Controlled Cabin Pressure in an Airplane

[aommaster]

Hi there!

 

I was just wondering about this:

 

In an airplane, the flight attendant usually tells you on those annoying videos that the pressure in the cabin is carefully controlled. what does that mean? Do they mean that the cabin pressure remains at normal atmospheric levels? If so, read on:

 

When the airplane ascends to a greater altitude, your ears start to pop, since there is a change in pressure. Doesn't that mean the pressure in the cabin is NOT controlled? As if the pressure WAS controlled, your ears wouldn't pop?

[Sayonara]

Maybe "controlled" doesn't mean "invariable".

[aommaster]

Possibly. I think they would control it to an extent that would be bearable to humans. But, yet, I am not sure!

[Cap'n Refsmmat]

There are vents on the airplane that let the pressure stay the same as the atmosphere. These are closed at 8,000 feet so people won't get hypoxia. When at cruising altitude, you feel the same pressure as being at 8,000 feet.

Because of that pressure difference, it puts strain on the fuselage, meaning they cannot keep it at sea level pressure because the whole thing would just go POP!

[YT2095]

Nice answer!

 

does that apply to Rockets too (the shuttle for instance) or is that a completely different setup?

[Cap'n Refsmmat]

The shuttle's pressure is constant unless there is going to be a spacewalk, where the pressure will be dropped to 10 PSI about 12 hours before the spacewalk, to reduce the risk of the bends. (spacesuits are only 4 PSI!!)

 

PS: on airplanes, they get extra air to put in the fuselage for heating and to maintain pressure from the compressor stage of the jets, where the air is already heated up from the compression.

[YT2095]

Cheerz

 

it`s nice to learn something new everyday!

[Cap'n Refsmmat]

Oh yes, and astronauts have to sit in the airlock and breathe pure oxygen for half an hour. (before the spacewalk)

[YT2095]

Oh yes, and astronauts have to sit in the airlock and breathe pure oxygen for half an hour. (before the spacewalk)

I guess that makes Smoking out the question for pre-EVA nerves then!

[Cap'n Refsmmat]

lol!

 

Unless you want to burn off 2 inches of your cigarrette in a second!

 

I believe Winston Churchill once decided that he had to be able to smoke in the unpressurized cabin of his airplane, so he had a special attachment made to his oxygen mask. Every once and a while it would feed the cigar too much oxygen, and POOF! 2 inches of it were gone before he realized what was going on!

[Martin]

Oh yes, and astronauts have to sit in the airlock and breathe pure oxygen for half an hour. (before the spacewalk)

 

 

Cap'n you mentioned that space suits are 4 PSI----pure oxygen I assume.

 

I seem to recall that the mercury program, or some early program pre-Apollo, had pure O2 atmosphere at maybe 30 percent normal pressure

 

do you recall? Yr memory probly better about this.

 

If humans were going to breath pure oxygen for several days at a time

what would be the healthiest pressure to have it at?

Would 4 PSI be right? Or maybe 3 PSI (partial pressure of o2 in normal atmosphere)?

Would health suffer from this? Or is O2 atmosphere actually healthy?

Is pure o2 atmosphere considered prohibitively dangerous because of

cigar-fires and the like.

 

I am thinking about the under-ice Callisto habitat.

pure o2 is available locally by electrolysis, but nitrogen or helium would be expensive to replace

[Cap'n Refsmmat]

Sorry. Don't remember.

[YT2095]

but nitrogen or helium would be expensive to replace

how so?

both are inert with respect to our breathing, we breathe in 78% nitrogen, and use non of it, the same volume is expelled

[Martin]

how so?

both are inert with respect to our breathing' date=' we breathe in 78% nitrogen, and use non of it, the same volume is expelled [/quote']

 

sure, one would recycle as much as possible,

but some loss (e.g. leakage, accident, airlocks possibly...) seems inevitable

and would have to be made up

so the replacement cost becomes an issue

 

my guess is that humans can thrive indefinitely in

a 4 PSI oxygen environment if everything else is OK and

if the safety problems can be managed

 

but would appreciate if anyone can confirm this

[Cap'n Refsmmat]

Airlocks do not let out gas, as the air inside them is pumped back into the cabin before being opened. The Space Shuttle contains liquid nitrogen tanks for the purpose to replace anything lost, however it is hardly necessary.

[Glider]

Pure oxygen is toxic to humans if given over long periods. It 'scorches' the lining of the lungs, causing the endothelieal cells of the alveoli to thicken in an attempt to reduce their exposure to it. Of course, once this has happened, it becomes very difficult to breathe at normal concentrations (although the lungs do revert in time). 'Weaning off' is one of the problems with patients that have been ventilated on high concentration O2 for a while.

[aommaster]

Sorry I couldn't asnwer earlier Something turned up! Thanx for all your asnwers. They are really helpful!

[Martin]

Pure oxygen is toxic to humans if given over long periods. It 'scorches' the lining of the lungs, causing the endothelieal cells of the alveoli to thicken in an attempt to reduce their exposure to it. Of course, once this has happened, it becomes very difficult to breathe at normal concentrations (although the lungs do revert in time). 'Weaning off' is one of the problems with patients that have been ventilated on high concentration O2 for a while.

 

Glider I want to make very sure I understand

I can see that O2 would be toxic if given at atmospheric pressure

that is 100 kPa or 15 PSI or whatever it is normally.

This the pressure in an oxygen tent at the hospital, for example, or

delivered by an ordinary respirator. I am not asking about that kind of

situation (where I can see why it could be toxic).

 

 

what i am inquiring about is o2 breathed at 20 percent of atmospheric normal

that is, around 3 PSI or 20 kPa pressure

 

this is the partial pressure of the oxygen in the air we customarily breathe

 

so intuitively, or naively, one would expect the chemistry to be about the same. the same numbers of O2 molecules would be hitting the

cells in the alveoli, with the same kinetic energy, as is usual for us

 

can you confirm that THAT would be toxic, namely being in an o2 atmosphere at 1/5 normal pressure?

[Glider]

No, you are absolutely right. The chemistry is the same at ATM of 3psi (155.1 mm Hg). The margin for neurological oxygen toxicity is higher, but for pulmonary oxygen toxicity, exposure to pO2 of around 0.5 ATA (Atmospheres Absolute: where 0.2 ATA is normal for oxygen) for >12 hours causes damage. However, by reducing the atmospheric pressure proportionately, this is avoided.

[Martin]

No, you are absolutely right. The chemistry is the same at ATM of 3psi (155.1 mm Hg). The margin for neurological oxygen toxicity is higher, but for pulmonary oxygen toxicity, exposure to pO2 of around 0.5 ATA (Atmospheres Absolute: where 0.2 ATA is normal for oxygen) for >12 hours causes damage. However, by reducing the atmospheric pressure proportionately, this is avoided.

 

Great! so my people on Callisto, when their population grows and they want to hollow out another ice cavern to live in

they dont have to bring atmosphere from earth

they can just electrolyze water and vent the hydrogen and

make themselves a 3 PSI O2 atmosphere for the cavern

 

they appreciate this news and send you their best regards

[Glider]

Tell them they're welcome, as long as they sheild all electrical equipment with inert gas, don't smoke, don't use power tools etc., etc..

[Cap'n Refsmmat]

Or, make sure everything is non-sparking.

No sparks means no fires.

 

Lockheed once had to work on a liquid-hydrogen fueled airplane, and so the building they worked in had to have absolutely no sparks- or- BOOM!!!! Liquid hydrogen explosion! So they got no-spark everything, even alarm systems.

[Martin]

Sounds like a tall order.

 

I wonder if Callisto ice has a percentage of ammonia or compounds, from which you could mine nitrogen for your atmosphere

I hate the idea of having to lug it all the way from earth