How long will a given supply of air last?

Hi, first post here, please be gentle.

I've tried researching this myself, referring to older posts from this
newsgroup, frighteningly technical nasa documents,
http://www.projectrho.com/rocket/rocket3g.html, the wikipedia and
various other sources. I've discovered a lot of arcane and
contradictory information and learned quite a lot but apparently not
enough. I think it's time I asked for some help, and I figured this
would be the best place to try. I'm not afraid of research, so if
someone would rather show me how to learn what I want to know rather
than spell it all out for me then tat would be gratefully received as
well.

I'm writing a short story set on a space station. Their air-recycling
system stops functioning (as these things always do in short stories)
and two people are left trying to figure out how long they can survive
on just the air surrounding them. I'd like to give them about 2 days.
Unfortunately, I can't simply say "they've got about two days" and
leave the rest of the details unspecified- The dimensions of the
station are critical, and I don't want to build it a hundred metres
across if they could continue breathing in there quite happily for
months.

Essentially, what I'd like to know but have been unable to work out is:

1) How long will a given volume of air, in metres cubed, support an
average human being?
2) (Less important) By what sort of percentage might that figure be
reduced by excercise/ exertion?
3) (Very important) Since I have a time period in mind for the
survivability of my protagonists, I'm assuming I can simply work out
the dimensions required to contain that volume of air and design my
station accordingly. Is this a valid way of going about this, or are
there other factors that make "air breathed in metres cubed per hour" a
meaningless expression for my purposes?

To nail down some of the other variables: I think I'd like to stick
with an "Earth-normal" mixture of gases at standard sea-level pressure.
It makes most sense for the story, for a number of reasons. All other
life-support systems, ie temperature regulation, air circulation, are
still functioning. There are no leaks (although I suppose I could make
some if necessary...)

wrote:
Hi, first post here, please be gentle.

I'm writing a short story set on a space station. Their air-recycling
system stops functioning (as these things always do in short stories)
Heck, it does that on a fairly regular basis on our real space station
too.
FWIW, there are 3-4 semi-independent components of air recycling.
Oxygen production, CO2 removal, trace contaminant removal, and
condensate removal. They may or may not be tied together.

I'd like to give them about 2 days.
Unfortunately, I can't simply say "they've got about two days" and
leave the rest of the details unspecified- The dimensions of the
station are critical, and I don't want to build it a hundred meters
across if they could continue breathing in there quite happily for
months.

For two days it would have to be a pretty small station. I don't have
numbers offhand, but from what I remember of the reports of various ISS
failures, it is quite a bit longer than that on ISS. If you look back
on the various Elektron, CDRA and Vozdukh failure reports, you might
get some ballpark numbers.
For example this story http://msnbc.msn.com/id/5953450/
suggests that an ISS crew of two could go 7 days without fresh oxygen
before flight rule limits were exceeded. You could survive for quite a
while after that. I suspect failure of the CO2 scrubber system would
cause problems more quickly, but again I don't have any numbers handy.

I know numbers for this sort of thing have been posted in sci.space.*
before, so searching the archives might be worthwhile.

Also, if you are going for realism, one would expect such a station to
have backup systems. For example, ISS has an oxygen generator, bottled
air and oxygen (in multiple different locations), and 'candle'
emergency oxygen generators. For CO2 removal, it has 2 independent
regenerative systems, and LiOH canisters for emergency. I would expect
any future space station to have at least one primary regenerative
system and a backup expendable system good for at least a week or two.

Beyond that, they would have the life support systems of any docked
spacecraft, and of EVA suits.

Of course, you can work around these things in a story... maybe part of
the station has to be closed off, so there is less volume available and
some backup items are inaccessible.

wrote:
Essentially, what I'd like to know but have been unable to work out is:
1) How long will a given volume of air, in metres cubed, support an
average human being?
2) (Less important) By what sort of percentage might that figure be
reduced by excercise/ exertion?
...


Take a look at:
http://www.newton.dep.anl.gov/askasc...9/eng99210.htm

However, I disagree with the latter part of that web page. There are
two factors that will greatly reduce the "survivability time":

1) The increasing percentage of CO2 does matter. At some point, when the
amoun of CO2 is high enough, the poor astronaut begins to feel dizzy, and
looses his/her consciousness. This happens significantly before that the
O2 reserves (from surrounding air) has depleted.

2) The human body can not survive in an air, which is low with O2. The
limit is usually ~14-16%, and with some high-altitude training one
can tolerate even ~11% oxygen (assuming that the pressure is sea-level STP),
for some short time. So, when the starting situation is that the O2-level
is 21%, you can barely use half of that before loosing your consciousness.
(however, the limit is not the percentage of O2, but the partial pressure)


Matti Anttila
http://masa.net/space/

1) How long will a given volume of air, in metres cubed, support an
average human being?

The average adult human uses 2 pounds/1 kilogram of oxygen per day.

At sea level air pressure and Earth-like air mix (80% nitrogen/20
oxygen), there's roughly 1 kilogram of oxygen in every 4 cubic meters
of air.

However, without carbon dioxide scrubbers, I think the limiting factor
will be CO2 build-up, not oxygen depletion. Normal 8-hour exposure
limits are 5000ppm (0.5%). When carbon dioxide levels reach 10%, you
can quickly (~10 minutes) pass out and die.

Actually, I'm not sure how quickly humans produce carbon dioxide. Some
inhaled oxygen goes into making water from food, while some goes into
making carbon dioxide. I just seem to recall that CO2 accumulation was
more of a problem on Apollo 13 than oxygen supplies. As a WAG, you
might say CO2 levels become a problem in half the time that oxygen
depletion would be a problem.

2) (Less important) By what sort of percentage might that figure be
reduced by excercise/ exertion?

I'd guess that you could estimate the increase by looking at the
calories the person is burning. The baseline metabolism of about 1500
calories/day/person represents the 1kg of O2 per day consumption. If
the person exerts 3000 calories in a day, double O2 consumption.
(Please, anyone feel free to correct me.) For caloric
expenditures...well, just hit any exercise or dieting website.

3) (Very important) Since I have a time period in mind for the
survivability of my protagonists, I'm assuming I can simply work out
the dimensions required to contain that volume of air and design my
station accordingly. Is this a valid way of going about this, or are
there other factors that make "air breathed in metres cubed per hour" a
meaningless expression for my purposes?

As I said, I think carbon dioxide will be a problem first, but carbon
dioxide production is linked to available oxygen. Estimating air volume
is a good place to start.

Note that unless your station is small, a lot of the air may prove hard
to access. Sure, there's fresh air in the all the cubic meters of
maintenance tunnels, but are your protagonists going to run around
looking for fresh air pockets?

Other stumbling blocks:

Emergency systems like oxygen candles and carbon dioxide scrubbers can
be very compact and lightweight systems. A few kilograms of each system
should give another man-day of life support. I mean, they're so simple
that it'd almost be reasonable to stick some in every corridor or
module of a station as back-up systems.

A tank of liquid oxygen stores pretty well and is a dense source of
fresh oxygen. Each liter of liquid oxygen is another man-day of oxygen.
You'd still have the CO2 problems, but fresh oxygen wouldn't be a
concern.

Also, if the protagonists have electricity and a few hours to set up an
electrolysis rig, a liter of water has almost a day's supply of oxygen
for one person. They'd have to separate and dump the hydrogen somehow,
but you should be able to get some oxygen from that. (Still have the
CO2 problem.)

Mike Miller

wrote in message
oups.com...

message snipped

Others have given the particulars on how much O2 a person need, and how much
CO2 will cause unconsciousness and death quickly.

If you have the available equipment CO2 is very easy to strip from an
atmosphere. It just requires a 'little' atmosphere processing.

If your protoganists have the time / equipment / motivation / energy /
sufficiently large heat sink all they need to do is routinely cool some part
of the atmosphere to ?78 °C. The CO2 will sublimate out as dry ice. Then
all you have do is sweep it up and put it somewhere air tight and safe where
it can't contaiminate the rest of the air supply.

If your protoganists have already gone the route of having to cracking water
via electrolisys to get their O2 then they can of cause burnt the CO2 and H2
produced in something called a Sabatier reactor. Out comes some H2O (that
can be reprocessed for electrolisys and much methane. Methane is slightly
easier to store (if you have big strong pressure vessels and refridgeration
gear) than dry ice. And if you get really deseperate you can invest a big
chunk of energy in pyrolising (burning with out oxygen) the methane and
getting carbon ash and H2, which you can send back to the Sabatier to turn
more CO2 in to Methane and H2O.

But for small groups of people, small spaces, and shortish periods of time
just putting a refridegration unit in the air conditioning loop is probably
easier ;-). And vent the dry ice via the toilet or put it in the airlock.

For more info on the Sabitier/electrolisys process look up Robert Zubrin's
Mars Direct proposal.

Regards
Frank Scrooby

On 5 Nov 2005 08:01:26 -0800, "
sprachen:

The baseline metabolism of about 1500
calories/day/person

I thought it was 2000 for an average woman, 2500 for a man.

Oh, actually ISWYM, the higher figure is average calorie use, not just
baseline (eg sleeping) level. But they'd be doing a fair lot of stuff
fixing the station, and just thinking can burn a good few watts.

------------------------------------------------------------------------

if love is a drug, then, ideally, it's a healing, healthful drug... it's
kind of like prozac is supposed to work (without the sexual side
effects and long-term damage to the brain and psyche)

Thanks for all the answers everyone.

Quoting Hop, Nov 5, 1:14 am :
FWIW, there are 3-4 semi-independent components of air recycling.
Oxygen production, CO2 removal, trace contaminant removal, and
condensate removal. They may or may not be tied together.

I knew it wouldn't be simple=-) Good job the only space stations I have
to design are imaginary ones.
I never realised that o2 generation and CO2 removal were seperate
processes. I assumed the CO2 scrubbers extracted the oxygen and dumped
it back into the atmosphere, killing two birds with one stone. Instead
they just store the CO2 in some form and then dump it into space/ ship
it back to Earth/ feed it to the plants, do they?


Also, if you are going for realism, one would expect such a station to have backup systems.

This is a very good point, but a somewhat inconvenient one. I can break
the backups easily enough, I guess (the malfunction in my story is not
accidental).



Now quoting Matti Anttila Nov 5, 12:31 pm :
1) The increasing percentage of CO2 does matter. At some point, when the
amoun of CO2 is high enough, the poor astronaut begins to feel dizzy, and
looses his/her consciousness. This happens significantly before that the
O2 reserves (from surrounding air) has depleted.

Quote from Nov 5, 4:01 pm :
However, without carbon dioxide scrubbers, I think the limiting factor
will be CO2 build-up, not oxygen depletion. Normal 8-hour exposure
limits are 5000ppm (0.5%). When carbon dioxide levels reach 10%, you
can quickly (~10 minutes) pass out and die.
...
might say CO2 levels become a problem in half the time that oxygen
depletion would be a problem.

This is valuable information to me. The whole story hinges around CO2
anyway, so this is the angle I will approach from: I will break the CO2
removal systems. Thanks.
I understand CO2 poisoning is quite unpleasant: Pain in the muscles
from lactic acid buildup and so on. Is that right?


As a WAG,

Wildly Accurate Guess?


Quoting Nov 5, 4:01 pm :
Note that unless your station is small, a lot of the air may prove hard
to access. Sure, there's fresh air in the all the cubic meters of
maintenance tunnels, but are your protagonists going to run around
looking for fresh air pockets?

Actually, yes. I'd very much like my station to be in the classic
"doughnut" shape (spinning for apparent gravity- think Blue Danube bit
from 2001), with my two characters involved in a long, pointless
pursuit around and around and around...
I realise that this introduces problems of its own (ie, if you build it
too small and spin it, you induce nausea) but the shape is important.


I mean, they're so simple
that it'd almost be reasonable to stick some [oxygen candles] in every corridor or
module of a station as back-up systems.

Yeah, this is another very sensible point, but potentially somewhat
troublesome. Do these oxygen candles burn like flares, or is the
reaction more slow and controlled? What is the chemical reaction
involved? Presumably, having oxygen locked up in an easily-released
form like that, it might be possible to engineer some kind of explosive
or torch-weapon from an oxygen candle. Having one character attack the
other with a weapon fashioned from their only source of survival might
add a nice touch of extra irony to their unfortunate situation.


Quoting Frank Scrooby Nov 7, 6:13 am :
If your protoganists have the time / equipment / motivation / energy /
sufficiently large heat sink all they need to do is routinely cool some part
of the atmosphere to ?78 °C. The CO2 will sublimate out as dry ice. Then
all you have do is sweep it up and put it somewhere air tight and safe where
it can't contaiminate the rest of the air supply.

Wow, I had no idea this kind of thing might be possible. Sadly, my
protagonists are seem to be too busy chasing one another to death to
work together for mutual survival.

I'll go back through this thread and re-approach my problem from a
broken CO2-scrubber perspective. This is good, since it would appear
that this problem (as opposed to o2 availability) would require a much
larger starting volume of fresh air to provide my planned 2 days, and a
larger starting volume means I can probably design a reasonable-sized
doughnut station. However, I can probably stretch things out to give
them an extra day or two,

Thanks again everyone. When I have the story written, I'll be sure to a
post a link to it in this thread.

wrote:
Thanks for all the answers everyone.
I never realised that o2 generation and CO2 removal were seperate
processes. I assumed the CO2 scrubbers extracted the oxygen and dumped
it back into the atmosphere, killing two birds with one stone. Instead
they just store the CO2 in some form and then dump it into space/ ship
it back to Earth/ feed it to the plants, do they?

Yes, ISS just collects it and dumps it into space. Of course, that
isn't the only way to do it. A fully closed system would recycle the
CO2. The C would eventually find its way back through the system as
food. Even so, one would expect such a system to have independent
subcomponents, with some margin in each step.

It is reasonable to assume that as we progress in space, we will move
toward more closed systems.

As Mike Miller pointed out, circulation is quite important. Again
looking to ISS as an example, they have a significant ventilation
system to keep air moving to all parts of the station (I've heard it
described as sounding like the inside of a data center). When the
screens get clogged or ducts get kinked, they see CO2 rise in certain
areas. ISTR on Mir, crew were advised to pick their sleeping spots near
a ventilation outlet, so they wouldn't wake up with a headache from
stale air. A spinning station might suffer from this less, as it would
have some natural circulation.
Yeah, this is another very sensible point, but potentially somewhat
troublesome. Do these oxygen candles burn like flares, or is the
reaction more slow and controlled? What is the chemical reaction
involved?
The reaction is quite vigorous... there was a fire on Mir caused by
these devices:
http://lubbockonline.com/news/032197/american.htm
http://spaceflight.nasa.gov/history/shuttle-mir/history/h-f-linenger-fire.htm
The chemical in question seems to be lithium perchlorate
http://en.wikipedia.org/wiki/Chemical_oxygen_generator
says they burn at about 500c

It has been suggested that your protagonists could generate O2 by DIY
electrolysis, but the experience with the electrolysis based oxygen
generators on Mir and ISS (Elektron) suggests this is a non-trivial
task. A spinning station probably makes it a bit simpler, as many of
the problems with Elektron result from the behavior of gas and fluid in
microgravity.

wrote:
I never realised that o2 generation and CO2 removal were seperate
processes. I assumed the CO2 scrubbers extracted the oxygen and dumped
it back into the atmosphere, killing two birds with one stone. Instead
they just store the CO2 in some form and then dump it into space/ ship
it back to Earth/ feed it to the plants, do they?

Mir and the ISS extract(ed) oxygen from exhaled/excreted water. (Your
body produces both CO2 and water as part of respiration, a result of
the combination of atmospheric oxygen with the carbon and hydrogen in
foods.) Though there have been proposals to crack CO2 for oxygen, I
don't believe the process has been actually used. Getting oxygen from
water is relatively easy.

Carbon dioxide is usually captured with chemicals (e.g., soda lime or
lithium hydroxide) or molecular traps (zeolite ceramics), then dumped
overboard.

I understand CO2 poisoning is quite unpleasant: Pain in the muscles
from lactic acid buildup and so on. Is that right?

You'd have to try your hand at googling. I was having difficulty
finding information on CO2 poisoning levels. I kept getting results
mixed in with carbon monoxide poisoning.

I realise that this introduces problems of its own (ie, if you build it
too small and spin it, you induce nausea) but the shape is important.

Cool, if you know that, that makes this explanation easier.

For semi-stomach-safe 3rpm design, you'd have a torus ~200m in
diameter. Assuming the donut is 10m square in cross-section, that's
~62800 cubic meters of air. It would take 2 people roughly 15700 days
to deplete that oxygen at normal exertion levels. Per prior
discussions, CO2 would be a problem first, but you're still looking at
more than a decade before the air becomes unbreathable.

Plus, you need to disable CO2 scrubbers that are probably present
around the station.

Is it possible to have a station-wide fire? Perhaps due to a faulty
oxygen line design? That would gulp down oxygen and produce plenty of
toxic gases.

Yeah, this is another very sensible point, but potentially somewhat
troublesome. Do these oxygen candles burn like flares, or is the
reaction more slow and controlled?

Well, they HAVE burned like flares on Mir, but that wasn't by design.


Good overview:
http://en.wikipedia.org/wiki/Chemical_oxygen_generator
Buy a few to play with and burn yourself:
http://www.molecularproducts.co.uk/v...cts/candle_33/
"Dud" candles on the ISS:
http://www.nasawatch.com/archives/20...ygen_cand.html

Mike Miller

Quoting Frank Scrooby Nov 7, 6:13 am :
If your protoganists have the time / equipment / motivation / energy /
sufficiently large heat sink all they need to do is routinely cool some
part
of the atmosphere to ?78 °C. The CO2 will sublimate out as dry ice. Then
all you have do is sweep it up and put it somewhere air tight and safe
where
it can't contaiminate the rest of the air supply.



I vaguely remember reading some science fiction book as a kid, might
have been "Island In The Sky" or some such (Lester delRey maybe, it's
been a long time ago) where there's some sort of malfunction in the life
support system. The intrepid hero plumbs up some tubing on the shady
side of the station and circulates air through it. The CO2 freezes out
and everyone is saved.