Why don't they run out of air in space?

Forgive me if this is a stupid question, but if one of my kids asked me I'm not sure what the right answer is. For example, why doesn't any given space station run out of air? As background, exhaled air is much the same in inhaled air, but some of the oxygen is converted to carbon dioxide.

Do they just carry additional air to replace the lost air, and somehow filter out the carbon dioxide? Or is there a chemical process to convert the carbon dioxide back to oxygen? Would such a process be possible on a mass scale, to address concerns of climate change?

I remember reading this article as a part of one of my high school science classes. Enjoy!

Quote: The Article

The primary source of oxygen will be water electrolysis, followed by O2 in a pressurized storage tank," said Jay Perry, an aerospace engineer at NASA's Marshall Space Flight Center working on the Environmental Control and Life Support Systems (ECLSS) project. ECLSS engineers at Marshall, at the Johnson Space Center and elsewhere are developing, improving and testing primary life support systems for the ISS.
Most of the station's oxygen will come from a process called "electrolysis," which uses electricity from the ISS solar panels to split water into hydrogen gas and oxygen gas.

NASA ECLSS

Quote: Wizard

Would such a process be possible on a mass scale, to address concerns of climate change?

What did all the climate changes in the past do
without man to be concerned about them? They
just happened and life went on.

They use systems similar to a rebreather in principle, but more sophisticated and with less expendable volumes.

Carbon dioxide is recycled via an absorption scrubber. The scrubber is then heated to high temperature to remove CO2 from it. CO2 is dumped overboard. There are two modules, one US and one Russian, doing it.
Oxygen is not carried in tanks, but in the form of water - it is 16 parts oxygen and only 2 parts hydrogen. Electrolysis is used to split it, H2 is dumped overboard.

Both processes are very energy-expensive. It's theoretically possible, but prohibitively energy-expensive, to split CO2 back into carbon and oxygen, by cracking it at several thousand Kelvins.

Quote: Wizard

Forgive me if this is a stupid question, but if one of my kids asked me I'm not sure what the right answer is. For example, why doesn't any given space station run out of air? As background, exhaled air is much the same in inhaled air, but some of the oxygen is converted to carbon dioxide.

Do they just carry additional air to replace the lost air, and somehow filter out the carbon dioxide? Or is there a chemical process to convert the carbon dioxide back to oxygen? Would such a process be possible on a mass scale, to address concerns of climate change?

Generally, any device that allows you to breathe your air more than once is called a "rebreather". The technology is widely used underwater, where you have pressure to contend with as well.

You have three principal steps in rebreathing.

1) Remove your exhaled carbon dioxide. This is accomplished through the use of a cannister of sodium hydroxide (Sofnolime). The carbon dioxide (gas) reacts with sodium hydroxide and calcium hydroxide to form calcium carbonate (solid).

2) Replace oxygen that you have consumed. Small tanks of pure oxygen or mixed gases (nitrogen-oxygen or helium-oxygen) inject fresh oxygen into the breathing loop.

3) Control the oxygen concentration in the breathing loop. Solid-state oxygen sensors monitor the partial pressure of oxygen in the breathing loop.


Wikipedia article on rebreather technology.
Company that specializes in rebreathers.

I think on a mass scale it would be very very expensive. They tried it in Arizona, and the volunteers were supposed to be sealed up for a month, but they quit because of safety concerns and decided to let some outside air in.

On a mass scale you would make liberal use of good old fashioned plants.

For SCUBA gear, only the military uses pure oxygen. Civilians cannot legally go over 50% oxygen, and for a practical matter most dive masters require far less (usually less than 30%). The danger of going into convulsions from oxygen toxicity at pressure are too severe. Even the special operations guys sometimes die on their mission.

Tell your kid that It's a very good question. I think that most space systems use liquid oxygen as a back up at the very least.

Quote: pacomartin

I think that most space systems use liquid oxygen as a back up at the very least.

Just gas bottles. Liquid oxygen would require energy to heat it up to safe temperatures, and if you have energy, you can as well use water.
But CO2 poisoning is a more difficult concern to address long-term than running out of O2.

Quote: pacomartin

For SCUBA gear, only the military uses pure oxygen. Civilians cannot legally go over 50% oxygen, and for a practical matter most dive masters require far less (usually less than 30%).

That's not strictly the case. Every closed circuit rebreather I've seen use pure oxygen (mind you, you don't see a lot of them). The diluent mixture is carried separately and added on the fly to maintain maximum safe concentration.
Nitrox or Trimix diving is not the same as a rebreather, they can't use pure O2.

Semi-closed circuits do use mixtures, you're probably talking about these. Semi-CCR are much cheaper and safer than CCR, and require minimal training, but they still do produce bubbles (but in less ridiculous quantities than aqualungs) and don't provide the dive time and depth limits of CCR.

Pure O2 rebreathers are used by the military for their cheapness and simplicity. However, they can't be used at significant depths, you can only go about 10m without running into issues, as little as 6m without special procedures.
Since tech and tec-rec divers are primarily interested in exploring greater depths than possible on compressed air, they have limited interest in pure O2 rebreathers. When they do, O2 rebreathers are easily homebuilt and the gas is easily and legally available.

The military uses rebreathers for 1) lack of bubbles, 2) compact size for basic models. Depth isn't sought in operations where pure O2 rebreathers are used.
There exist specialized military rebreathers for serious diving that are on par with civilian CCR in quality if not better, such as CDLSE. I've never seen one, though they are not illegal to own, but most of the cost goes towards silencing and strictly non-magnetic materials.

Quote: EvenBob

What did all the climate changes in the past do without man to be concerned about them? They just happened and life went on.

Well life went on, but most of the species died. Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.

This particular species does not care to go extinct.

Quote: EvenBob

What did all the climate changes in the past do
without man to be concerned about them? They
just happened and life went on.

Removal of a large number of the causes of excess CO2 production, to such a level that the CO2 -> O2+C converter were able to reduce the levels of carbon dioxide in the atomosphere again. It's possible that such a feedback mechanism may not always work (in either direction).

Quote: pacomartin

Well life went on, but most of the species died.

You act like that's a bad thing. We won't go extinct, we're
the last major species this planet will ever produce. The
earth finally got it right.

Quote: thecesspit

Removal of a large number of the causes of excess CO2 production,

You do realize that the vast majority of the effort
to combat climate change it to pay shit bag 3rd
world countries reparations for all 'harm' we've done
them. Do you know what a 'shakedown' is?

Quote: YoDiceRoll11

I remember reading this article as a part of one of my high school science classes.

It is kind of interesting that primarily at least the stored O2 is in the form of water. It has always surprised me that getting rid of the CO2 is more urgent than introducing more O2.

Quote: Wizard

Is there a chemical process to convert the carbon dioxide back to oxygen? Would such a process be possible on a mass scale, to address concerns of climate change?

On Earth the 'filtering' of CO2 is accomplished by photosynthesis. It's possible we could suck up more CO2 by fertilizing nutrient starved areas of the oceans. Disposing of excess CO2 is done by great 'sinks' on Earth, such as the Amazon jungle/swamps. These are so efficient that some scientists are now thinking the climate change problem isnt CO2 so much as release of stored methane, a much worse greenhouse gas by a large factor. It gets released when an area that had been cold warms up; this has a synergistic effect.

I don't think he acted like it's something bad. It's just part of nature. If the surroundings change and you are not able to adept quick enough you die.

Your other statements are pretty bold:

"We won't go extinct, we'rethe last major species this planet will ever produce"

We might go extinct. Actually I think it's pretty safe to say that we will go extinct at some point in time.
And what exactly is a MAJOR species ? I'd like to hear a definition of that please....

"The earth finally got it right."

What exactly did the earth got right ? And does the mud we live on really does have an intend ?

Thanks for the scientific replies. I appreciate the help.

If the climate change argument goes much further I'll split it off to another thread.

The planet provides much better ways of cleaning CO2 out of the atmosphere than scrubbers which use alot of energy which in turns produce alot more carbon (or nuclear waste, or hydroelectric production, or windmills, what have you).

Planting trees work, but you would have to plant about 350 trees for each person in the United States as a healthy tree absorbs about 13 pounds of carbon per year but each of us create about 4600 pounds per year.

Figuring out how to increase the carbon sinks in the oceans would help.

Quote: P90

Oxygen is not carried in tanks, but in the form of water - it is 16 parts oxygen and only 2 parts hydrogen.

Really? That's very odd. Granted hydrogen makes up for a minuscule part of the mass, but it is extra mass. Of course, water can be carried in anything, while LOX needs insualted tanks and such. But electrolysis uses up power, too, once you're in orbit. That's fine for the space station, but how do the Soyuz capsules manage?

The Shuttle, BTW, carried hydrogen and oxygen in separate tanks, and used fuel cells to generate power and water.

Anyway, as far as air goes, I think exhaled air contains only 4% CO2. So you'd need a reserve eventually, however it's carried.

Little known fact: air pressure is very emaningful. When you climb to, say, 2,200 meters above sea level, the atmosphere has the same composition as at sea level, still 21% Oxygen. but as the air pressure is lower, there are less oxygen atoms making up that 21%. That's why some people feel they don't get enough air when they first arrive at a high-altitude city: because they're not getting enough oxygen. the body adapts by, among other thing, producing more red blood cells.

Anyway, the early US rockets were a lot less powerful than the early Soviet models. So NASA had to save mass any way possible. One such way was to fill the capsules with 100% oxygen at a lower pressure. The Soviets used air at higher pressures. This NASA approach saved mass, but made the air inside much more suited for combustion. This resulted in the tragic fire that claimed the lives of three astronauts in an Apollo rehearsal on the pad at KSC.

The Shuttle did use air. I'm not sure what Skylab used.

Quote: EvenBob

You do realize that the vast majority of the effort
to combat climate change it to pay shit bag 3rd
world countries reparations for all 'harm' we've done
them. Do you know what a 'shakedown' is?

Way to change the subject. You asked what happened in the past when CO2 levels rose. I don't recall there being a shakedown documented in the fossil record.

I didn't even say that "climate change is occurring now" or even "CO2 levels are increasing right now". Mostly because I know it's a contentious point, and I can't be arsed to rehack that argument one more time.

Way to project, Bob.

Quote: Wizard

Thanks for the scientific replies. I appreciate the help.

If the climate change argument goes much further I'll split it off to another thread.

Photographers are often interested in re-breather technology since it reduces the amount of bubbles produced by their SCUBA equipment. This allows them more time to photograph animal life without the disturbing noises that compressed air produces.

I don't know what kind of SCUBA you've done,but you seem to enjoy photography. Bahamas are an excellent place to rent new technology.
Underwater Exploration in Bahamas

With compressed air (approximately 20% oxygen), you have to dive to 10 atmospheres until you begin to worry about oxygen toxicity. That is a depth of (10-1)*33 feet of sea water. It's essentially a non-issue because long before you get to those depths you have the much better known problems with Nitrogen gas (i.e. the bends). Nitrogen is not toxic, but it makes you essentially "drunk" underwater, which can be dangerous in and of itself. The effect is often called "nitrogen narcosis" or "narced" for short. The problem is if you don't surface properly or if you stay too long then you can get the buildup of gases in your joints (or in a worst case in your brain). A "resort dive" usually limits your depth to 60', but many professional dive boats are asking people to stay shallower than 100'. A hundred feet is conservative, but it helps their insurance. Professional divers are not allowed to go deeper than 175' if they are breathing compressed air without a hyperbaric chamber on board the ship. Since the cost of one of those units is so extraordinary, there are very few professional dives below that depth,

There is a range of technologies just to combat these problems other than breathing compressed air. Nitrox is perhaps the simplest solution. You breath a gas mixture which has a greater than normal percentage of oxygen. If your mix has 50% oxygen then you will have less headaches and your tank will last much longer. However the dangerous depth for oxygen toxicity is much shallower. It is now (10/2.5-1)*33 or 99'. The factor of 2.5 comes from oxygen being 2.5 times the percentage as normal air (ie. 50%/20% = 2.5). It is illegal for a recreational diver to breath NITROX with more than 50% oxygen. In reality a professional tour operator will probably only permit 35% at the most. You will also be asked to sign a log for every dive that says that you understand the dangers of toxicity, and you will not go beyond a certain depth. Oxygen toxicity is usually fatal, while nitrogen narcosis is often detected and you can recover.

A military special operations diver will often breath pure oxygen. It is possible to build a closed system that uses pure oxygen that lasts for up to 9 hours. This permits the special ops diver to use an open craft and invade a harbor, plant a bomb, and leave without surfacing and producing any bubbles which allows him not to be detected and shot. But he must stay shallower than 25' for the entire operation. As a practical matter, this confines operations to night. The danger of making an error and going too deep and dying in a convulsion from oxygen toxicity is relatively high.

AP diving makes the most widely used closed circuit rebreather technology diving equipment under the brand "Inspiration".

Another reason to use rebreathers is for cave diving (another specialty). One of the big problems with caves is that if you stir up too much sediment you can get disoriented and die in the cave.

Quote: Nareed

Really? That's very odd. Granted hydrogen makes up for a minuscule part of the mass, but it is extra mass. Of course, water can be carried in anything, while LOX needs insualted tanks and such. But electrolysis uses up power, too, once you're in orbit. That's fine for the space station, but how do the Soyuz capsules manage?

Yes, but in the end water offers the highest ratio of oxygen to total weight.

The Soviet program used air at normal atmospheric pressure with chemical CO2 scrubbers, NASA had pure oxygen at low pressure until Skylab. From Skylab on, it's air. Both stored oxygen as a liquid, shared between life support systems and the engines where present. Skylab had regular liquid tankage. Mir Station used a water electrolysis system, that was later redesigned and installed on ISS.

Quote: Nareed

Anyway, as far as air goes, I think exhaled air contains only 4% CO2. So you'd need a reserve eventually, however it's carried.

It's the reverse - breathing can only reduce air concentration by about 4%. So if 21% is going in, 16.x% is going out, then 12.x%, then 9.x%.

Quote: P90

It's the reverse - breathing can only reduce air concentration by about 4%. So if 21% is going in, 16.x% is going out, then 12.x%, then 9.x%.

If you don't scrub the CO2 off the air. But even if you do, some oxygen gets locked up in CO2 and you'll run out eventually. That's why you need a reserve to replenish the oxygen aboard.

Quote: Nareed

If you don't scrub the CO2 off the air.

It's whether you scrub CO2 or not, CO2 is a separate issue. Too much CO2 is simply toxic, that's why you can't just keep recycling the same diluent and adding oxygen to replace what's spent. A simple plastic bag you can breathe in and out of can extend your underwater time (without SCUBA) slightly. Semi-closed circuits reuse some of the air, like said plastic bag, but with replacement supply.

Quote: pacomartin

It's essentially a non-issue because long before you get to those depths you have the much better known problems with Nitrogen gas (i.e. the bends). Nitrogen is not toxic, but it makes you essentially "drunk" underwater, which can be dangerous in and of itself.

That's two distinct concerns, nitrogen narcosis and decompression sickness. Nitrogen toxicity occurs when you rapidly go below a certain depth. Say, you're qualified for 40 meters, you decide that a quick dip down to 45 meters won't hurt, and then at 45 meters your judgement is rapidly impaired enough that you don't care anymore.

So the recreational limit is usually maintained at 30 meters, at least as long as you aren't your own DM with a buddy. 60ft/18m is very conservative, but resort dives do it because entry level certification is only to 18m, and they don't want to split the groups. They'll be liable for potential mishaps if they let you go deeper than you're allowed to, and they'll have to cook the logs.

"Illegal" is an overly strong word. You aren't going behind bars for breathing the wrong mixture. (And since most diving destinations aren't even within US jurisdiction, no one else is either). However, if you are not trained in Nitrox diving, you can't be officially provided with Nitrox at all, although less scrupulous operators may allow people to use Nitrox without proper training, because there isn't a lot to train in. Nitrox-2 (36%) is very common in rec diving, because it can use the same equipment as air and stays within the limits of basic level certifications.

Quote: pacomartin

A military special operations diver will often breath pure oxygen. It is possible to build a closed system that uses pure oxygen that lasts for up to 9 hours.

Commercial/research diving CCR can do this as well, and deeper. It's not as much the advantages of pure O2 as simplicity and cheapness. You don't need all the sensors, mixing units, computer controllers. O2 rebreathers are easily operated manually with minimal training and maintenance.

Not the same story with computerized mixture CCR like on the picture. You need a couple months' training just to use one of these... and about half an hour maintenance per dive hour. System cost is similar to a small car, for some models not so small.
For maximum dive length, commercial divers use saturation diving and an underwater or onboard hyperbaric chamber, it's not that complex. With saturation diving, one can stay at significant depth almost indefinitely. But decompression can take a week and more, so it's never going anywhere else.

The fun part is in the lower right corner of the NASA diagram. This is probably solar-powered hydrolysis (Electrical Fractioning of H2O into H2 + O2). Venting the H2 outside is rather necessary.