Author Topic: ISS Breathing Mix  (Read 14016 times)

Offline Noldi400

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ISS Breathing Mix
« on: March 09, 2013, 01:22:36 PM »
Hi, all. I have a quick question if there's anyone around who knows the answer:

I know the ISS uses a oxygen-nitrogen mix for breathing air; is that in the usual atmospheric mix and pressure?

And why did they go to an O2/N2 mix?  Are there known health problems from breathing a pure oxygen atmosphere long term?

I know health/medical matters are supposed to be my area, but I seem to have been absent the day they taught space medicine in Paramedic school.   8)
"The sane understand that human beings are incapable of sustaining conspiracies on a grand scale, because some of our most defining qualities as a species are... a tendency to panic, and an inability to keep our mouths shut." - Dean Koontz

Offline BazBear

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Re: ISS Breathing Mix
« Reply #1 on: March 09, 2013, 03:08:09 PM »
The ISS uses a ~78% O2/21% N2 atmosphere at about 14.7 psi. AFAIK the main reason to use that mixture is to lower the risk of fire, and the fact that the pure O2 at about 5 psi made hearing speech for any distance (such as in Skylab) difficult due to the thin atmosphere. I can't find anything about any medical issues due to pure oxygen use being discovered during Skylab. Of course, after the Apollo 1 fire NASA no longer used 16 psi pure O2 on the pad, instead slowly bleeding off pressure and increasing the O2 ratio until it was at about 5 psi (the partial pressure of O2 at seal level). I believe the reason they didn't use a 14.7 psi sea level mix in the upper atmosphere and in space was that the spacecraft weren't designed to handle that kind of pressure differential.
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Offline Allan F

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Re: ISS Breathing Mix
« Reply #2 on: March 09, 2013, 03:54:55 PM »
The sea level mix would have made the use of spacesuits very difficult, because the astronaut would be subject to decompression sickness due to nitrogen dissolved in body fluids. The spacesuits used about the same pressure internally as the LM. It would also have added complexity to the spacesuits and space vehicles, because nitrogen would have to be supplied to re-pressurize after each EVA. The use of pure oxygen didn't offer all these difficulties, and the distances involved internally in the spacecraft did not pose a problem with regard to sound propagation in the low pressure.
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Offline Nowhere Man

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Re: ISS Breathing Mix
« Reply #3 on: March 09, 2013, 04:41:07 PM »
Guys?  It's O2 and N2 with subscripts, not superscripts.

Fred
Hey, you!  "It's" with an apostrophe means "it is" or "it has."  "Its" without an apostrophe means "belongs to it."

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Offline Noldi400

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Re: ISS Breathing Mix
« Reply #4 on: March 09, 2013, 05:00:58 PM »
Guys?  It's O2 and N2 with subscripts, not superscripts.

Fred
Yeah, those square oxygen molecules hurt your throat, doncha know.
[/selffacepalm]
"The sane understand that human beings are incapable of sustaining conspiracies on a grand scale, because some of our most defining qualities as a species are... a tendency to panic, and an inability to keep our mouths shut." - Dean Koontz

Offline Noldi400

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Re: ISS Breathing Mix
« Reply #5 on: March 09, 2013, 05:04:22 PM »
The sea level mix would have made the use of spacesuits very difficult, because the astronaut would be subject to decompression sickness due to nitrogen dissolved in body fluids. The spacesuits used about the same pressure internally as the LM. It would also have added complexity to the spacesuits and space vehicles, because nitrogen would have to be supplied to re-pressurize after each EVA. The use of pure oxygen didn't offer all these difficulties, and the distances involved internally in the spacecraft did not pose a problem with regard to sound propagation in the low pressure.
Right answer, wrong question. I'm asking whether there was a specific reason why the low-pressure O2 atmosphere that was used through Apollo was replaced by a oxy/nitro mix.  Health reasons? Russian influence?
"The sane understand that human beings are incapable of sustaining conspiracies on a grand scale, because some of our most defining qualities as a species are... a tendency to panic, and an inability to keep our mouths shut." - Dean Koontz

Offline Allan F

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Re: ISS Breathing Mix
« Reply #6 on: March 09, 2013, 05:50:59 PM »
Then all I got is crew comfort. IIRC the russians spacecrafts have mostly been full of surface mix. Maybe some structual rigidity benefits?
Well, it is like this: The truth doesn't need insults. Insults are the refuge of a darkened mind, a mind that refuses to open and see. Foul language can't outcompete knowledge. And knowledge is the result of education. Education is the result of the wish to know more, not less.

Offline BazBear

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Re: ISS Breathing Mix
« Reply #7 on: March 09, 2013, 06:13:39 PM »
Guys?  It's O2 and N2 with subscripts, not superscripts.

Fred
Whoops! ;D
"It's true you know. In space, no one can hear you scream like a little girl." - Mark Watney, protagonist of The Martian by Andy Weir

Offline BazBear

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Re: ISS Breathing Mix
« Reply #8 on: March 09, 2013, 06:14:24 PM »
Guys?  It's O2 and N2 with subscripts, not superscripts.

Fred
Yeah, those square oxygen molecules hurt your throat, doncha know.
[/selffacepalm]
Yes, they would, wouldn't they? ;D
"It's true you know. In space, no one can hear you scream like a little girl." - Mark Watney, protagonist of The Martian by Andy Weir

Offline Donnie B.

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Re: ISS Breathing Mix
« Reply #9 on: March 09, 2013, 08:48:09 PM »
The ISS uses a ~78% O2/21% N2 atmosphere at about 14.7 psi.

Are you sure this isn't the other way around?  If not, why would they use such a highly O2 enriched mix at 1 atmosphere pressure?

Quote
(the partial pressure of O2 at seal level).

I believe seal level is a bit below sea level, at least during certain parts of the year :)

Offline BazBear

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Re: ISS Breathing Mix
« Reply #10 on: March 09, 2013, 09:58:42 PM »
The ISS uses a ~78% O2/21% N2 atmosphere at about 14.7 psi.

Are you sure this isn't the other way around?  If not, why would they use such a highly O2 enriched mix at 1 atmosphere pressure?

Quote
(the partial pressure of O2 at seal level).

I believe seal level is a bit below sea level, at least during certain parts of the year :)
Yes, the other way around, doh!*face palm* Guesss I shoouda prooof reaad.  ;)
"It's true you know. In space, no one can hear you scream like a little girl." - Mark Watney, protagonist of The Martian by Andy Weir

Offline Chew

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Re: ISS Breathing Mix
« Reply #11 on: March 09, 2013, 10:21:35 PM »
The ISS uses a ~78% O2/21% N2 atmosphere at about 14.7 psi.

Are you sure this isn't the other way around?  If not, why would they use such a highly O2 enriched mix at 1 atmosphere pressure?

Quote
(the partial pressure of O2 at seal level).

I believe seal level is a bit below sea level, at least during certain parts of the year :)
Yes, the other way around, doh!*face palm* Guesss I shoouda prooof reaad.  ;)

The other way around? Now you're saying seals can fly?

Offline ka9q

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Re: ISS Breathing Mix
« Reply #12 on: March 10, 2013, 12:23:02 AM »
I got interested in this a while back so I did a fair bit of reading. Here's what I learned.

There are three main advantages to air at sea level pressure over reduced pressure O2:

1. Reduced fire hazards.
2. No health issues
3. The Russians were already using it.

Actually, the shuttle was designed to use sea level air back when the notion of a joint US-Russian mission was still a joke. So air was a natural for the station even if the Russians hadn't joined us. Had we used anything else, incompatibility with the Russian atmosphere would have been a big problem. The different atmospheres was probably the biggest single technical problem that had to be overcome in ASTP.

Although the breathability of a cabin atmosphere depends mainly on the O2 partial pressure, the fire hazard depends more on the O2 percentage, regardless of the total pressure. A diluent gas reduces the fire hazard by carrying away heat. I've seen some films from Sealab showing what happens when they try to strike a match. Basically, nothing happens.

Everybody knows about the intensive fire safety campaign conducted on Apollo after the Apollo 1 fire. The hazard was certainly greatest before launch when the cabin was at (or somewhat above) sea level pressure, but it was still present in space at 5 psi cabin pressure. So everything that went into an Apollo cabin had to be carefully vetted for flammability in pure O2. That would have been completely impractical for the shuttle and especially the ISS, both of which were much bigger than the Apollo cabins and would carry lots of scientific experiments built by many different people.

O2 is very toxic at high partial pressures; it's a good example of too much of a good thing. Anything over 2-3 atm is likely to cause seizures; this has killed many divers. Although it seemed safe enough for the few weeks of an Apollo mission, nobody knew if 5 psi pure O2 was safe for much longer periods, and tests seemed to show some mild physiological changes. So for this reason and also to reduce the fire hazard, Skylab used a compromise atmosphere of 74/26 O2/N2 that could still be acommodated with the Apollo CSM.

Offline Dalhousie

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Re: ISS Breathing Mix
« Reply #13 on: March 10, 2013, 04:45:05 AM »
It also reduces the experimental variables on the human organism, one of the main subjects of ISS research.  Not having the enhanced fire risk of O2 enriched air also means a much wider range of items can be used off the shelf.


Offline ka9q

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Re: ISS Breathing Mix
« Reply #14 on: March 10, 2013, 11:17:16 AM »
That's a very good point, though if the goal were to understand the human organism under the conditions of a long-term interplanetary space flight you'd want to use whatever system was most likely to be used for such a mission.

There are some definite drawbacks to sea-level air:

1. Extra complexity. You need two gas supplies and O2 partial pressure sensors as well as total pressure sensors. On the ISS, O2 is made by electrolyzing water (including that recovered from urine) but N2 has to be sent up. You don't metabolize it, but you do have to replace that lost to leakage and through airlocks. (I suppose that N2 could also be made from urine (specifically the urea) but I doubt that's being worked on at the moment...)

2. The cabin walls, windows and seals have to be much heavier. (I occasionally have nightmares about being on the ISS when it suddenly depressurizes...)

3. EVAs are a real pain. To avoid the bends you have to go through a tedious pure-O2 prebreathe and decompression protocol. On Apollo, you could just put on your suit and go.

I'm not sure, but the fire hazard inside a Shuttle/Soyuz/ISS pressure suit might actually be greater than it would be in a reduced-pressure pure O2 atmosphere.

I know that in technical diving it is common to breathe pure O2 during your last decompression stop just below the surface to flush out the N2 a little faster. (You can't breathe it any deeper than a few meters, as it would be toxic.) On the ISS, you could also speed up the process by breathing pure O2 at 1 atm before you start lowering the pressure. Physiologically this would be tolerable for a few hours, but it would greatly worsen the consequences of a fire within the suit.