Decompression question

[ex_matelot]

Apologies if this appears bone but..

I've often heard that in the event of a decompression "only x seconds of useful consciousness left" etc. How does that work exactly? I'm thinking along the lines of ability to hold breath etc.

Thanks

[BOAC]

Depending on the actual height it may not even be possible to 'hold your breath', but in any case lower down the amount of oxy in your lungs will not sustain you for very long. I guess if you are a pearl diver............................

[Noah Zark.]

If a cabin depressurises, you won't be able to hold your breath, as the air in your lungs will be at whatever the pressure was before the event. As the air goes out of the cabin, you will involuntarily exhale (if you didn't, your lungs would probably burst!) until the pressure stabilised in the cabin.
It is the same with scooby divers. (or, say, someone trapped in a submarine, and then escaping.). Because they are breathing air under pressure, to stop the water pressure at depth caving their bodies in, if they were to come to the surface quickly (for a moment ignoring other dangers like the 'bends') the pressurised air in their lungs would expand as they got nearer the surface, and they would be exhaling air all the way to the surface.

[ex_matelot]

thanks for the replies.


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[Mr Optimistic]

I always smile at the ....then breathe normally.....instruction. Dont think so :E

[MarkerInbound]

Ah, where to start? Humans live at the bottom of an ocean of air. Because of all the air stacked up above, the air near sea level is compressed about 14 psi. While we don't feel this pressure, in our lungs this pressure drives oxygen into the blood stream. If you reduce the pressure not as much O2 is pushed into the blood. You can make up for a reduced pressure to some extent by increasing the amount of O2. While what we breath is only 21% O2, what you get out of the oxygen masks is 100% O2. But you go up up high enough, even breathing 100% O2 won't help. The masks the pilots have in the cockpit have an emergency setting that feeds O2 under pressure to the mask so the oxygen can cross into the bloodstream. The yellow Dixie cup masks don't have this function but they should keep you from being brain damaged for a few minutes. One of the factors limiting how up an airliner can fly is how fast it can get down to a breathable altitude. Early DC-9s were limited to FL350 not due to any wing or engine limits but because of the time it took to get down to 14,000 feet.

[BOAC]

what you get out of the oxygen masks is 100% O2 - not correct! It is ambient air supplemented by oxygen UNLESS '100%' is selected - and this is only available for crew.

[vctenderness]

- not correct! It is ambient air supplemented by oxygen UNLESS '100%' is selected - and this is only available for crew.

Except on Ryanair where they have to pay for it!!

[PAXboy]

I think we all know that - as far as FR shareholders are concerned - MoL is infallible ... :p

[MarkerInbound]

- not correct! It is ambient air supplemented by oxygen UNLESS '100%' is selected - and this is only available for crew

I'll grant you there can be a difference what is supplied to the mask and what you get out of it. For the last 25 years and three companies the policy has been for crew masks to be in 100%.