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MaxRahder

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(I hope this question hasn't come up recently!)

I am a private pilot who dives occasionally. I've seen guidelines such as this one http://faculty.washington.edu/ekay/altitude.html, but many guidelines seem overly conservative. Why isn't flying after diving treated like additional loss of depth? For example, flying 3,000' is about 3" of Mercury or about 0.1 atmosphere. Can't that be factored into one's interpretation of depth tables? But I haven't seen that done. Why not? What's different about the change in pressure due to flying and the change due to diving? Let me take an example from the guideline (above)--it says that one should wait 8 hours before flying to 4,000 feet. That's about 4" of Mercury. Isn't that like diving to about 4 or 5 feet and ascending? What am I missing?
 
Hmm.. trying to apply logic to the dive industry. This should be interesting.
 
To carry the nitrogen question one step further: During your safety stop, (or during your deco stops if you're doing that kind of diving), you've spent time hanging around, off-gassing enough to get out of the water and continue your life on the surface. Make ONE dive for forty minutes, do a three-minute safety stop, and BAM: No flying for you for TWENTY-FOUR hours! Something doesn't add up here, or else maybe there's something that I'm misunderstanding... (Yeah, probably the latter)
 
I think the issue is that there is very little clinical research on the effects of flying after diving. Thus, the dive industry is reluctant to make any claims or guidelines.
 
The basics go all the way back to Haldane, who discovered that one could approximately halve the pressure and have reasonably low probability of caisson's disease or DCS. That's equivalent to doing a long dive to 33' and then ascending to sea level. Nowdays that is considered too risky or aggressive and minimum bends depth is in the low 20' range for most models.

At 8000' the air pressure is about 75% of sea level. To apply that same "double the pressure" rule would limit you to doing a long dive to about 3/4 of 33' or 25'. The most simplistic explanation of the altitude diving rules is that, by using an equivalent depth about 4% higher for every 1,000' of altitude, you will keep the RATIOs of pressure more or less the same as the equivalent sea level dive. Or to phrase it another way, you just use "atmospheres of pressure" for measuring your depth, but use the actual atmospheric pressure at you altitude instead of standard sea level atmospheric pressure.

Think RATIOs of pressures rather than absolute pressures and everything makes more sense. (Actually, some things like bubble surface tension and water vapor pressure really are absolute pressure, but the dominant terms are ratios between bottom pressure and surfacing pressure or pressure at altitude).

Flying after diving is similar, but not quite the same as diving at altitude, because your very slow tissues are loaded up at 1atm rather than the reduced pressure of altitude.
 
I am a private pilot who dives occasionally. I've seen guidelines such as this one http://faculty.washington.edu/ekay/altitude.html, but many guidelines seem overly conservative. Why isn't flying after diving treated like additional loss of depth? For example, flying 3,000' is about 3" of Mercury or about 0.1 atmosphere. Can't that be factored into one's interpretation of depth tables? But I haven't seen that done. Why not? What's different about the change in pressure due to flying and the change due to diving? Let me take an example from the guideline (above)--it says that one should wait 8 hours before flying to 4,000 feet. That's about 4" of Mercury. Isn't that like diving to about 4 or 5 feet and ascending? What am I missing?
You’re not missing anything, you are quite correct. And one way to handle the question it to assume that all your diving is being done at 3,000 ft. If your going to fly at 3,000 and you use a 3,000 ft altitude table to dive on, then all is well.

By way of background, the original military recommendation was that you could fly in a pressurized aircraft if your were in group D or below. That was later changed to group C since it turned out that if you were a D diver, but almost into E, you could not ascend to 8,000 feet.

I’ve occasionally used the fact that 2 hours of surface breathing pure oxygen will take you from group N to group A to my advantage.

The recreational community has had bad experience with flying after diving. There were recommendations for 12 hours and then 18 hours and now it’s 24 hours. Some feel that this had more to do with poor ascent technique and inability to properly use tables and/or computers that it did with failure of the group C concept. Some of the more cynical observers think that the 24 recommendation was produced by legal shy docs and grabbed onto by some resorts that would just assume have you out of the water for a day of your vacation.
 
I am also a diver/pilot, and if you are interested in this take an altitude diving class. We covered all of this. There are tables that can be used for this sort of thing.

Personally, I don't go near it. As a pilot, you have two-fold risk:

1. Risk of DCS
2. Being able to still fly the plane if you take a hit.

Personally I'm more worried about #2.

Also, the guy who taught my class had a DCS hit on a plane once. It had nothing to do with diving, as he hadn't been in the water for a couple weeks. He was just dehydrated and such, and on a commercial flight he got a hit, and recognized the mild symthoms because of his diving experience/education. IIRC, he had skin bends and got two black eyes during the flight.
 
mattboy:
I think the issue is that there is very little clinical research on the effects of flying after diving. Thus, the dive industry is reluctant to make any claims or guidelines.

Duke Hyperbaric Medicine does a large amount of Flying After Diving studies.
When I took a tour there they said that they have gotten it down to a 4-hour surface interval with no issues on a normal multiple recreational dive setting (I can't remember the specifics of the dive though).
 
Here's the NOAA ascent to altitude tables.

You'll need to use the NOAA tables, as well as the NOAA residual tables to yield the correct pressure group for use with the Ascent to Altitude table.

We do an equivalent (to flying in a light plane) after diving every weekend. We dive at Monterey, California (sea level), then drive home to Reno, Nevada (4,500') via Donner Summit (7,000'). Limiting dives on the travel day to one dive, max of 60' and using 36% has given all of us excellent results...nary a niggle.

I believe that using a dive schedule for altitude, or setting your computer to altitude, then, diving that schedule at sea level will not give the results you wish. The schedule assumes you have a reduced surface pressure, and calculates the nitrogen loading from depth+altitude pressure. Since you would be at sea level, depth+sea level pressure will result in higher nitogen load than assumed in the model, thus increasing chances of a hit.

All the best, James
 

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