Just curious if any flight attendants know, about the infamous Oxygen Mask bag...they always says "the bag MAY not inflate." What is the purpose of the bag, and when DOES it inflate, if used?

This is a completely wild a** guess, but here goes my total stab in the dark (loooking forward to seeing if I'm right or not)

The O2 coming out of the oxygen generators is probably at fairly high pressure and certainly at high velocity. By having that bag between you and the high pressure source, it allows the O2 to enter a far less constricted area (the entire bag vs. the thinnish tube) and therefore to lose velocity and also provide some extra volume so when you are exhaling you aren't fighting the outbound pressure.

The bag would inflate somewhat if you were exhaling, if the pressure in the cabine were really low, or if there were very few bags activated (I understand there is a small metal pin that blocks the flow of oxygen that when you pull the mask firmly toward you yanks out the pin and starts the flow of oxygen allowing you to put your mask on before assisting others...) and the O2 generators are going full tilt boogie, there would be excessive pressure in the system vs. having every mark on the plane activated.

Just my guess. Any A&P mechanics lurking out there that know the real answer?

--PP

What is said is "may not fully inflate". Which is true. It's said to prevent passengers from expecting a giant inflated air bag ala a microwave popcorn bag. The oxygen is flowing, even if the bag does not fully inflate.

In general a bag connected to an oxygen delivery device, be it a mask or cannula serves as an external reservoir of oxygen. For example, with a simple nasal cannula your nasal cavity serves as the oxygen reservoir. Even when you're not taking a breath in the cannula delivers new oxygen to your nasal cavity. When you breathe in, room air (21% oxygen at sea level) is entrained along with whatever concentration of oxygen you have in your nasal cavity (>21% oxygen) and delivered to your lungs.

You can move this reservoir externally as well by using a mask over your face (the space between the mask and your face plus your nasal/oral cavity become the reservoir) or a mask with a bag which adds an external reservoir. The bag on the aircraft oxygen mask is simply providing a reservoir of 100% oxygen that you can then breathe in. If you breathe in faster than the oxygen delivery device is delivering oxygen the bag will not inflate since you're drawing in all of the delivered oxygen plus ambient air entrained around the mask edges. If you're breathing slower than oxygen is delivered to your bag/mask then the bag will inflate assuming you have a tight seal of the mask against your face. That's unlikely given the single elastic strap and the fact that the system should be designed to allow you to breathe in room air even with the mask fully secured to your face. That'll keep you from suffocating once the oxygen generator runs out.

Since you're likely going to be breathing much faster due to the adrenergic surge from simply having emergency equipment deployed it's unlikely you'll be breathing slower than the rate of delivered oxygen. A normal adult inhales/exhales 6-8 liters of air a minute and can easily double that rate. I don't believe for a minute you're getting that much oxygen from your mask although I don't have the actual technical specs on hand.

I should add, while there's still 21% oxygen at altitude, the partial pressure of delivered oxygen (and hence what you're obsorbing) is lower at altitude. Hence you need more oxygen when you go higher up. The whole point of this is to keep your arterial blood oxygen level high enough to keep you from passing out. Aircraft are normally pressurized to 8,000 feet above sea level.

At sea level barometric pressure is 760mmHg, with the partial pressure of oxgen at 159mmHg. At 8,000 feet this drops to 564mmHg and 118mmHg respectively. At 30,000 feet it is 226mmHg and 47mmHg.

Now compare your sea level PaO2 (the amount of oxygen in your blood) to your levels at 8,000 feet and 30,000 feet. Let's say you have a "normal" range PaO2 of 90mmHg and no lung disease. At 8,000 feet you have a PaO2 of 56mmHg or just before the steep part of the oxyhemoglobin curve where your body gets really upset at you. At 30,000 feet you're estimated to have a PaO2 of 23mmHg which is pretty much guaranteed to make you pass out.

Actually on UA the usual thing on the taped announcements is "Though the bag does not inflate, oxygen will flow to the mask".

just out of interest would a very rapid decompression at 30,000ft be enough to make CO2 come out of solution in the blood, I know bottles of soda swell on normal descent.

Since the partial pressure of CO2 at 30,000 feet is less than the partial pressure of CO2 at sea level I don't think any CO2 will be released from the blood. My understanding of decompression sickness is that it's governed by Henry's law which states that at a fixed temperature the amount of gas dissolved in a liquid (i.e. blood) is directly proportional to the partial pressure of that gas. Since the partial pressure of CO2 is higher at sea level than at 30,000 feet you'd expect more CO2 to begin dissolving into your blood as you descend.

Also keep in mind your body is never at 30,000 feet. The cabin is pressurized to 8,000 feet.

Yeah, you aren't going to get bent going down. It is possible to get bent going up but the normal reaction to a decompression is to dive to safe altitudes, there isn't going to be enough time for it to be a problem. It's normally only an issue for high altitude jumpers in the military.

thanks,I was just idly musing, about the how altitudes would affect P CO2 and whether a rapid descent would increase or decrease P CO2. Now if the bag and mask had a perfect seal it would be more interesting.

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Thanks for informative post Lurker1999.