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Apollo Discussions => The Reality of Apollo => Topic started by: Noldi400 on March 09, 2013, 01:22:36 PM

Title: ISS Breathing Mix
Post by: Noldi400 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)
Title: Re: ISS Breathing Mix
Post by: BazBear 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.
Title: Re: ISS Breathing Mix
Post by: Allan F 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.
Title: Re: ISS Breathing Mix
Post by: Nowhere Man on March 09, 2013, 04:41:07 PM
Guys?  It's O2 and N2 with subscripts, not superscripts.

Fred
Title: Re: ISS Breathing Mix
Post by: Noldi400 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]
Title: Re: ISS Breathing Mix
Post by: Noldi400 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?
Title: Re: ISS Breathing Mix
Post by: Allan F 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?
Title: Re: ISS Breathing Mix
Post by: BazBear on March 09, 2013, 06:13:39 PM
Guys?  It's O2 and N2 with subscripts, not superscripts.

Fred
Whoops! ;D
Title: Re: ISS Breathing Mix
Post by: BazBear 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
Title: Re: ISS Breathing Mix
Post by: Donnie B. 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 :)
Title: Re: ISS Breathing Mix
Post by: BazBear 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.  ;)
Title: Re: ISS Breathing Mix
Post by: Chew 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?
Title: Re: ISS Breathing Mix
Post by: ka9q 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.
Title: Re: ISS Breathing Mix
Post by: Dalhousie 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.

Title: Re: ISS Breathing Mix
Post by: ka9q 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.
Title: Re: ISS Breathing Mix
Post by: raven on March 10, 2013, 02:56:36 PM
Oh gods, a fire inside a spacesuit. :o
Thanks, I needed a new nightmare. My old ones were getting kind of boring.
Title: Re: ISS Breathing Mix
Post by: Dalhousie on March 10, 2013, 08:03:27 PM
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.

Everything has advantages and disadvantages.  At present clearly the advantages are seen outweigh the disadvantages.  I don't think there re has been any serious suggestion to return to pure O2 for the main spacecraft. With respect to 1) leak rates are nominally about 0.05% of cabin volume per day, so even over long missions the amount does not add up to much, although airlock purges will increase this significiantly.

2) indeed, although I don't have the numbers to hand, the percentage increase is small, I think and decreases proprotionally with larger volumes.


3) Indeed it is, as a diver I wonder if there is not excesive caution being shown with this.  However on the ISS there are not that many EVAs, two or three every six months. On a Moon or Mars station EVAs would happen several times a week, so this could be an issue, although we can note that prebreath approach (more below) seemed to be acceptable on Hubble servicing missions with similar or greater EVA rates.  However doing it month in and month out might be a different story.

Some studies have suggested a sealed EVA prep room with reduced pressure and enriched O2 would help the prebreathe process.  Also on the Moon and Mars, apart from servicing EVAs happening every few months, most EVAs wpould be associated with logn distance excursions with pressurised vehicles.  These would operate with O2 preduced pressure atmospheres, allowing quicker EVAs (especially with suit ports).

Quote
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 don't follow you, sorry.  Russian EVA suits (principally the Orlan) have always operated at 400 mb pressure with pure O2,  The US shuttle ISS suit (EMU) operates with only 296 mb of pure O2. The Apollo suit was even lowe, only 255 mb pure O2.  They are all reduced pressure pure O2 suits.

Quote
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.

This is more or less what is done on the ISS with the US suits, although the crews don't actually breath high pressure O2 in the suit, but wear masks beforehand. Orlan does not need a prebreathe, apparently

Title: Re: ISS Breathing Mix
Post by: ka9q on March 11, 2013, 01:10:12 AM
Everything has advantages and disadvantages.
Sure, at least the stuff we're discussing. If something has either all advantages or all disadvantages, we wouldn't have to talk about it...

Quote
With respect to 1) leak rates are nominally about 0.05% of cabin volume per day, so even over long missions the amount does not add up to much, although airlock purges will increase this significiantly.
I haven't seen the Shuttle or ISS figures, but on Apollo I was surprised at how much O2 was budgeted for leakage, both from the pressure suits and from the cabins. It could be as much as or even more than the metabolic consumption. The actual figures were generally less than the nominal, but still nonzero. So unless seals have gotten a lot better, I wouldn't be surprised to see the mass loss rate on the ISS be considerably more because of its higher total pressure and the greater volume per crewmember. On the other hand, its larger size means more volume to surface area, which would proportionately reduce leakage rates.

Quote
2) indeed, although I don't have the numbers to hand, the percentage increase is small, I think and decreases proprotionally with larger volumes.
Sure, because for many shapes the volume increases faster than the surface area.
Quote
3) Indeed it is, as a diver I wonder if there is not excesive caution being shown with this.
I'm surprised to hear you say that. I keep hearing horror stories of divers experiencing decompression sickness sometimes even when they follow the standard tables.

DCS, fortunately mild, has definitely happened in space. Michael Collins wrote about his experience with it on his Gemini flight and how he was determined to avoid it on Apollo 11. He didn't report it at the time because he thought it would get him thrown off flight status. That's too bad, because it's a human physiology thing that can hit anyone, and you can allow for and prevent it if you know it's a problem.
Quote
I don't follow you, sorry.  Russian EVA suits (principally the Orlan) have always operated at 400 mb pressure with pure O2,  The US shuttle ISS suit (EMU) operates with only 296 mb of pure O2. The Apollo suit was even lowe, only 255 mb pure O2.  They are all reduced pressure pure O2 suits.
Yes, but I wasn't talking about a fire hazard during the actual EVA but during the preparation. You're decompressing to suit operational pressure, and to speed it all up you're probably breathing O2-enriched air (or pure O2) to flush out the N2 in your tissues more quickly.

The Apollo astronauts faced the same (fortunately theoretical) hazard when they prebreathed pure O2 at 1atm or more in their suits starting at suit-up, through the walk-out and trip to the pad, and while waiting in the CM for launch.

Quote
This is more or less what is done on the ISS with the US suits, although the crews don't actually breath high pressure O2 in the suit, but wear masks beforehand.
Okay, that's probably the best approach as it limits the high pp-O2 to just the mask and its supply hoses, and I presume they can be made oxygen-safe.

Quote
Orlan does not need a prebreathe, apparently
Why not? It uses pure oxygen too, right? The pressure might be a little higher than the American suits, but I can't imagine it's so high that DCS isn't an issue.
Title: Re: ISS Breathing Mix
Post by: Dalhousie on March 11, 2013, 08:27:48 AM
I haven't seen the Shuttle or ISS figures, but on Apollo I was surprised at how much O2 was budgeted for leakage, both from the pressure suits and from the cabins. It could be as much as or even more than the metabolic consumption. The actual figures were generally less than the nominal, but still nonzero. So unless seals have gotten a lot better, I wouldn't be surprised to see the mass loss rate on the ISS be considerably more because of its higher total pressure and the greater volume per crewmember. On the other hand, its larger size means more volume to surface area, which would proportionately reduce leakage rates.

Maybe technolgy has improved since Apollo, all I know is that nominal leak rates in a recent study were 0.05% per day with an upper limit of 0.14% per day.

[quoteI'm surprised to hear you say that. I keep hearing horror stories of divers experiencing decompression sickness sometimes even when they follow the standard tables.[/quote]

There is no such thing as a completely safe decompression.  However incidence is very low.  A survey of scientific diving in Australia showed no cases in over 15,000 dives. In the US the incidence is about 1-2 every 10,000 dives by recreational divers who will be much sloppier in their proceedures.  In all cases the pressure changes are far greater than would be experienced by astronauts, so yes, even after reading some detailed justifications I do wonder whether excessive caution is being shown.

Quote
DCS, fortunately mild, has definitely happened in space. Michael Collins wrote about his experience with it on his Gemini flight and how he was determined to avoid it on Apollo 11. He didn't report it at the time because he thought it would get him thrown off flight status. That's too bad, because it's a human physiology thing that can hit anyone, and you can allow for and prevent it if you know it's a problem.

Do you have a reliable source for this?  All I have found so far with Google are unattributed sources.

Quote
The Apollo astronauts faced the same (fortunately theoretical) hazard when they prebreathed pure O2 at 1atm or more in their suits starting at suit-up, through the walk-out and trip to the pad, and while waiting in the CM for launch.

ISS astronauts are not, as far as I know, ever breathing one atm O2.

Quote
Orlan does not need a prebreathe, apparently
Why not? It uses pure oxygen too, right? The pressure might be a little higher than the American suits, but I can't imagine it's so high that DCS isn't an issue.
[/quote]

It's higher pressure and presumably the Russians don't think a prebreathe is neccessary.  They don't seem to have had any problems either.
Title: Re: ISS Breathing Mix
Post by: ka9q on March 11, 2013, 12:14:10 PM
Michael Collins wrote about his Gemini DCS experience in his book Carrying the Fire. He experienced the classic joint pains of the bends.
Title: Re: ISS Breathing Mix
Post by: Noldi400 on March 11, 2013, 03:58:18 PM
DCS, fortunately mild, has definitely happened in space. Michael Collins wrote about his experience with it on his Gemini flight and how he was determined to avoid it on Apollo 11. He didn't report it at the time because he thought it would get him thrown off flight status. That's too bad, because it's a human physiology thing that can hit anyone, and you can allow for and prevent it if you know it's a problem.

Do you have a reliable source for this?  All I have found so far with Google are unattributed sources.

Quote
Michael Collins wrote about his Gemini DCS experience in his book Carrying the Fire. He experienced the classic joint pains of the bends.

Specifically (since it's lying right here by my computer...

About 8 hours into the flight, trying to go to sleep:

"Speaking of hurting, my left knee hurts, a throbbing ache that began a couple of hours ago, gradually worsened, and is now holding steady at a moderate but very uncomfortable level of pain. I think it is nitrogen coming out of solution in the tissues... The reason I make this diagnosis is that the pain is exactly like ones I have felt before in altitude chambers. ...what to do now(?). Discuss it or try to ignore it? I have a vivid picture of the avalanche of medical conferences one quick complaint will produce... everything short of a house call. ... What can they tell me to do, besides take a couple of aspirin?"
Eight hours, four aspirin, and a couple of hours of spotty sleep later:

"... when I awake I note with relief that my knee pain has nearly disappeared."




Title: Re: ISS Breathing Mix
Post by: Dalhousie on March 12, 2013, 04:09:41 AM
Michael Collins wrote about his Gemini DCS experience in his book Carrying the Fire. He experienced the classic joint pains of the bends.

Thanks to both you and Noldi400

Was it his opinion that he suffered DCI, or was it diagnosed?  Lots of things can cause joint pain after all.
Title: Re: ISS Breathing Mix
Post by: ka9q on March 12, 2013, 04:51:14 AM
I don't think he reported it at the time. I remember reading in papers on life support research that there was only one reported case of DCS (decompression sickness) among American astronauts and it was reported well after the fact, so it must have been a reference to Collins.

You saw Collins' own explanation that he self-diagnosed DCS by the exact match to known DCS he had experienced in altitude chambers. That it went away within a day is a strong indication he was right.

Title: Re: ISS Breathing Mix
Post by: ka9q on March 12, 2013, 04:56:08 AM
Last May I had a conversation about DCS with Skylab-3 astronaut Owen Garriott. I asked him if they had to take any special precautions when flying (e.g., in a T-33) after performing underwater training. I don't remember his exact answer but I do remember being surprised that the wait period was so short compared to the recommendations I had read. It was a number of hours vs a day.

I don't know what they breathe during underwater training, but judging from the sizes of Skylab and the ISS and the pools I suspect it's not pure O2.

Title: Re: ISS Breathing Mix
Post by: Noldi400 on March 12, 2013, 12:24:44 PM
You saw Collins' own explanation that he self-diagnosed DCS by the exact match to known DCS he had experienced in altitude chambers. That it went away within a day is a strong indication he was right.

Also, he experienced identical pain on Apollo 11, although to a much lesser degree. He stated that this time he had "wasted no time in suiting up" so as to maximize his preoxygenation time, so it's pretty certain he was correct in his diagnosis.

Last May I had a conversation about DCS with Skylab-3 astronaut Owen Garriott. I asked him if they had to take any special precautions when flying (e.g., in a T-33) after performing underwater training. I don't remember his exact answer but I do remember being surprised that the wait period was so short compared to the recommendations I had read. It was a number of hours vs a day.

I don't know what they breathe during underwater training, but judging from the sizes of Skylab and the ISS and the pools I suspect it's not pure O2.

I wonder how deep that tank/pool is. Back in the early '70s the restrictions of flying after diving were generally less conservative  than they are now, and (this is a guess)  I'll bet they were using USN dive tables, which were less conservative than sport diving tables; if the training was done at 30' or less, that was considered to cause very minimal nitrogen saturation, even if you were down for 2 or 3 hours.

As far as I can find, dive/flight guidelines were largely educated guesses and varied wildly from one group to the next. It looks like the USN guideline was indeed two hours.
Title: Re: ISS Breathing Mix
Post by: BazBear on March 12, 2013, 08:42:35 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?
Only the level seals!
Title: Re: ISS Breathing Mix
Post by: Dalhousie on March 13, 2013, 04:12:55 AM
I don't think he reported it at the time. I remember reading in papers on life support research that there was only one reported case of DCS (decompression sickness) among American astronauts and it was reported well after the fact, so it must have been a reference to Collins.

You saw Collins' own explanation that he self-diagnosed DCS by the exact match to known DCS he had experienced in altitude chambers. That it went away within a day is a strong indication he was right.

It's still a non-medical self diagnosis.  DCS pains generally take much longer to go away than a day without recompression.

If he was right then this is is most unusual, suggesting some predetermining condition.