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Izunya
04-27-2009, 08:21 AM
The first question is about microgravity. I understand that if you light a candle in microgravity and turn off the ventilation, the flame goes out, smothered by its own carbon dioxide. But if you have people in a spaceship, in microgravity, with the circulation temporarily offline—would the same thing happen to them if they didn't move? I'm pretty much trying to find out whether a sleeping person would be in danger. Or rather, more immediate danger than everyone else.

The second question is sort of silly. Does anyone have any idea what a soda would do in a habitat on the moon? Would it fizz up all at once, like you'd just shaken the bottle, and then go flat? Or would it act more or less normal?

Izunya

Pilot
04-27-2009, 08:54 AM
The first question is about micro gravity. I understand that if you light a candle in micro gravity and turn off the ventilation, the flame goes out, smothered by its own carbon dioxide. But if you have people in a spaceship, in micro gravity, with the circulation temporarily offline—would the same thing happen to them if they didn't move? I'm pretty much trying to find out whether a sleeping person would be in danger. Or rather, more immediate danger than everyone else.

The second question is sort of silly. Does anyone have any idea what a soda would do in a habitat on the moon? Would it fizz up all at once, like you'd just shaken the bottle, and then go flat? Or would it act more or less normal?

Izunya

As to the first question, since the human being respirates, there should be no chance for the CO2 to gather about their air source. Even the gentle act of breathing should provide enough movement in the air to create a flow. Consider that your breath is warm coming out of the lungs, and warm air rises, which should add to the movement of air even if only minimally.

The second question is also about CO2, the gas that provides the carbonation. It is under pressure, even after you open it here on Earth. On the moon, the lack of pressure would cause it to expand, perhaps even violently. That holds true in the open reaches of the moon. If you were in a shelter that allowed you to breathe, it would be about the same as here on Earth. That subject is all about air or atmospheric pressure, not gravity.

Mac H.
04-27-2009, 08:55 AM
The first question is about microgravity. I understand that if you light a candle in microgravity and turn off the ventilation, the flame goes out, smothered by its own carbon dioxide. But if you have people in a spaceship, in microgravity, with the circulation temporarily offline—would the same thing happen to them if they didn't move? I'm pretty much trying to find out whether a sleeping person would be in danger. Or rather, more immediate danger than everyone else.Our lungs move quite a bit of air .. if you fall asleep with your head into the pillow then you are basically rebreathing a lot of your CO2. However, your body compensates by simply breathing harder, which will increase the air movement.

For the sake of sci-fi story, though, it wouldn't surprise me if the guy who is asleep wakes up with a hell of a headache .. perhaps due to the increased CO2.


The second question is sort of silly. Does anyone have any idea what a soda would do in a habitat on the moon? Would it fizz up all at once, like you'd just shaken the bottle, and then go flat? Or would it act more or less normal?I'm assuming that the habitat (being habitable) is pressurised. So it should act just like on earth. (However, the pressure would be lower than sea-level - perhaps equivalent to the cabin pressure on a 747. So that is probably a fair comparison.)

Another question, though, is that with the extreme expense of taking anything to the moon, what the hell is a can of soda doing there?

Mac
Note: I noticed that I cross-posted with Pilot.

There is one correction to Pilot's entry, though:

Consider that your breath is warm coming out of the lungs, and warm air rises, which should add to the movement of air even if only minimally
Warm air rises because the denser cold air is dragged down by gravity. It wouldn't 'rise' in micro-gravity, because which way is up !?

benbradley
04-27-2009, 09:17 AM
I think the sleeping person in zero gee/microgravity would be okay, as breathing is sucking air in then pushing it out, and the exhale would always mix with "fresh" air that gets inhaled, and would circulate the air just enough so that each inhale would have a reasonable amount of fresh oxygen. If the body detects not enough oxygen, I suspect it will breathe deeper and/or more often, which may wake the person, as opposed to it being a danger. But that's just a guess.

I suspect the soda would act as "normal" as any other fluid on the Moon (rocking back and forth much more slowly in the glass than in Earth's gravity), though the air pressure may have something to do with whether it overfizzes. Air pressure in space or on the Moon would likely be substantially less than that at sea level, and more like that at a higher altitude.

Ruv Draba
04-27-2009, 10:30 AM
On earth, cooler air displaces warmer air because it's more dense, and sinks lower. Under gravity this creates convection currents -- cool air has more oxygen and replaces the warm air which has less. The convection currents feed the flame fresh oxygen and this is one reason that fires burn brighter with chimneys -- a chimney ducts the air-flow created by pressure and gravity.

Without gravity the warm air will still expand due to its higher pressure, and cool air will mingle with it, but the convection currents won't occur so readily. Without some sort of air-current the candle-flame will expire -- so in low gravity a candle-flame can survive near an air-vent, but might die in a room full of still air.

Our diaphragm is a kind of bellows. It creates its own air-currents, and our bodies also warm the air that we breathe, which can also help create currents. In practice though, rooms need air circulation to remain habitable. Whatever removes the carbon dioxide, methane, hydrocarbons from plastics, radon from electronics, and restores oxygen should also create air currents.

The rate of fizz in a soda bottle as you open it depends on the difference between the pressure in the bottle and the pressure in the environment. The higher the difference the more the fizz. At extremes, the gases in a liquid can fizz out so violently that the liquid itself boils out -- much like shaking a soda bottle then opening it. Not only that, but the boiling point of a liquid itself drops as you decrease pressure. These phenomena are familiar to jet-pilots, scuba-divers etc... who work at different pressures. The key factor is air-pressure though, rather than gravity. If the walls of your space home are strong enough you can have whatever pressure you like inside it. At regular earth atmosphere, your soda should fizz normally.

Pilot
04-27-2009, 11:19 AM
There is one correction to Pilot's entry, though:
Warm air rises because the denser cold air is dragged down by gravity. It wouldn't 'rise' in micro-gravity, because which way is up !?

Excellent point, Mac. Not to mention dead on the money. I was writing faster than I was thinking. Thanks. :)

geardrops
04-27-2009, 09:16 PM
I'm a computer engineer, for starters, not a chemist or a physicist, but I did reasonably well in those courses throughout my life.


The first question is about microgravity. I understand that if you light a candle in microgravity and turn off the ventilation, the flame goes out, smothered by its own carbon dioxide. But if you have people in a spaceship, in microgravity, with the circulation temporarily offline—would the same thing happen to them if they didn't move? I'm pretty much trying to find out whether a sleeping person would be in danger. Or rather, more immediate danger than everyone else.

Gravity is not really your issue here, the main problem is the amount of oxygen in the air in a sealed environment.

A candle will go out in an environment without oxygen simply because it needs oxygen to burn. Once it has consumed all the oxygen, it goes out, lacking the necessary components to maintain the chemical reaction.

People, obviously, consume oxygen and expel carbon dioxide. A sleeping person, however, breathes and metabolizes at a slower rate than a waking person, and therefore will actually survive longer than a waking person in an environment depleted of oxygen.

As for the air moving around, this becomes a problem of fluid dynamics and I did not study this in enough depth to tell you much about it.


The second question is sort of silly. Does anyone have any idea what a soda would do in a habitat on the moon? Would it fizz up all at once, like you'd just shaken the bottle, and then go flat? Or would it act more or less normal?

It'd act the same, as far as the carbonation in the soda goes. I believe, anyway. This is where I'm not sure. I'm wondering if perhaps the composition of the atmosphere affects how quickly sodas lose their carbonation, or maybe the lessened gravity will have an impact on how quickly the carbon dioxide moves to the surface of the soda and enters the atmosphere.

Lhun
04-27-2009, 11:29 PM
As for the air moving around, this becomes a problem of fluid dynamics and I did not study this in enough depth to tell you much about it.Well air still has friction in zero gravity so it stopy moving eventually. The interesting question is how big the "bubble" would be a human would interact with. I.e. breathing would not mix the air of a complete room, only within a certain radius. No idea how big that'd be though.

It'd act the same, as far as the carbonation in the soda goes. I believe, anyway. This is where I'm not sure. I'm wondering if perhaps the composition of the atmosphere affects how quickly sodas lose their carbonation, or maybe the lessened gravity will have an impact on how quickly the carbon dioxide moves to the surface of the soda and enters the atmosphere.While the composition of the air (or specifically, the partial pressure of the CO²) affect the rate with wich CO² exchange happens, that's not what causes bubbles. Bubbles happen because the soda has a CO² concentration that is only stable under higher pressure. Similar to how water can hold more salt in solution when its hot.
Anyway, in zero gravity but normal air pressure a soda behaves much like on earth, except that the bubbles don't rise (obviously) so it should slowly turn from water into a kind of foam. Or possibly the air bubbles merge into bigger ones until they break through the surface and the cohesion keeps the water together. Can't really say, i don't think there's a way to find out without trying. Though you might try youtube.

Izunya
04-27-2009, 11:29 PM
Thanks, everyone!

MacH wrote: Another question, though, is that with the extreme expense of taking anything to the moon, what the hell is a can of soda doing there?

It would have been manufactured there. We're not talking about a single base; my setting has several Lunar cities. I'm trying to work out the quirks of living there.

Thanks also for the idea of the CO2 headache.

dempsey wrote: Gravity is not really your issue here, the main problem is the amount of oxygen in the air in a sealed environment.

Well, the air circulation going out is a crisis—part of a larger, ongoing crisis, actually. My main character is emotionally wrung out and just got hit by a major worldview shift—she's going to be tired. I was trying to figure out how alarmed she should be by her own tiredness.

Of course, she could start wondering if it's a sign of oxygen deprivation . . . [evil author mode] heheheh. Yes, that works.

Again, thanks, everyone.

Izunya

Izunya
04-28-2009, 12:01 AM
Posting again because Lhun's reply wasn't up when I started writing . . .


Well air still has friction in zero gravity so it stopy moving eventually. The interesting question is how big the "bubble" would be a human would interact with. I.e. breathing would not mix the air of a complete room, only within a certain radius. No idea how big that'd be though.

Yeah, this is what I was wondering. The consensus seems to be that human lungs create a bit of a breeze and thus a decent-sized bubble—not immediately fatal, in other words. (Air circulation going out would be eventually fatal for everyone, whether they're asleep or not—but fear not, my characters will get it running just in time for the next emergency.) I still wonder if NASA has ever tried to work out the bubble radius mathematically. When I googled, I couldn't find anything except the bit where the candle goes out.


While the composition of the air (or specifically, the partial pressure of the CO²) affect the rate with wich CO² exchange happens, that's not what causes bubbles. Bubbles happen because the soda has a CO² concentration that is only stable under higher pressure. Similar to how water can hold more salt in solution when its hot.
Anyway, in zero gravity but normal air pressure a soda behaves much like on earth, except that the bubbles don't rise (obviously) so it should slowly turn from water into a kind of foam. Or possibly the air bubbles merge into bigger ones until they break through the surface and the cohesion keeps the water together. Can't really say, i don't think there's a way to find out without trying. Though you might try youtube.

Actually, these two questions are for two different stories. The soda wouldn't be in zero gravity, it would be in one-sixth—in a city on our moon—and mainly, I just wanted to know if they would even have soda. It's one of those worldbuilding things that might make it into the finished product in a single line, if that—but it Must. Be. Right. Or else crazy author goes crazier.

From what everyone else has said, they'd probably be able to have soda, so long as they can produce it themselves and not ship it up from Earth. Although it might taste a bit weird to an Earther, because they wouldn't be using corn syrup—corn being a big, greedy plant that takes up water and space that could be used for ultra-compact gengineered soybeans . . .

Y'know, I overthink these things. A lot. I'm not sure whether that makes me a good author or a good cat-vacuumer.

Izunya

Pthom
04-28-2009, 03:09 AM
Soda (the beverage) isn't dependent on corn syrup. I used to make root beer, using Hires flavor syrup, cane sugar, champagne yeast. (Used to, because Hires no longer sells the syrup). The yeast eats the sugar and in an anaerobic environment, produces CO2. I have to say the slight yeasty flavor enhanced rather than detracted from the taste. Never having made it using corn syrup, I couldn't say if it tasted better or worse. The manufacture of beer is similar: yeast eats sugary stuff (malt) aerobically for awhile to produce alcohol, then before the sugar is all gone, works anaerobically to produce the desired amount of CO2. Beer can be made from any sugar bearing plant material--most beer is made from barley malt--such as rice, wheat, beets... . Even soybeans (bleah).

The bigger danger would be getting the sugar/yeast concentration balanced so that you don't blow off the bottle caps. My forays into root beer production resulted in several episodes of violent de-cap-itations before I figured out the proper ratio. Flying bottle caps could be dangerous, depending on the material of the environment enclosure.

In any case, you may not have soda, but I'd think the likelyhood is high that you'll have beer. You probably have adults who desire the fruit of the barley but may not be patient enough to distill whiskey. :D