Breathing at high altitude flights...

[Externet]

How does it work ?  What equipment is on board jetliners to provide enough oxygen for the occupants ?

Pressurizing cabins is one thing, but where does the amount of oxygen consumed comes from ?  Never seen at ground support equipment, any oxygen filling operation.

There may be oxygen cylinders for the masks that drop from overhead on emergencies, but that has to be unrelated.

[fiveworlds]

Quote

Pressurizing cabins is one thing, but where does the amount of oxygen consumed comes from ?

The same compression used in the engine forces air through the cabin while the engine is running. If the engine is not running a chemical is used in the masks that drop from overhead.

[Janus]

50% of the air is brought in from the outside and compressed, and 50% is recirculated cabin air.

[Externet]

Thanks.

OK, the air brought in by engines is of poor 7% oxygen content, plus the recirculated is also depleted of O2 by occupants;  so 7% is still enough oxygen at 30,000+ feet altitude  instead of 21% for passengers to not experience anoxia as for engines to combust ?

I have to assume the physical  inactivity of passengers is what makes a difference to alpinists in feeling effects in flights of many, many hours duration.

Does the ISS gets 'refills' from shuttles or is all the oxygen fully recycled ?

[YaDinghus]

5 minutes ago, Externet said:

Thanks.

OK, the air brought in by engines is of poor 7% oxygen content, plus the recirculated is also depleted of O2 by occupants;  so 7% is still enough oxygen at 30,000+ feet altitude  instead of 21% for passengers to not experience anoxia as for engines to combust ?

I have to assume the physical  inactivity of passengers is what makes a difference to alpinists in feeling effects in flights of many, many hours duration.

Does the ISS gets 'refills' from shuttles or is all the oxygen fully recycled ?

That doesn't sound entirely correct. For 7% O2 to be indistiguishable from 21% at sea level, cabin pressure would have to be 3 Atm. However, continental flights should be at an altitude where outside pressure is about 1/3 atm, while having the same oxygen concentration, it would be equivalent to the partial pressure of 7% Oxygen at sea level. You would not live for a very long time with that partial pressure of Oxygen, no matter your physical activity, unless you've just completed 2 months of intense altitude training.

10 hours ago, Janus said:

50% of the air is brought in from the outside and compressed, and 50% is recirculated cabin air.

Compressing the outside air however will equalize the O2 partial pressure, and jet engines reach compression levels far beyond what is needed for this task. In regards of fuel use for cabin air compression, it would amount to a rounding error

[StringJunky]

9 minutes ago, YaDinghus said:

That doesn't sound entirely correct. For 7% O2 to be indistiguishable from 21% at sea level, cabin pressure would have to be 3 Atm. However, continental flights should be at an altitude where outside pressure is about 1/3 atm, while having the same oxygen concentration, it would be equivalent to the partial pressure of 7% Oxygen at sea level. You would not live for a very long time with that partial pressure of Oxygen, no matter your physical activity, unless you've just completed 2 months of intense altitude training.

In the 'death zone', above 25000-26000ft where it's 7%, a seasoned mountaineer won't last more than 48 hours there without supplementary oxygen. Basically, you are slowly dying over that period until you do die.

[YaDinghus]

2 minutes ago, StringJunky said:

In the 'death zone', above 25000-26000ft where it's 7%, a seasoned mountaineer won't last more than 48 hours there without supplementary oxygen. Basically, you are slowly dying over that period until you do die.

Thanks for that bit of Info 

[Externet]

YaDinghus :  Am lost if you are suggesting pressurizing the cabin affects / "equalizes" the percentage of O2... Does it ? 

Once over the Andes as passenger, had a altimetre on hand and cabin pressure was under 1 Atm, perhaps equivalent to 3000 feet.  But cannot remember precisely. Another ocassion also over the Andes, as 'copilot' , pilot and I put oxygen masks at certain 18,000ft? altitude.

StringJunky :  That is the point. Been in 13 hour flights (London to San Francisco) like 6 times with no recognizable sympthoms of partial anoxia.

Edited July 19, 2018 by Externet

[StringJunky]

3 minutes ago, YaDinghus said:

Thanks for that bit of Info 

In a way, oxygen-free climbers are holding their breathe, knowing that they will die if they hang around too long when they shoot for the summit of K2 and Everest into the death zone.

[Janus]

1 minute ago, Externet said:

YaDinghus :  Am lost if you are suggesting pressurizing the cabin affects / "equalizes" the percentage of O2... Does it ? 

Once I had on hand an altimeter flying as passenger above the Andes, and cabin pressure was not sea level, was less.  Cannot remember how less than 1 Atm,  perhaps an equivalent to 3000 ft, but unsure, was too long ago.

StringJunky :  That is the point. Been in 13 hour flights (London to San Francisco) with no recognizable sympthoms of partial anoxia.

Compressing the outside air before it is used in the ventilation system raises the partial pressure of the oxygen. The percentage of oxygen doesn't lower with altitude ( it still makes up ~21% of the gas mixture), there just are fewer gas molecules per given volume at high altitude. At sea level the partial pressure of oxygen is ~3 lb/in³.   An atmosphere of pure oxygen at 3 lb/in³  is perfectly breathable. (This is what early astronauts used to breathe).   At 1/3 atm, the partial pressure of oxygen is 1 lb.in³.   If you take such air and compress it up to 1 atm, you also bring the oxygen partial pressure back up to  3 lb/in^3.

 At an equivalent pressure of 3000 ft, air is still quite breathable even for fairly strenuous activity, so for sitting in an aircraft it is quite sufficient, as it only lowers the partial pressure to 2.6 lb/in³.

[YaDinghus]

1 minute ago, Janus said:

At an equivalent pressure of 3000 ft, air is still quite breathable even for fairly strenuous activity, so for sitting in an aircraft it is quite sufficient, as it only lowers the partial pressure to 2.6 lb/in³.

Continental flights are that low? I didn't know... I usually only fly transatlantic, which is 30 k ft min...

[StringJunky]

1 minute ago, Janus said:

Compressing the outside air before it is used in the ventilation system raises the partial pressure of the oxygen. The percentage of oxygen doesn't lower with altitude ( it still makes up ~21% of the gas mixture), there just are fewer gas molecules per given volume at high altitude. At sea level the partial pressure of oxygen is ~3 lb/in³.   An atmosphere of pure oxygen at 3 lb/in³  is perfectly breathable. (This is what early astronauts used to breathe).   At 1/3 atm, the partial pressure of oxygen is 1 lb.in³.   If you take such air and compress it up to 1 atm, you also bring the oxygen partial pressure back up to  3 lb/in^3.

 At an equivalent pressure of 3000 ft, air is still quite breathable even for fairly strenuous activity, so for sitting in an aircraft it is quite sufficient, as it only lowers the partial pressure to 2.6 lb/in³.

Perhaps I should have said "..equivalent to 7% oxygen".

[YaDinghus]

5 minutes ago, Janus said:

3 lb/in³

Is it really per cubic inch? Pressure is usually per sqare inch... 

[Janus]

1 minute ago, YaDinghus said:

Continental flights are that low? I didn't know... I usually only fly transatlantic, which is 30 k ft min...

I was referring the 3000 ft elevation reading Externet got on his altimeter while in flight.  This just means that the cabin pressure was at a 3000 ft equivalent pressure and thus had an equivalent oxygen partial pressure

[Externet]

13 minutes ago, YaDinghus said:

Continental flights are that low? I didn't know... I usually only fly transatlantic, which is 30 k ft min...

3000 ft is the guessed altitude or equivalent pressure I read on my altimetre as a passenger above the Andes on a pressurized cabin.  Read again.  Also brought to guesstimate as cabin pressure at 30,000+ft by Janus.

Also destination of flight being Quito, cabin pressure had to be gradually brought to same as landing site.

 

Edited July 19, 2018 by Externet

[Janus]

1 minute ago, YaDinghus said:

Is it really per cubic inch? Pressure is usually per sqare inch... 

Right, it should be in²  Somewhere in the back of my head I was thinking in air volume when I typed that. Bit of a brain fart.

[Externet]

OK, got it... I think

1/3 of oxygen means also 1/3 of nitrogen.  Compressing brings up the density of both to a tolerable level as in ~3000 ft.

My misconception may had been believing that only oxygen was less dense at altitude, as if it was a stratified column of air from hearing "There is less oxygen at high altitude"   Well, yes, but there is less nitrogen too, actually being "There is less air at high altitude"  -There is less O2 because the whole air is less dense.-   That is why compressing takes care of it.