You’ve heard the spiel in the airplane, and most likely ignored it last time you boarded a commercial airliner.
“If there is a loss of pressure in the cabin, oxygen masks will fall from the overhead. To start the flow, pull down on the mask and pull elastic strap behind head. Though the oxygen is flowing, the bag may not inflate.”
For those who have never paid attention, the oxygen mask consists of a tube hanging from the roof of the airplane that attaches to a flat plastic bag which has another tube out the back of it that ends in a cup-shaped mask that looks uncomfortable and leaky.
So, if the bag won’t inflate, what’s it for? (What could be its function if it would inflate?) At first, second, or even third glance it doesn’t seem to have any real function (although I haven’t examined one carefully).
It is there because at that altitude, there is less oxygen in the air than you are used to breathing. Possibly too little for you to be able to breathe properly and avoid passing out.
The bag doesn’t inflate because it doesn’t have to. The oxygen flows anyway. Why does it matter whether it inflates or not?
Here’s a few WAG’s:
•It looks cool.
•It buffers any variations in the pressure and flow of oxygen to the mask.
•It provides a temporary space for some of the exhaled air, thus making it less likely that breathing out suddenly will blow the mask off your face.
The higher you are, the less pressure will be in the cabin if depressurization occurs. If the oxygen system isn’t built to take this into account, my bet is that the bag makes up for variations in pressure, like squink said.
Now that I write that, though, I’m having a hard time wrapping my mind around how it’d work.
I think the mask+oxygen makes you breathe the same air over and over, with a slow but steady stream of oxygen added to keep the air reasonably fresh. If you just have a tube with no bag, you can only inhale very slowly. That would be unpleasent and distracting.