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The characters in my world use oxygen spheres like the top answer to this post (a leather balloon filled with air) to safely traverse land filled with smoke. How would you know when you are running out of air? Only medieval technology allowed.

Edit: This is a balloon you can strap to your back. A leather chord attaches the balloon to a mask on your mouth. I want enough air to last about 2 hours, so assuming a human breathes 440 liters of air every 15 minutes, as the post I linked assumes, the balloon would be about 4 cubic meters. Also removed bad assumption that oxygen is more dense than c02

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  • $\begingroup$ It's a leather bag filled with air under pressure. I'd say that it is quite obvious when the gas inside gets depleted. Hint: how can you tell whether a rubber tyre is filled with air under pressure or not? $\endgroup$
    – AlexP
    Commented Apr 9, 2020 at 21:51
  • $\begingroup$ @AlexP That was my first thought as well, but I just want to confirm thats how it works. Because, us as humans breathe out air (c02), so I thought maybe it won't sag because of that air we are blowing back out? A tire can sag because nothing is putting any air into it and the air inside leaks out. $\endgroup$ Commented Apr 9, 2020 at 22:18
  • $\begingroup$ You don't blow back CO2 into the bag. You blow CO2 into the surrounding atmosphere. (You cannot blow black CO2 into the bag, because the bag is under pressure.) $\endgroup$
    – AlexP
    Commented Apr 9, 2020 at 22:20
  • $\begingroup$ So, you would have to take a breath from the balloon, then take it off your mouth to blow out, and then put it back on for your next breath? I was kind of hoping my characters could just attach it to their mouth and breathe normally, from that answer in the post I linked they were using it for diving so I thought that was possible. $\endgroup$ Commented Apr 9, 2020 at 22:26
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    $\begingroup$ Oh no, not like that. They do it like divers do, with a no-return valve. Look how divers with aqualungs breathe. Air comes from the cylinders, into a regulator; when the diver breaths in, air flows into their mouth; when the diver breaths out, the no-return valve closes and the CO2-filled air escapes into the environment. (I was serious saying that you physically cannot blow back into a container under pressure. Your chest cannot generate more than about 2 psi or 0.15 atm of pressure.) $\endgroup$
    – AlexP
    Commented Apr 9, 2020 at 22:29

2 Answers 2

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Bring a canary with them. When it dies, find fresh air fast.

Yes, it's a trite answer, but it's also what we did back before we had technological ways of monitoring air quality.

If they're allowed sufficient scientific knowledge, when they start getting headaches, that's another indicator. (Especially as CO₂ may build up to problematic levels faster than O₂ is depleted.)

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  • $\begingroup$ Haha thats a clever idea! However, I'm not sure it quite fits, because where would you put the canary? Could it fit inside the oxygen balloon? Or could it be hooked up to the balloon somehow? $\endgroup$ Commented Apr 9, 2020 at 22:21
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    $\begingroup$ You don't necessarily need to know any science behind oxygen deprivation to learn that headaches are an early warning sign of imminent death. It won't take too many deaths to learn an approximation of how long you can use this breathing apparatus safely, and people will notice that headaches are common when you're about to run out of time. $\endgroup$
    – Ryan_L
    Commented Apr 9, 2020 at 22:58
  • $\begingroup$ @DanielHicks, if people fit, I think a canary will also. My impression was we're talking about something large enough for one to several people to fit in at once. It would almost have to be, anyway, to hold a useful volume of air. $\endgroup$
    – Matthew
    Commented Apr 10, 2020 at 14:25
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    $\begingroup$ Oh let me clear that up, its more like an oxygen tank that you can hold on your back. Instead of metal cylinder its a leather balloon for medival times. A leather chord attaches the balloon to a mask you put on your mouth. According to the answer in the link I posted you don't actually need that much volume to last 2 hours. When I said "get out" I meant to get out of the land filled with smoke, not the balloon. $\endgroup$ Commented Apr 10, 2020 at 18:02
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Pressure shouldn't change much in a closed system as O2 is converted into CO2 as both O2 and CO2 will behave much like ideal gasses.(http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/idegas.html) I would need to do a lot of fancy math to prove the concept, but how about lighting a candle in a closed jar and counting how long it takes to burn out? This can be complicated by lots of factors but in theory, as O2 concentration decreases, burn time would go down. (http://people.math.harvard.edu/~knill/pedagogy/waterexperiment/) This is a little off-the-cuff, so it would need some confirmatory research.

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  • $\begingroup$ PS I was assuming an environment of air (Think hot-air balloon), not just a bladder with some air in it. For a different kind of solution to your problem, try candles that MAKE oxygen. (chlorine.americanchemistry.com/Science-Center/…) Hard to justify with Medieval tech, but maybe some exotic animal has a fat that can be made into special tallow. $\endgroup$
    – DWKraus
    Commented Apr 10, 2020 at 4:21
  • $\begingroup$ I have mixed feelings. On the one hand, a burning candle may indicate that you still have enough oxygen. On the other, that flame is probably using up your available oxygen much faster than you would prefer, and I'm also not sure you won't run into problems with too much CO₂ first, which the candle won't help with. Oh, and it's also adding its own CO₂ (and maybe even-worse CO)... $\endgroup$
    – Matthew
    Commented Apr 10, 2020 at 14:28
  • $\begingroup$ It's a fair point. A candle is small, and in a big enough environment is consuming a sample. You draw blood from anemic people to see what their hemoglobin is, even though it takes a little hemoglobin out of them. Of course you'll run into problems with CO2 in a closed system, but medieval people aren't going to be able to generate sciency engineeringly sophisticated solutions that fix all the downsides. Downsides make good writing, anyway. $\endgroup$
    – DWKraus
    Commented Apr 10, 2020 at 14:49
  • $\begingroup$ It won't work. The human will be dead looooong before the candle stops burning. The human (and animal, in general) respiratory system is very very much more sensitive to the partial pressure of CO2 than to the partial pressure of O2. If one tenth of the O2 is converted into CO2, and the CO2 is not somehow scrubbed out from the air, the human dies. That's why they used canaries in the old days; the canary will die a little before the human, so if your canary abruptly stops singing it's time to leave the area quickly quickly. $\endgroup$
    – AlexP
    Commented Apr 10, 2020 at 18:12
  • $\begingroup$ Kind of see your point ( kimberlymoynahan.com/2012/04/…) Not QUITE as bad as that, but not good. Test would have to be very sensitive to be useful. $\endgroup$
    – DWKraus
    Commented Apr 10, 2020 at 19:57

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