressure by being in a chamber or by breathing different gas mixes but that is not applicable because when you ascend in the airplane to 8000ft. that is being done for you. Descending in the plane later doesn't affect anything because you are going from .75ata to 1ata again which is the same as descending in depth.

Basically what I am saying/asking is that if you PLAN the dive for that depth and account for the extra nitrogen load you will get from ascending to altitude is it safe to do that dive. What I have gotten so far from what I can interpret is a way to accurately simulate diving at 8000ft. at sea level. What I am looking for is reasoning behind whether or not it is possible to dive at sea level, follow an acceptable dive profile for 8000ft., add in the extra nitrogen load you will gain from ascending to 8000ft. and then actually getting in a plane and flying.

Kstnbike

 

[Kstnbike]

After a lot of searching I found part of the answer here.

http://www.diversalertnetwork.org/medical/articles/article.asp?articleid=20

The test they did here was diving to 60 feet for 55 minutes which is the NDL for that depth on the RDP. After a 3 hour surface interval they ascended to 8000ft. From their results there was a 8.3% rate of DCS in 36 trials.

Kstnbike

 

[evensplit]

Cabin altitude schedules and pressurization systems vary depending on the aircraft.

Rapid pressure fluctuations are possible with throttle adjustments particularly at lower altitudes and lower power settings. Ever had your ears pop at altitude when the aircraft started a descent?

 

[Kstnbike]

Cabin altitude schedules and pressurization systems vary depending on the aircraft.

Rapid pressure fluctuations are possible with throttle adjustments particularly at lower altitudes and lower power settings. Ever had your ears pop at altitude when the aircraft started a descent?

Rapid fluctuations are possible as is the risk of depressurization at 35000ft (which would just about screw anyone) but the maximum commerical airliner pressurization allowable is 8000ft. Descending would decrease your risk of DCS not increase it. There could be a possibility of micro bubbles getting all screwed up in your body due to the fluctuations but descending shouldn't affect you.

Kstnbike

 

[747pilot]

A little background on aircraft pressurization systems. The 8000' number is the highest altitude that US carriers fly their cabins at. This is approximately 8.7 pounds of differential pressure at normal cruise altitudes. I believe the biggest threat is if there is a rapid depressurization. I have never had a rapid depressurization in my 14,000 hours BUT I have had planes not pressurize after takeoff. With a light payload the cabin altitude, which follows the aircraft climb rate which could be as high as 4500'/minute. There is a warning system in Boeings that sounds off after the cabin altitude reaches 10,000'. At a climb rate of 4500'/ min., the cabin could easily go as high as 12,000' before the pilots could react. This is the most likely non normal situation. A rapid decompression could take the cabin altitude up to 45,000 feet in seconds. That would be a worse case senario. Highly unlikely but not something I would want to do after a dive. It would take the crew about 5 to 6 minutes to get the plane back down to an altitude where people could breath without O2 masks. I personally think this is why DAN and others recommend 24 hours SI before flying.
To answer someones question about flying a small plane after diving, I've never done that but there is a mountain pass that's between home and Puget Sound. I always wait at least 8 hours before driving over.

 

[fisherdvm]

I don't know much about planes, but the few military planes I've been on sure seems to have alot more "fluctuation" than civilian planes. Do they have different standards?

 

[anzac65]

...I have never had a rapid depressurization in my 14,000 hours BUT I have had planes not pressurize after takeoff.......

Wow 747pilot ! I hope you're not really a junior member, being as it appears that someone lets you drive a Boeing.....

 

[knotical]

Is the NOAA time-to-ascend table accessible online?

yes:
http://www.ndc.noaa.gov/pdfs/AscentToAltitudeTable.pdf
caveat: The pressure group designations are NOAA’s, so the Ascent to Altitude table should be used in conjunction with their other tables:
http://www.ndc.noaa.gov/pdfs/NoDecoAirTable.pdf