PDA

View Full Version : Why don't Oxygen Mask "bags" inflate?


Hartington
26th Aug 2008, 15:39
The subject says it all really. Why bother with the bag if it doesn't (apparently) do anything?

TownsvilleRefueller
26th Aug 2008, 15:52
They do act as a reservoir for a bit of the gas, but they dont inflate like you'd expect a life jacket to do.

Nashers
26th Aug 2008, 16:56
it will also show that the oxygen is flowing through the system to the mask.

Hartington
26th Aug 2008, 17:42
If you watch "Medical Dramas" the bags you see there inflate and deflate as the patient breathes. How does the bag on the passenger mask

"show that the oxygen is flowing through the system to the mask"

I've always interpreted the safety briefing as saying "the bag will remain inert, unmoving"

SNS3Guppy
26th Aug 2008, 18:47
If you watch "Medical Dramas" the bags you see there inflate and deflate as the patient breathes.

Different kind of mask...a rebreather mask, vs. the nonrebreather mask in the airplane. Medical administration of oxygen, a drug, is a little difference than emergency supply of oxygen in an airplane.

The bag may or may not inflate. The mask is a simplified non-rebreather mask with a maximum potential delivery rate of about 22 lpm flow. The bag acts as a resorvoir which does two things; it enables a larger volume of oxygen to be drawn with a breath, and it helps prevent flow-through wastage of oxygen between breaths.

How full the bag gets depends on one's rate of breathing. In a time of crisis, one tends to breathe in a panic; it's one of the first controls to go. The crisis claims our breath. That is, our body has been practicing breathing for our entire lifetime, but in a crisis the process of simple respiration is accelerated in a panic. Rather than allowing time for the bag to inflate, we suck the air down as fast as we can...and we see no inflation.

I worked as a firefighter. This involved wearing self contained breathing apparatus...SCBA gear. I've seen firefighters empty a tank in five minutes. It's not hard to do, breathing hard, working hard. It should last about fifteen, and it can, if one breathes carefully and slowly. This takes practice however, and conserving air isn't first and foremost on the average passenger's mind.

Rather than giving a detailed explaination, the briefing is usually "the bag may or may not inflate, but you will be receiving oxygen none the less." As a passenger, you really don't need to know the flow rate or function of the mask. Just that it's delivering oxygen, and that you shouldn't worry if you don't see the bag inflate. You're told that to prepare you...so you don't have to question it later.

The bag does two things, then. Given a little time and a good seal on the mask, it will inflate between breaths. However, if there's no seal on the mask (as often there isn't), then enough oxygen is escaping the the mask that air only flows through the bag, rather than inflating it. Not a big deal. If you do get a good seal, rather than simply letting the pressure build in the mask and then push out and be wasted, the bag acts as a resorvoir. It contains the oxygen between your respirations, allowing it to go unwasted.

It's worth noting that at altitude it's not the amount of oxygen that's important to you; it's the pressure. Vapor at any altitude has a pressure, and for any given altitude, oxygen makes up about 21% of that pressure. At sea level and for most people anything up to 10,000 to 14,000, the partial pressure of the oxygen in the atmosphere is sufficient to pass through the cell walls in your lungs and participate in the respiration process...the exchange of waste materials from your bloodstream for fresh oxygen for your hemoglobin.

At higher altitudes, even though you may be supplied a higher concentration of oxygen, insufficient partial pressure may be available to permit the respiration process. Simply put, the gas (oxygen) is there, but you can't really use it. Never the less, get on it as soon as you can and stay on it. At higher altitudes, the time of useful consciousness is very low, close to 5 to 15 seconds during a rapid decompression. The potential for loss of consciousness is very high, and you'll need that mask on your face as you descend...even if you're not awake for the descent.

You're always told to pull the mask toward you. It took some time to dawn on me that this wasn't just a way of telling me to put the mask on. You must pull the mask because there's a lanyard attached to the mask. This lanyard pulls a pin in the valve assembly above your seat, allowing oxygen to flow to the mask. When the masks deploy, it's done because oxygen pressure has been let into the lines leading to the manifold above your seat, and that oxygen pressure releases the latch. It doesn't provide oxygen to the mask, however, until you pull on it, pulling the pin in the manifold and causing the oxygen to flow. For the masks simply hanging down, there's no oxygen flowing out of them, and you won't see the bags inflate on those either.

Where oxygen is very, very important is smoke in the cabin. You can live without water for days, without food for weeks, and without sex for...unfortunately a very long time. But not oxygen. Just minutes. When synthetics begin to burn they form toxic gasses which range from Phosgene gas to carbon monoxide to hydrochloric acid...which constitutes when the smoke contacts your mucus membranes such as your eyes, nose, mouth, and airway. Smoke can overcome you very quickly, so getting on the oxygen is important. This isn't a time to worry if the oxygen is flowing because the bag looks flat...so you're told in advance that it may or may not inflate.

Hartington
26th Aug 2008, 22:30
I've never had to use a mask (and I hope I never will). I always knew that I had a short time to use it or lose it and hoped I would respond well if the need arose. I hope I can now do it "properly" after that explanation.

Checkboard
27th Aug 2008, 10:52
Smoke can overcome you very quickly, so getting on the oxygen is important. This isn't a time to worry if the oxygen is flowing because the bag looks flat...so you're told in advance that it may or may not inflate.

The masks are designed to provide supplemental oxygen at altitude. That means they enrichen the cabin air you breathe through the mask with a bit of extra oxygen - but most of the volume of air you breathe while wearing the mask comes from the cabin around you.

They are not designed for use in the event of smoke in the cabin (they don't filter cabin air, or prevent you from breathing cabin air while wearing them) so they are not deployed for smoke events.

There have been several designs for individual fairly cheap smoke filter & hoods like the Evac-U8, but these were shown to admit Carbon Monoxide and were recalled two years ago. The crew on board have oxygen generator smoke hoods available to them.

SNS3Guppy
27th Aug 2008, 11:01
That would depend on the aircraft, and on the company. I've worked for several operators that did manual mask deployment during a cabin environmental emergency.

As for what the crew has available, again, it depends on the operation in question. We don't use smoke hoods with built in generators, except for firefighting. Otherwise, walkaround bottles are used.

Checkboard
27th Aug 2008, 11:19
Portable O2 bottles and the drop down masks are not recommended for fume/smoke events by any manufacturer that I am aware of, portable O2 bottles have the same limitations, and using one may convince a crew member to enter or remain in an environment in the belief that they are safe, when in fact they are not.

All the airlines that I have flown for recommended the smoke hoods for both fire fighting and dealing with dangerous goods fume events (as the hood works, while the bottle doesn't).

The Real Slim Shady
27th Aug 2008, 16:30
With a cabin fire the last thing I would do would be to drop the O2 masks. Why introduce another source of energy?

SNS3Guppy
27th Aug 2008, 18:33
Oxygen isn't a source of energy. It's an oxidizer. Oxygen supplied to the manifold doesn't get released into the cabin except where the pin is pulled for a specific mask. Where the pin is pulled, the mask is in use, and where the mask is in use, the oxygen is used in respiration by the user...not contributing to a fire. Additionally, to increase fire intensity, the oxygen would have to be applied at the fire site.

As stated before, I've worked for several operators which had policies and procedures for deploying masks any time environmental contamination was a problem (eg, smoke) or loss of atmosphere occured (depressurization). That's one of the reasons there's a provision to manually drop the masks; to give the crew the option when it's not released by the altitude switch. A cabin altitude event is not the only reason to deploy the masks.

Without the mask, one is breathing cabin air anyway. With the mask, one is breathing cabin air with a higher percentage of oxygen, and that's the point.

glawkshuter
27th Aug 2008, 19:43
That little bag holds your expended air and new air in reserve... I think Guppy's airbag would be a little bigger then most....maybe his O2 mask is made by Hefty. :)

Checkboard
27th Aug 2008, 21:27
Without the mask, one is breathing cabin air anyway. With the mask, one is breathing cabin air with a higher percentage of oxygen, and that's the point.

In fume/smoke events there isn't a problem with the amount of O2 in the air, so adding a bit doesn't help any. (That is why they only help in situations with a lack of O2 - i.e at altitude.) In fact, the lung doesn't use all of the oxygen available at normal (below 10,000 feet) pressure, and expires the amount not used - which is why EAR (Expired Air Resuscitation, "mouth to mouth") works.

The problem is the combustion poisons present - which the mask doesn't alleviate . As before - the O2 masks available in the cabin don't help in smoke/fume events, and may indeed hinder by convincing a user without proper education that they do help.

toothpic
5th Sep 2008, 03:45
On most modern aircraft the oxygen is produced chemically in a canister, operated by the pull of a lanyard which releases the operating mechanism and fires trhe cartridge.
As one canister willl normally supply 3 or 4 masks, then the mask hanging down next to you in the vacant seat will (if its fed off the same canister) be releasing oxygen as well. it only needs 1 mask to be pulled for the others in the group to operate.

Dream Land
5th Sep 2008, 06:19
As usual, great explanations by SNS. :ok:

SNS3Guppy
6th Sep 2008, 03:53
That little bag holds your expended air and new air in reserve


Although glockshuter was a troll and was banned, it's worth pointing out that he or she was describing a rebreather mask, which isn't found in an aircraft.

The bag on an aircraft mask is a prebreather resorvoir; it doesn't take in any expended air, but only serves as a resorvoir for oxygen flow coming from the aircraft system. It's a non-rebreather mask; it doesn't take any exhaled air from the user. Only oxygen flows in through that bag. If one were to plug the mask inlet, or have a tight seal and a good valve on the mask, the bag would inflate. Non-inflation simply means the user is consuming the flow before it has a chance to inflate the bag. Not a big deal.

On most modern aircraft the oxygen is produced chemically in a canister, operated by the pull of a lanyard which releases the operating mechanism and fires trhe cartridge.

As one canister willl normally supply 3 or 4 masks, then the mask hanging down next to you in the vacant seat will (if its fed off the same canister) be releasing oxygen as well. it only needs 1 mask to be pulled for the others in the group to operate.


Depends on the system and the aircraft. From the user perspective, ususualy pulling the pin attached to the mask lanyard, ("pull the mask toward you") only allows oxygen to flow to that mask. The pin allows the flow valve to open to that mask only.

Oxygen flow typically opens the mask doors and drops the masks, and oxygen is flowing to the masks through a manifold (depending on the aircraft type and system) based on a pressure switch's response to cabin pressure altitude. When the cabin altitude reaches a preset value (typically 10,000' to 14,000'), the masks drop...because oxygen is being provided to them. Alternately, the crew has the capability of deploying the masks.

Some systems may use a gang valve system at the common mask attach manifold, some don't. The method of delivering the oxygen, acuating the system, etc, is not universal and varies with aircraft type and manufacturer. The differences aren't significant to the user. From a passenger perspective, if you get on the airplane and the mask drops, you grab it, pull it toward you to pull the pin in the manifold, and begin breathing. The differences are significant to the maintenance personnel who work on the system, and generally only then if they work on more than one system.

Not all systems use chemical generators, either.

tedatkins
21st Sep 2012, 17:02
OK guys I am new here so please be kind. I make all the high altitude O2 systems for the people who climb Everest and jump out of aircraft at the edge of space. I use the same gear as I supply as I also climb and jump.
To ans the question above is simple. The O2 is supposed to flow into the bag while you breathe out ready to be used when you breathe in, giving a bolus of O2. It is commonly known that this never happens. This is because gas will take the easiest route to atmosphere. The valve which controls the flow from the bag into the mask requires a lighter pressure to open than the pressure required to inflate the bag. So the gas flows to the mask takes a look at the bag and decides its too much work so simply lifts the valve and flows into the mask; constantly.
So yes the idea of the bag is in most cases useless, not just in aircraft but also in paramedic casualty use. Because O2 is cheap no one has looked hard at getting better value from it. The true cost of the gas is in the logistics. A cylinder of O2 at the South Col on Everest is worth $1000; it costs $3 only to get filled in Delhi. So we take the opposite view on Everest and we have to get as much 'bang for our buck' as possible and that's why Topout O2 systems were invented. The gossamer bag is protected inside a plastic vessel vented to atmosphere and has near nil resistance to filling. So the gas really does collect and you really do get a bolus delivery. This is further enhanced by weighting the valves to ensure that the O2 is delivered in the first part of the breath as only the first part reaches the lower lung where the O2 transfer can take place. The second part of the breath fills the 'anatomic dead space' or the pipe work down to the lungs. No transfer can take place here (there are no alveoli, thankfully) so it would be wasteful to oxygenate this space.
Ted Atkins