No fly vs Time to deSat?

[Turtle Dude]

Just a general, "I was wondering" question...

I was playing with my OSTC and noticed that the No-Fly time is set, as a default, to 60% of the Time to De saturate.

I, like most, stick rigidly to the conservative guideline of don't fly within 24h of a dive, but just wondered if anyone had a better informed view on the general risks with flying and the relationship with the time to desat?

[Lazlo]

I suppose there might be a relationship between desat time and no-fly time, but I haven't heard of any studies that tried to examine that aspect.

Obviously the concepts are related, but how? Is it even an interesting question to ask - in the sense that would knowing the answer tell us anything useful?

The latest no-fly guidelines are summarised here:

DAN Divers Alert Network

[NotDeadYet]

The no fly time is just a calculation like any other deco stop. The computer should work out what the m-value at 8000ft (~0.75ATA from memory) would be then work out how long you would need to stay at sea level in order for your tissues to drop to those levels. It's no different to how the computer works out how long you should stay at 9m before ascending to 6m.

It isn't based on the total desat time but it probably works approximately enough to be able to say that no fly time is x% of desat.

8000ft is chosen because it's the minimum equivalent air pressure allowed in a commercial airliner cabin.

When i was writing deco software I asked DAN, they said forget calculations and go for at least 24hrs.

[Woz]

BSAC 88 tables you need to be a B before flying. Which can be as soon as 2 hours of getting out from the water. If you've done a deco dive then it's a bit longer.



4 hours.

[Houseplant]

i've never understood the term "desaturation" on a computer. i'm saturated with nitrogen right now and am neither underwater nor apres dive. perhaps someone could explain.

[Blu DL]

as I understand it - nitrogen saturation is the point at which no further nitrogen can be dissolved in the blood or body. Obviously the higher the ambient pressure (deeper) the more nitrogen can be held in the blood and tissue. But at any depth this is a finite amount.

AIUI - desaturation is the point at which in - normal 1 bar atmospheric conditions - your body content of nitrogen has reached a dynamic equilibrium - ie you are no longer off gassing - and you have reached a normal stable level of n2
.

I guess the 'de-saturation' tag is purely relative to the higher pressure conditions you were at on the dive. You would only desaturate completely at 0 bar - in space - but then the lack of atmospheric pressure would make your blood boil away rapidly so I guess this theory would be less important at that time

maybe someone can correct this if wrong ???!!

[chrisch]

Personally I can't fly. I can however wave my arms like a demented twat, but fly - no.

Chris

[nigelH]

AIUI - desaturation is the point at which in - normal 1 bar atmospheric conditions - your body content of nitrogen has reached a dynamic equilibrium - ie you are no longer off gassing - and you have reached a normal stable level of n2

The trouble with that definition is that it's an exponential. Always getting nearer but never getting there. You need some definition of 'near enough' or you wait forever.

I like the idea of it being the zero meter stop before going to less than 1 bar but, again, what research has been done on this? I assume NASA has some as I hear space suits run at 0.2bar of O2 and you have to deco before a space walk. If this is true I wonder if there are published results.

[NotDeadYet]

The trouble with that definition is that it's an exponential. Always getting nearer but never getting there. You need some definition of 'near enough' or you wait forever.

That's if you do it based purely on half times. Use the Schreiner equation and it will reach equilibrium. Haldane's theory was that six half-times were near enough to say equilibrium had been reached.

I like the idea of it being the zero meter stop before going to less than 1 bar but, again, what research has been done on this? I assume NASA has some as I hear space suits run at 0.2bar of O2 and you have to deco before a space walk. If this is true I wonder if there are published results.

IIRC Buhlmann did research at altitude. There is also tons of info on the NASA website. Not just for space walks but bends also occurred on high altitude flights in unpressurised aircraft. I seem to remember something about Blackbird or U2 pilots having to do oxygen breathing before take off and ascent.

[Gav6280]

Blackbird or U2 pilots having to do oxygen breathing before take off and ascent.

They still do, or at least James May did on the telly.