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I say "breathable gas" because what's in the tank can be a variety of mixtures and I don't know a single word for gas mixture you can safely breathe.

Scuba divers and astronauts have tanks of breathable gas and as they maneuver in their respective aqueous and vacuous environments consume some of it, expel CO2, H2O, CH4, H2 and other things into it, and their gear eventually expels leftovers into the environment.

Is it possible to, in a simple way, compare and contrast these two systems?

For example answers to Would've the CO2 scrubbers failed before the O2 supply ran out during an Apollo moonwalk? discuss CO2 scrubbers and normal scuba divers don't have those that I am aware of (rebreathers are a separate topic).

Memories of watching The Undersea World of Jaques Cousteau include special gas mixtures for their deepest dives, I'm primarily interested in the least exotic type of scuba diving, the kind a beginner would learn.

Question: In simple terms, how does the way space suits manage breathable gas differ from how scuba gear does it?

Linking to existing answers that cover each aspect in more detail is strongly encouraged. What I need here is a view from 100,000 feet. There's no need to reproduce what's already been explained well in existing posts. Thanks!

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  • $\begingroup$ The cabin-atmosphere tag usage guidance begins "atmosphere in a spacecraft or spacesuit that an astronaut would breathe..." so it's appropriate here. space.meta.stackexchange.com/a/1580/12102 $\endgroup$ – uhoh Jun 25 at 1:39
  • $\begingroup$ This feels less like a question and more like a homework assignment. It's very broad and rather open-ended. Scuba and space suits are radically different in almost every aspect and this question doesn't really put any effort into focusing the answers on any particular topic. Your questions should be reasonably scoped. If you can imagine an entire book that answers your question, you’re asking too much. $\endgroup$ – J... Jun 25 at 13:48
  • $\begingroup$ @J... pease re-read the post all the way through; that's literally already been addressed! $\endgroup$ – uhoh Jun 25 at 14:35
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Intravehicular spacesuits are worn inside the cabin in case of emergencies, particularly during ascent and descent. The Mercury suits were manufactured by Goodrich. Nearly all other IV suits have been manufactured by the David Clark Company: Gemini, Apollo 1, Shuttle ejection seat suit, Shuttle LES, and Shuttle ACES (all an evolution of the Gemini design).

IV suits are fairly similar to scuba gear. In particular:

  • The air source is different. Scuba air comes entirely from a tank. IV suits get their air through an umbilical to the cabin environmental system.
  • IV suits also have had a bottle of oxygen built into the suit as a backup. Scuba only occasionally uses second tank; the usual backup is to have another diver nearby to share air with. IV suits have never been able to take air from another suit.
  • The pressure and composition (i.e. percent oxygen) of the incoming air has varied among both scuba and space programs.
  • Where the incoming air is distributed varies. Most scuba divers breathe through a mouthpiece; less common are masks that provide air to the entire face. Early IV suits had a seal around the face or the front side of the head. Modern IV suits have the seal at the neck.
  • Where the exhaled air goes is quite different. Exhaled scuba air goes into the surrounding water. In the David Clark suits, exhaled air passes through a one-way valve in the seal of the face/head area, entering the body of the suit. There is then a pressure valve in the body of the suit which then expels exhaust air into the ambient air of the cabin. Thus both scuba and IV suits are open systems.
  • Scuba air does not help control body heat. Although the air in IV suits provides some ventilation to the body of the suits, these suits are primarily cooled by a liquid cooling undergarment.

Extravehicular spacesuits are used for spacewalks and moonwalks. Some of the David Clark suits for Gemini were modified for EV use. All other EV suits have been made by ILC Dover: Apollo, Shuttle EMU, and the ISS EMU.

EV suits are more comparable to rebreathers:

  • The air source may be an umbilical to the cabin environmental system (Gemini, Apollo launch, Apollo service module spacewalk), or from a Portable Life Support System (Apollo moonwalk, Shuttle, ISS).
  • All EV suits have had a backup oxygen supply built into the suit or PLSS. Apollo suits could also share resources with another suit (BSLSS).
  • Incoming air in EV suits is distributed both to the face and to liquid cooling - ventilation undergarments.
  • Exhaled air either returns through the umbilical, or to the PLSS. In either case, the air is scrubbed for CO$_2$, humidity removed, oxygen added, and temperature conditioned so that it may return to the suit. Thus EV suits are closed systems.
  • Air helps cool EV suits; in particular, it carries sweat away to help cool the astronaut.

To the best of my knowledge, the above distinctions also apply to the Russian Sokol (IV) and Orlan (EV) suits.

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    $\begingroup$ Re Scuba only occasionally uses second tank. SCUBA divers need a secondary air supply, primarily meant to be used by someone else in the group. You have a choice of a pony bottle or an "octopus". The latter, more common with modern (split regulator) design, is another secondary regulator on the end of a longer hose resembles the octopus tentacle as you swim. But if you look at the bulky regulators in an old TV show like Sea Hunt, I think that a spare bottle would be more practical back then. $\endgroup$ – JDługosz Jun 25 at 14:26
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    $\begingroup$ @JDługosz: Your comment only reinforces my point that one tank on each person is more common than two. $\endgroup$ – DrSheldon Jun 25 at 14:59
  • $\begingroup$ It is today, with modern split-regulator design. But at the time the space suits were designed? Skimming through Wikipedia, I only find that "over time they were improved and became dominant" but no specific years. The Sea Hunt and Cousteau style was still iconic in the 60's. Oh, I'm also reminded that the buoyancy control bladder can serve as another emergency source of air. $\endgroup$ – JDługosz Jun 25 at 15:15
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    $\begingroup$ Re "Scuba air does not help control body heat.", the problem in almost all diving is retaining body heat, thus wetsuits & drysuits that insulate the body. Spacesuits, especially those used for EVA, of course have the opposite problem, since they're surrounded by vacuum. $\endgroup$ – jamesqf Jun 26 at 4:13
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As a scuba diver I got some remarks.

A space suit is designed as a closed system with constant pressure. Exhaled oxygen is reused and the exhaled carbon dioxide is scrubbed. Only the oxygen used by the astronaut is replenished.

Most scuba gear are open systems, exhaled breathing gas is not reused. The bubbles rising to the surface are all the exhaled gas. No scrubber is used. Scuba gear is used at a pressure between 1 and 5 bar. Deep divers up to 15 to 30 bars.

Very few scuba divers are using closed systems, no bubbles, exhaled oxygen is reused, a scrubber removes exhaled carbon dioxide. If they want to go deeper than about 6 m, they can't use pure oxygen, they a mixture of oxygen with nitrogen or helium. If the system delivers a mix with too few or too much oxygen for the actual depth, the user might be killed.

If an open scuba systems fails, the diver will notice it in any case and may do something to survive.

If a closed scuba systems fails by too few or to much oxygen or too much carbon dioxide in the loop, the diver might not notice that at all and will be killed without warning. The diver will notice if there is to few or too much gas available, but he does not feel anything when oxygen content is wrong or there is too much carbon dioxide.

There is a huge difference in the necessary gas mass. An open scuba systems may need about 3 kg of air for a dive of about one hour. An Apollo astronaut had 0.45 kg of oxygen for up to 4 hours, about 0.1 kg per hour. 30 times and up to 150 times more mass per hour for the open scuba system.

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    $\begingroup$ I was under the impression that too much carbon dioxide was very noticeable, very unpleasant well before toxic, but that's at 1 bar. I'm thinking of the CO2 scrubber canister scenes in Apollo 13 and answers to Do ISS astronauts notice, or experience symptoms of elevated CO2 levels? Do they wear monitors regularly? $\endgroup$ – uhoh Jun 25 at 8:00
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    $\begingroup$ @uhoh There have been cases of scuba divers using closed systems killed by to much carbon dioxide cause by exhausted scrubber. These divers were experts but did not feel anything before their death. If they felt anything, they would have done something against it. There was a german scuba diver I did know personally from a lecture. He was the German expert of closed systems of that time. I was asked after his death if I know somebody to perform a gas analysis of the remaining gas on his oxygen tanks. I found a chemist at the local university where I was working. $\endgroup$ – Uwe Jun 25 at 8:20
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    $\begingroup$ @uhoh There have been many cases of vintners dying in fermenting cellars of too much carbon dioxide. These vintners did not notice anything, otherwise the would have left the cellar before loosing consciousness. $\endgroup$ – Uwe Jun 25 at 9:16
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    $\begingroup$ The trouble with (normal, open-system) SCUBA is that the amount of air used goes up with the pressure, so you go through it so fast at depth. The EV suits used a much lower pressure, so you would need to extrapolate to compare the performance. $\endgroup$ – JDługosz Jun 25 at 14:33
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    $\begingroup$ @JDługosz, Amount of air used per breath goes up with pressure in open-loop SCUBA. You can compensate for that somewhat, by training yourself to breathe more slowly at depth. $\endgroup$ – Solomon Slow Jun 26 at 4:01
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There are several differences, and I'm quite sure the list that follows is not complete.

  • Pressure.
    A scuba diver is subject to more than one atmosphere. Pressure increases about one atmosphere for every ten meters of water depth. Scuba divers have to worry about nitrogen narcosis, decompression sickness, and oxygen toxicity. Astronauts breath nearly 100% oxygen at reduced pressure. This means astronauts on the ISS do need to go undergo pre-breathing prior to an EVA to avoid decompression sickness, but other than this, astronauts do not need to worry about the pressure challenges faced by scuba divers.
  • Cost of breathing air.
    Even trimix is dirt cheap compared to the vastly more expensive cost of the air breathed by astronauts. Even with what SpaceX has done to reduce launch costs, it is still very expensive to send anything into space, and that of course includes breathing air.
  • Exhausted air versus recycled air.
    Scuba divers simply release their exhaust breath into the water. That exhausted air still contains quite a bit of oxygen, but air (even trimix) is cheap. Because sending anything into space is very expensive, the air in a spacesuit (or a spacecraft) is recycled. The carbon dioxide created by human metabolism is captured and possibly expelled. The oxygen that remains is recycled.
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    $\begingroup$ @uhoh, your request here for a more rigorous answer and your request in the question for "in simple terms" are at odds with one another. Which is it? $\endgroup$ – David Hammen Jun 25 at 5:02
  • $\begingroup$ "off topic..." was meant as a flag to warn that the following was in some way unrelated to the current post. "...but this question..." links to an orbital mechanical question in Physics SE. So "in simple terms" applies to this page, "a more rigorous answer" applies to the linked question in Physics SE. Sorry for the confusion! I'll delete up the first comment now and this one in due course. $\endgroup$ – uhoh Jun 25 at 5:36
  • $\begingroup$ There is a very small number of scuba divers using closed systems with no release of their exhaust breath into the water. $\endgroup$ – Uwe Jun 25 at 10:09
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    $\begingroup$ You are wrong, ISS astronauts do not need pre-breathing prior to an EVA to avoid nitrogen narcosis. Nitrogen narcosis is impossible at pressures lower than about 2 to 3 bar. Nitrogen narcosis is possible when breathing air at a pressure above 3 to 5 bar at a depth of 20 to 40 m. Prebreathing before an EVA is only needed to reduce the risk of decompression sickness. $\endgroup$ – Uwe Jun 25 at 21:42
  • $\begingroup$ @Uwe Fixed, thanks. $\endgroup$ – David Hammen Jun 25 at 22:58
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The previous answers go into great (and accurate) detail about SCUBA, but since you specifically asked about the kind a beginner would use:

The breathable mixture is plain air, compressed into a single tank.

As mentioned, there may be one or two regulators (mouthpieces), with the second serving as a backup in case the first fails, or to assist another diver whose air has run out or whose regulator has failed.

In any case, part of beginner training is "buddy breathing", learning how to share a single regulator safely between two divers in the event of an emergency.

Another bit of training is learning what to do if your air runs out. You learn (and are tested on) how to ascend safely from a depth of 60 feet/20 meters with no air supply. Knowing how easy this is, you're less likely to panic should your air run out.

Beginners are taught not to descend past 90 feet/30 meters and are drilled on the use of dive tables to ensure that they don't spend too much time at depths of over 30 feet.

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  • $\begingroup$ "You learn (and are tested on) how to ascend safely from a depth of 60 feet/20 meters with no air supply". What insane organization has such a training? CMAS and PADI certainly don't. You need several minutes to safely ascend from 20m after you spent some time down there. Very few people will have the air to do so and will simply race to the top with very dire consequences including death. Also if you ever end at 20m with no air, so many things went wrong beforehand, that you should never end up in such a situation if properly trained. $\endgroup$ – Voo Jun 28 at 8:09

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