ISS Breathing Mix

[raven]

Oh gods, a fire inside a spacesuit.
Thanks, I needed a new nightmare. My old ones were getting kind of boring.

[Dalhousie]

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).

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.

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

[ka9q]

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...

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.

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.

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.

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.

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.

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.

[Dalhousie]

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.

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.

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.

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.

[ka9q]

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

[Noldi400]

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.

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."

[Dalhousie]

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.

[ka9q]

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.

[ka9q]

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.

[Noldi400]

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.

[BazBear]

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?

(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!

[Dalhousie]

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.