General / Off-Topic geek question about space/scuba diving

[CMDR Sgt.Code]

So, chatting to my old man ealier and discussing how long a standard scuba cylinder (12L) would last in space.
working on the theory that for every ~10 metre depth is 1 atmosphere of compresion so say 30 metres is 3 times the pressure than at sea level, the deeper you go the more compressed your air supply gets and the faster you consume it so standard tank can last up to about an hour at 5-10 metres depth but only about 30 mins and 30 metres, my question is how long would that air supply last in space? no atmosphere means no compression on the cylinder so my tinking is about 1.5 hours maybe 2hours depending on the breathing rate of the user?

 

[Micha]

Well, you'll still need to breathe at more or less atmospheric pressure if you use normal air, otherwise there's not enough oxygen to keep you conscious/alive. But if you increase the Partial Pressure of oxygen in the air you breathe, you can use much lower pressure. For example, astronauts on spacewalks breathe pure oxygen since their suits are pressurised to only 1/3rd of atmospheric pressure.

But keeping it a standard SCUBA cylinder using a normal mix of ~79% nitrogen / 21% oxygen, 12l tank filled to 220 bar, you've got 2640 litres of air. You can actually breathe it until it's emtpy as there's no opposing pressure.

At rest, you go through about 8 litres of air per minute.

This gives you about 330minutes, or 5-and-a-half hours, of air with an open-circuit breathing device.


In practice you'd never use an open-circuit breathing device in space as the exhausting air would push you all over the place unless you were anchored onto something. Astronauts effectively use rebreathers, so a fairly small oxygen cylinder, and CO2 scrubbers. This gives them roughly 6-8 hours of EVA due to needing much more than just air to keep an astronaut alive; you also need active heating and cooling and various other bits of machinery.

 

[Screemonster]

It's nothing to do with compression of the cylinder itself. Whether in open air or underwater, the gas in the cylinder is many many times the pressure of whatever is outside it.

The regulator (mouthpiece) of the SCUBA gear has a diaphragm valve that supplies air from the tank at equal pressure to the outside environment - if you were under 10M of water and the gas in the mouthpiece were only at 100kPa, you wouldn't be able to inhale it. The water pressure around your body would compress your lungs. (this actually happens to free divers - the air in their lungs is at surface atmospheric pressure, so their lungs actually shrink during the dive)
Because of this, when you take a breath at 30M below, you're draining a breath of a time at 400kPa (roughly) instead of 150-200 for a shallow dive / 100 at the surface. More pressure per volume = more mass of gas removed from the tank.

Since you wouldn't want to actually expose yourself to "external" pressure while in space, you'd use a pressure-sealed suit and your breathing apparatus would supply air at whatever pressure that suit operated at. On the other hand, since you're not operating at higher pressures, you won't have to worry about the oxygen toxicity issues of using an enriched air mix either.