Another flying after diving Q

[Dan]

Then this recent DCI incident makes me concern about other issues besides just having enough surface interval: Heidi Uh Oh

 

[rjack321]

During the flight, in case of prolonged lost cabin pressure your 15ft foot dive because an extreme high altitude dive and may (hypothetically) get you bent. "

Going from a cabin pressure of 8,000ft to 32,000ft (or about that) will instantly bend everyone on the plane, divers or not.

 

[Altamira]

Going from a cabin pressure of 8,000ft to 32,000ft (or about that) will instantly bend everyone on the plane, divers or not.

No it won't. Military crew members get trained in an altitude chamber several times during their flying career. The altitude chamber is primarily used to demonstrate the effects of a loss of cabin pressurization and subsequent hypoxia on human physiology, especially the effects of reduced O2 pressure on your ability to think, react, do even simple tasks, and how short of time you may have before you pass out. At some point in the training, the cabin pressure was increased to approximately 8000', while simulated being at an actual altitude of approximately 35,000'. Without warning, the cabin pressure instantaneous shot to 35000' simulating a sudden, complete loss of cabin pressurization. While I cannot say no one ever got bent in this training, it never happened to any crew member of any of my military squadrons. In addition, crew members and passengers involved in rapid decompressions in commercial aircraft are not routinely given DCS treatment, other than using the aircraft supplied O2 during the emergency descent.

 

[RainPilot]

The pressure at 18000 feet AMSL is 1/2 a bar so from sea level to 18000 instantly would put you on the edge of saturation, which is why we had to pre breathe O2 while on the ground before doing test flights above 18000 unpressurised. The difference from 8000 to 35000 is not usually enough to bend someone who is at a normal tissue loading, especially if they have been at 8000 for a while offgassing. Add in that the procedure for a sudden loss of pressure is to descend below 10000 and we don't get a lot of DCS on aircraft from non-divers.

Also, when we did HALO jumps the crew and the meat-bombs were on 100% O2 from engine start to allow off-gassing during the climb phase.

As to the OP question, a way to pad your safety margins a bit might be to breathe a low N2 mix on the surface after the dive. My Perdix allows me to select a gas on the surface, so when I know I might be flying sooner than planned I will often finish up the nitrox tanks on the boat ride back, if I have my deco stage with me I will rotate between the 50% and the 100% as we come back. It makes a HUGE difference to the tissue loadings. Basically accelerated deco on the surface.

I wouldn't use that method to "cheat" the flying rules but as more of a "belt and braces" approach, same as setting the PDC to Air while diving 32%.

 

[rjack321]

The risk of flying after diving has nothing to do with loss of cabin pressure (which is exceptionally rare).

 

[RainPilot]

The risk of flying after diving has nothing to do with loss of cabin pressure (which is exceptionally rare).

Agreed. It is to do with a fairly rapid ascent to 0.7 ATA (8000 ft cabin altitude). If there is a loss of pressure, the world of hurt grows exponentially.

What we often forget is that NDL doesn't REALLY mean "No" decompression. What it means is that we have a low-enough gas loading to finish deco on the surface (hence SIT and letter groups etc etc). However the gas gradient on the surface breathing air is a VERY inefficient way of off-gassing. Thats why I treat all dives as deco dives and do a VERY slow ascent from 6m (20ft), I would rather off gas in the water than on the boat.

 

[Shotmaster]

The best layman' example of what happens is the simple coca cola example. Dissolved CO2 in a liquid. When it is cold and you open it slowly, micro bubbles appear, rise to the top and off gas to the atmosphere. Very controlled release of gas. It takes quite a bit of time before the soda goes flat. Open one that is hot and the bubbles are larger, and very active. Shake it up and open it and the off gassing is violent. Most "our" experience is the former. When my friend got bent, it was a shock. Like most I just thought that 24 hours out of the water was sufficient. I came to realize that a lot remains unknown. While most would never see the same flying conditions which caused my friends accident. It made me wonder long and hard about the 24 hours for commercial flying. His bends caused brain damage and occurred just after he exited the aircraft. He was unconscious when he got to the ground and his AOD had fired,pulling the ripcord. He had gone rapidly from the ground to 12,500 ft in an unpressurized C130. These type aircraft have an incredible rate of climb. My personal opinion is that flying after diving should be looked at again. My personal delay is 48 hours. I fly a lot for work and have noticed that my coke served on the plane goes flat almost before I can drink it.

 

[EFX]

However the gas gradient on the surface breathing air is a VERY inefficient way of off-gassing. Thats why I treat all dives as deco dives and do a VERY slow ascent from 6m (20ft), I would rather off gas in the water than on the boat.

I don't know what your criteria is for efficiency. As I understand it, going from some depth to the surface as fast as possible gives the greatest efficiency because the gradient (the pressure difference between the tissues and inspired air) is the greatest. Doing this you run the risk of exceeding the safe off-gassing rates for the medium tissue compartments (TC). The fast TC's can keep up with the pressure change as long as the ascent rate is not too fast. Coming up too slow allows the slow TC's to on-gas. Using my spreadsheet you can try different profiles to see which TC is controlling.

 

[halocline]

I have certainly done similar things many times, and for whatever it is worth, I'm still here.

Kind of depends on how long your flight is. On the hop from Little Cayman to Grand Cayman I wouldn't even think about it. If I was flying Sydney to London, that might make me a tad more cautious.

Usually the puddle jumpers don't go very high in altitude, far lower than the 8000 ft typical long distance commercial flights are pressurized to. I'm not sure about the plane from Little to Grand Cayman, but I bet it doesn't get much over 1000 ft.

Back to the OP, it's a crapshoot. Flying after diving is probably the least predictable aspect of deco theory, hence DAN's blanket recommendations of 12 and 18 hours after single and multiple dives, respectively. Personally I'd be pretty reluctant to violate those.

 

[PfcAJ]

Agreed. It is to do with a fairly rapid ascent to 0.7 ATA (8000 ft cabin altitude). If there is a loss of pressure, the world of hurt grows exponentially.

What we often forget is that NDL doesn't REALLY mean "No" decompression. What it means is that we have a low-enough gas loading to finish deco on the surface (hence SIT and letter groups etc etc). However the gas gradient on the surface breathing air is a VERY inefficient way of off-gassing. Thats why I treat all dives as deco dives and do a VERY slow ascent from 6m (20ft), I would rather off gas in the water than on the boat.

Off gassing is most efficient on the surface on a recreational dive.

Ambient pressure is lower which is what drives decompression.