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We may all be familiar with the part of the cabin safety briefing where they say that, in the event of a reduction in cabin pressure, oxygen masks will drop from a compartment above our heads, yadda yadda, the bag may not inflate. What is the purpose of the bag?

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I made a very quick Google and didn't find much; Wikipedia doesn't answer the question. In any case, I think the site would benefit from having an answer as I'm sure it's something others are curious about. –  David Richerby Apr 14 at 7:24
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A quick google for images lead to this page: daerospace.com/OxygenSystems/OxygenSystemContinuousFlow.php which is in sync with Jan Hudecs answer. –  PlasmaHH Apr 14 at 13:49

3 Answers 3

up vote 25 down vote accepted

Oxygen in passenger masks is either generated by oxygen generator or released from pressurized container via reduction valve. In either case it is released at fixed rate. But human breath is discontinous. So the bag acts as a buffer. You inhale the oxygen accumulated in it and then it slowly refills while you are exhaling.

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so what happens if you breath "too fast" or "too slow"? Would the bag overinflate and pop if you were breathing too slow? Would you pass out from not enough oxygen if you were breathing too fast? –  n00b Apr 14 at 17:33
    
So, in reality, the bag will probably inflate a little bit but potentially not enough to be really noticeable? –  David Richerby Apr 14 at 17:39
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@n00b: It will not pop, because the flap connecting it to the mask will open due to overpressure just as it does when you inhale and the mask is not pressure-tight, so you'll just waste a bit of oxygen. The amount dispensed is enough to keep anybody usefully conscious. You'd only need more oxygen if you were doing some intense physical activity in which case you'd become tired and stop before running out of oxygen so much as to pass out. –  Jan Hudec Apr 14 at 17:47
    
@DavidRicherby: I think it will usually be noticeable, but it will almost never be full. –  Jan Hudec Apr 14 at 18:10
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@JanHudec That's not exactly true. Oxygen consumption can also spike if you become very anxious (understandable in a plane accident) and hyperventilate. Your body would be using more oxygen than the pump supplies, and you would faint (which can be dangerous). –  Superbest Apr 15 at 7:23

In addition to what our friend Jan Hudec mentioned:

Wikipedia

The mask may also have a concentrator or re-breather bag that may or may not inflate depending on the cabin altitude, which has (in some instances) made passengers nervous the mask was not providing adequate oxygen, causing some to remove them, who thereby suffered hypoxia.

AOPA

The partial rebreather is the most common. With these, there's an external plastic bag that inflates each time you exhale. The purpose of the bag is to store any unused oxygen, so that it can be inhaled with the next breath.

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Airliner "Dixie cups" are not partial rebreather masks, though. The bag works differently in each design. –  egid Apr 14 at 14:31

I can't actually track down the purpose of the bag; however, the FAA Oxygen Equipment guide states that it is not for the purpose of mixing exhaled air with pure oxygen:

The phase-sequential continuous-flow mask looks similar to a general aviation re-breather. However, both masks function differently, and the phase sequential mask allows the user to go to higher altitudes. This mask uses a series of one-way ports that allow a mixture of 100% oxygen and cabin air into the mask. Exhalation is vented to the atmosphere; as a result, the bag does not inflate. [emphasis mine] This mask can be safely used at emergency altitudes up to 40,000 feet.

Because of the construction, the bag seems to act as a buffer, as Jan Hudec states in his answer.

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