The Plastic Bag Will Not Inflate
Jul 25, 2005 | 2:21 pm
#1
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The Plastic Bag Will Not Inflate
Then why the heck is it there on the oxygen mask?
Jul 25, 2005 | 2:33 pm
#2
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The plastic bag will inflate partially.From past expereinces it seems that passengers tend to panic is it does not completly inflate.
Of course i might be wrong
Of course i might be wrong
Jul 25, 2005 | 3:47 pm
#4
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HuH?
Originally Posted by FoPAA
To mix pure oxygen with the ambient air.
If there was ambient air, you would not need need a mask.
Am I missing some Gas Law of Physics here?
Hot Air Beef: The safety video on US carriers uses some sugar coated PC language like "In the event of a change in cabin pressure, a mask yada yada yada."
Don't want to scare grandma by letting her know that planes are made by people, are not perfect, can and do BREAK sometimes, least of all during the Safety Briefing for Her Own Good.
My compliments for straight talking truth to Singapore Airlines who recently told me "In the event we HAVE A LOSS OF CABIN PRESSURE. . ."
Thanks SQ for having the guts to say what actually happens.
Jul 25, 2005 | 3:56 pm
#5
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Google is your friend:
http://www.straightdope.com/classics/a3_140.html
Dear Cecil:
Here is one I have been trying to find the answer to for years. I have asked flight attendants on airplanes all over the world. No one knows. No one even hazards a wild guess.
Why doesn't the plastic bag inflate? Since it doesn't, what is it for? I am speaking, of course, of the oxygen mask that will drop in case of emergency and that you are supposed to tie securely around your face before attending to infants or children. The plastic bag attached to the mask never inflates, and what's more, they make a point of telling you it won't inflate. This to me is more perplexing than some of the early undeciphered scripts I study. --Thomas P., director, Program in Aegean Scripts and Prehistory, University of Texas at Austin
Cecil replies:
Doc, your prayers have been answered. First an inside secret: the bag does inflate, but only when you exhale.
Here's the deal. Passenger oxygen masks give you a continuous flow of oxygen (as opposed to oxygen on demand, which only flows when you inhale). The oxygen obviously can't flow into your lungs while you're exhaling, so if there weren't some way to store it temporarily it would have to be vented wastefully. The bag makes this unnecessary. When you start exhaling, your breath plus the incoming O2 flow into the bag. When a certain pressure is reached the bag stops filling and the rest of your exhaled breath, which contains more carbon dioxide, is vented through a port in the mask.
The flight attendants make a point of telling you the bag won't inflate (right away, that is) because of an incident years ago. An airplane lost cabin pressure, the oxygen masks dropped down, and the passengers put them on--but when they noticed the bags didn't inflate, they figured the masks weren't working and took them off. Bad idea. Thus the warning. Simple, eh? Now, you want a hand with those Aegean scripts?
--CECIL ADAMS
Here is one I have been trying to find the answer to for years. I have asked flight attendants on airplanes all over the world. No one knows. No one even hazards a wild guess.
Why doesn't the plastic bag inflate? Since it doesn't, what is it for? I am speaking, of course, of the oxygen mask that will drop in case of emergency and that you are supposed to tie securely around your face before attending to infants or children. The plastic bag attached to the mask never inflates, and what's more, they make a point of telling you it won't inflate. This to me is more perplexing than some of the early undeciphered scripts I study. --Thomas P., director, Program in Aegean Scripts and Prehistory, University of Texas at Austin
Cecil replies:
Doc, your prayers have been answered. First an inside secret: the bag does inflate, but only when you exhale.
Here's the deal. Passenger oxygen masks give you a continuous flow of oxygen (as opposed to oxygen on demand, which only flows when you inhale). The oxygen obviously can't flow into your lungs while you're exhaling, so if there weren't some way to store it temporarily it would have to be vented wastefully. The bag makes this unnecessary. When you start exhaling, your breath plus the incoming O2 flow into the bag. When a certain pressure is reached the bag stops filling and the rest of your exhaled breath, which contains more carbon dioxide, is vented through a port in the mask.
The flight attendants make a point of telling you the bag won't inflate (right away, that is) because of an incident years ago. An airplane lost cabin pressure, the oxygen masks dropped down, and the passengers put them on--but when they noticed the bags didn't inflate, they figured the masks weren't working and took them off. Bad idea. Thus the warning. Simple, eh? Now, you want a hand with those Aegean scripts?
--CECIL ADAMS
Jul 25, 2005 | 4:07 pm
#6
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Originally Posted by Flaflyer
At 40,000 feet, WHAT ambient air?
If there was ambient air, you would not need need a mask.
Am I missing some Gas Law of Physics here?
If there was ambient air, you would not need need a mask.
Am I missing some Gas Law of Physics here?
When the masks drop, the oxygen to the masks is delivered from either a system of high pressure bottles or generated from a chemical reaction. If there is an emergency requiring the use of masks, the flight crew will descend rapidly to an altitude where people can breathe without the use of supplemental oxygen, say 10,000 feet or so.
I asked a first officer about the plastic baggies on the mask and he told me that the reason they are there is because the oxygen system on the aircraft is fixed flow (meaning its on all the time). To help regulate the flow of oxygen, the baggie can collect oxygen when the passengers are not breathing in and release the collection oxygen when they are breathing in. He said that if everything is working right with the emergency O2 system then the bags will typically not inflate or only inflate part way. He also said that they have a few minutes to get down to a breathing altitude and that in all of his flights he has never had the masks deploy.
Jul 25, 2005 | 4:26 pm
#7
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Originally Posted by Flaflyer
...
[1]At 40,000 feet, WHAT ambient air?
[2]If there was ambient air, you would not need need a mask.
[3]Am I missing some Gas Law of Physics here?
....
[1]At 40,000 feet, WHAT ambient air?
[2]If there was ambient air, you would not need need a mask.
[3]Am I missing some Gas Law of Physics here?
....
[1] PLENTY of it. Lot and lots. When space shuttles reenter the earth's atmosphere, they generate hot plasma from the heat of reentry WAY above 40,000 feet. Relatively speaking, at 40,000 feet the ambient air is VERY thick. And, it has almost exactly the same oxygen content as air at sea level. So, no probs for the atmosphere. There are, however, probs for us air-breathing mammals, and it does have to do with the partial pressure. As referred to above, the problem is the air is so thin to us, that there are just not enough oxygen molecules in each breath to work for us. Also, the reduced pressure does have an effect, also.
[2] Just untrue. "Ambient air" does not define how much air there is. It can be very thin, for us, but it is still there. If you had an instrument good enough, you could find ambient air in deep space...of course, it would be so close to a pure vacuum, that it would not mater to any but the most sensitive instruments. ...not to mention our lungs! See the definition of ambient air and you will se that is basically means (and I am somewhat oversimplifying here) the air that completely surrounds us. So, you have to pick a spot at whichyou do not consider any air to be surrounding us. I won't split hairs as to whether this ceases to be at the troposphere, the ionosphere, the edge of space, or half way to Alpha Centauri, but I assure you there is air at 40,000 feet.
[3] Yes/no. See above answers, there are laws of gasses that tell us how thin gas gets at certain places. It depends on where you draw the line at where the atmosphere stops...the laws will tell you at what point that will occur.
Just to recap, the extreme thickness of the atmosphere even at 40,000 feet is usually the problem, not the extreme thinness of it. WHich is why SR-71s and their ilk flew to past 80,000 feet, why the shuttle has to dissipate the heat generated in reentry at past 100,000 feet, and so on.
Our lungs are the problem, not the lack of air, or oxygen.
Jul 25, 2005 | 4:39 pm
#8
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Originally Posted by Flaflyer
At 40,000 feet, WHAT ambient air?
If there were no air then the jet engines would not run.
If there were no air then their would be no air to compress to pressurize the cabin.
Roughly half of the atmosphere is below 18,000'. Most, but not quite all, of the atmosphere is below 100,000'.