Author Topic: The case for lower pressure Mars habitats  (Read 19881 times)

Offline Robotbeat

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The case for lower pressure Mars habitats
« on: 01/29/2017 12:49 am »
This is a home to the splinter discussion about below sea level pressure habitats on Mars.
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Offline Vultur

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Re: The case for lower pressure Mars habitats
« Reply #1 on: 01/29/2017 02:06 am »
I really don't see any reason to use sea level pressure for anything off Earth.

Denver is ~1 mile elevation, ~12 psi and that's no problem at all. Altitude issues are IIRC very rare below 8000 ft., ~10.9 psi. If you screened people for susceptibility, you could get away with less, but it might not be worth it.

At 12 psi, that's ~2.5 psi oxygen. You could do something like 30% O2/70% N2 (~8.35 psi total pressure) or 25% O2/75% N2 (10 psi total pressure).

Didn't Skylab even use something like 5 psi 75% O2/25% N2?

Offline Paul451

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Re: The case for lower pressure Mars habitats
« Reply #2 on: 01/29/2017 02:45 am »
Chris, you branched this from discussions of colonies. But this section is also for missions (it's in the name, even). You might want to better specify the terms of your topic.

Offline Robotbeat

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Re: The case for lower pressure Mars habitats
« Reply #3 on: 01/29/2017 04:26 am »
I think it applies to both, but mostly to settlements.

10-12psi would be a pretty conservative compromise. 10psi is what Shuttle operated at to significantly reduce EVA pre-breathing times. But I think we can safely go to around 7-8psi without much trouble. 1-2psi if you have a pure oxygen mask could be done once you're acclimated, might have some interesting use cases (like a very large greenhouse, a temporary enclosure to build or fix some other structure or equipment, or maybe Hellas Basin itself in the early days of terraforming).

I'm very interested in what the lower bound is once you start thinking creatively WRT blood doping, etc. But conservatively, I would say 10psi off the bat, 7-8psi with some creativity.
« Last Edit: 01/29/2017 04:28 am by Robotbeat »
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Offline Vultur

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Re: The case for lower pressure Mars habitats
« Reply #4 on: 01/29/2017 07:38 am »
I think it applies to both, but mostly to settlements.

10-12psi would be a pretty conservative compromise.

Oh, definitely. 12 psi is what I was saying there's essentially zero cost to using -- I am a sea level native who's not exceptionally fit and I don't even notice a difference at ~1 mile, though it becomes quite evident at ~8000ft.

But altitude sickness can be tricky and you'd want to pre-screen people (have training in a high-altitude location for example) if you wanted to go above 8000 ft. equivalent in oxygen partial pressure. (Which can still mean significantly lower total pressure if you use an enriched oxygen mixture - 8000 ft is about 10.9 psi so about 2.28 psi O2. At 30% O2 that's 7.6 psi total.)

Quote
1-2psi if you have a pure oxygen mask could be done once you're acclimated,

IIRC you can't actually do that because of alveolar gas pressure (CO2 plus vapor pressure of water at body temperature). You lose almost 1 psi total pressure because of that.

I guess you could use 3 psi pure oxygen and get a bit over 2 psi, which is enough for acclimated people.

Offline guckyfan

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Re: The case for lower pressure Mars habitats
« Reply #5 on: 01/29/2017 08:26 am »
1-2psi if you have a pure oxygen mask could be done once you're acclimated,

I wonder why they do 3 psi in spacesuits when a lower pressure would have so big advantages with flexibility. There must be a physiological reason.

Edit: Just saw Vultur argue about this one post ahead of mine.
« Last Edit: 01/29/2017 08:30 am by guckyfan »

Offline Robotbeat

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Re: The case for lower pressure Mars habitats
« Reply #6 on: 01/29/2017 10:25 am »
1-2psi if you have a pure oxygen mask could be done once you're acclimated,

I wonder why they do 3 psi in spacesuits when a lower pressure would have so big advantages with flexibility. There must be a physiological reason.

Edit: Just saw Vultur argue about this one post ahead of mine.
EVAs are strenuous and astronauts usually not altitude trained.

Partial pressure of oxygen at Everest is 1psi. People do climb it without supplementary oxygen. I was trying to determine a lower bound. As in, what's THE minimum pressure that could be tolerated given any kind of countermeasures? The Armstrong Limit is 0.9psi.
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Offline Rei

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Re: The case for lower pressure Mars habitats
« Reply #7 on: 01/29/2017 11:38 am »
Indeed, it's partial pressure that matters, not absolute pressure.  I have a reference somewhere around here for the health effects of removing nitrogen from the air while maintaining O2 levels constant; it's a bit dated, but the only negative one known was reduced effectiveness at removing contaminants from the lungs via coughing.  Many of the effects were actually beneficial - breathing takes less energy, for example, and CO2 is more effectively removed from the bloodstream.

The main reason that you see environments like ISS at atmospheric pressure is that it simplifies docking with vehicles sent from the surface - it means that your crew vehicle can start at 1 atm, remain at 1atm in space, and dock with a 1 atm station, without ever having to change their internal pressure or gas mixture.  But when you're talking something as far removed from Earth as a habitat on another planet, arguing for 1atm does  become difficult.

As a side note, plants in general seem indifferent to or appreciative of the removal of nitrogen.  They can also tolerate significantly lower total pressures than humans can, albeit with some negative consequences once you start dropping the O2 too far, and obviously they can no more tolerate life below the Armstrong limit than we can.  Low pressure greenhouses have been argued for, with the main argument against them being the difficulty of working in space suits (I'm kind of fond of the idea of having the plants be "mobile" - on rails, or able to be set on carts, so that you can bring forward those that you want to work on into a large airlock so they can be worked on in shirtsleeves)

Now, what I'm wondering: why would people use PSI?  I don't get that. I understand atmospheres (nice whole numbers, easy to understand), pascals (easy physics calculations, and 1atm is only a bit over 100kPa so it's easy to convert), bar (exactly 100kPa, so likewise)... all of that's easy to understand.  But why would you use PSI?  They're not at all close to whole numbers with pascals or atmospheres, they're not convenient for physics calculations... what's the reason?
« Last Edit: 01/29/2017 11:53 am by Rei »

Offline clongton

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Re: The case for lower pressure Mars habitats
« Reply #8 on: 01/29/2017 12:13 pm »
The difficulty of working in spacesuits in low or zero pressure is only because they are pressurized, which makes them less flexible. There are solutions to this being worked on in various places that employ a hard suit where pressure against the skin is maintained by a close-fitting shell with flexible joints where the body needs them. the only part of the suit that is actually pressurized is the helmet. Suits designed this way would eliminate completely the pressurization difficulties of EVAs on a planet's surface or in spacecraft-based missions. The helmet would be pressurized to the spacecraft/living/working areas ambients. The living/working areas could be pressurized with whatever the ideal ambient pressure and oxygen mix ratios turns out to be without regard to what was "outside" or frequency of EVAs.

Because of the TM level of the suits so far, we are unlikely to see these used in near term missions but the work continues in earnest with the eventual goal of completely replacing the pressurized EVA/space suit.

Once these types of suits become commonplace spacecraft and base design will become less complex in terms of the O2 percentage and ambient atm pressure.
« Last Edit: 01/29/2017 12:22 pm by clongton »
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Offline RonM

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Re: The case for lower pressure Mars habitats
« Reply #9 on: 01/29/2017 02:01 pm »
Now, what I'm wondering: why would people use PSI?  I don't get that. I understand atmospheres (nice whole numbers, easy to understand), pascals (easy physics calculations, and 1atm is only a bit over 100kPa so it's easy to convert), bar (exactly 100kPa, so likewise)... all of that's easy to understand.  But why would you use PSI?  They're not at all close to whole numbers with pascals or atmospheres, they're not convenient for physics calculations... what's the reason?

US Customary units are still used in the United States. Of course, everyone here would be better off using SI metric, but that means changing from a system they have always used. That's difficult, even when the new system is better.

Kilopascals are confusing to Americans too.

It would be easier to discuss pressure in atmospheres.

Offline KelvinZero

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Re: The case for lower pressure Mars habitats
« Reply #10 on: 01/29/2017 02:18 pm »
Now, what I'm wondering: why would people use PSI?  I don't get that. I understand atmospheres (nice whole numbers, easy to understand), pascals (easy physics calculations, and 1atm is only a bit over 100kPa so it's easy to convert), bar (exactly 100kPa, so likewise)... all of that's easy to understand.  But why would you use PSI?  They're not at all close to whole numbers with pascals or atmospheres, they're not convenient for physics calculations... what's the reason?
I was going to suggest they convert every pound to 120 Arabic silver dirhams  :)

Yeah I would find 1atm or (about) 100kPa much easier. I think they are saying 2/3 earth sea-level is pretty conventional (shuttle) but if you are a bit daring 1/2 earth sea-level could be ok. Probably bump up the oxygen fraction just a bit, not attempting sealevel partial pressure though, due to fire risk.

Offline Robotbeat

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Re: The case for lower pressure Mars habitats
« Reply #11 on: 01/29/2017 02:20 pm »
kPa isn't too bad. 101kPa is 1 atmosphere. So kPa is roughly atmosphere in percent. Pretty easy to remember.


I'm bilingual when it comes to units. I usually prefer SI, but lots of reference material for space suits, etc, is in psi.
« Last Edit: 01/29/2017 02:22 pm by Robotbeat »
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Offline RonM

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Re: The case for lower pressure Mars habitats
« Reply #12 on: 01/29/2017 02:26 pm »
1-2psi if you have a pure oxygen mask could be done once you're acclimated,

IIRC you can't actually do that because of alveolar gas pressure (CO2 plus vapor pressure of water at body temperature). You lose almost 1 psi total pressure because of that.

I guess you could use 3 psi pure oxygen and get a bit over 2 psi, which is enough for acclimated people.

Yes, human physiology limits.

Quote from: wikipedia
Generally, to supply enough oxygen for respiration, a space suit using pure oxygen must have a pressure of about 32.4 kPa (240 Torr; 4.7 psi), equal to the 20.7 kPa (160 Torr; 3.0 psi) partial pressure of oxygen in the Earth's atmosphere at sea level, plus 5.3 kPa (40 Torr; 0.77 psi) CO2 and 6.3 kPa (47 Torr; 0.91 psi) water vapor pressure, both of which must be subtracted from the alveolar pressure to get alveolar oxygen partial pressure in 100% oxygen atmospheres, by the alveolar gas equation.[2] The latter two figures add to 11.6 kPa (87 Torr, 1.7 psi), which is why many modern space suits do not use 20.7 kPa (160 Torr; 3.0 psi), but 32.4 kPa (240 Torr; 4.7 psi) (this is a slight overcorrection, as alveolar partial pressures at sea level are slightly less than the former). In space suits that use 20.7 kPa, the astronaut gets only 20.7 kPa − 11.7 kPa = 9.0 kPa (68 Torr; 1.3 psi) of oxygen, which is about the alveolar oxygen partial pressure attained at an altitude of 1,860 m (6,100 ft) above sea level. This is about 78% of normal partial pressure of oxygen at sea level[citation needed], about the same as pressure in a commercial passenger jet aircraft, and is the realistic lower limit for safe ordinary space suit pressurization which allows reasonable capacity for work.

https://en.wikipedia.org/wiki/Space_suit

So, 3 psi (20.7 kPa, 0.2 atm) is about as low as you should go. Since people going out in a spacesuit are most likely to be working, 4.7 psi (32.4 kPa, 0.32 atm) is more practical. If you're not in good shape, just walking around in the spacesuit will be strenuous, even in lower gravity.

We need to keep in mind we're talking about living in Mars habitats, not pushing the limits of human endurance.

kPa isn't too bad. 101kPa is 1 atmosphere. So kPa is roughly atmosphere in percent. Pretty easy to remember.

Sounds good. Typing all three like I did above is too much trouble.

Online Johnnyhinbos

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Re: The case for lower pressure Mars habitats
« Reply #13 on: 01/29/2017 04:51 pm »
I had written the following in the Amazing Martian Habitats thread, but this sounds like a better place to repost it... (I work in ATA, but feel free to substitute 101kPa for every 1 ATA as you see fit!)

Quote
Yes, it's important folks think in terms of partial pressures and not percentages. As I mentioned upthread there's a lot to be gained from reducing the overall pressure while maintaining (or near to) the PP O2.

The makeup gas(s) used to backfill the remaining pressure required is an interesting challenge. Humans need at minimum 0.16 Atmospheres Absolute (ATA) PP O2. Air at 1 ATA (ie sea level) contains 0.21 ATA PP O2.

Mars atmosphere is right around 0.0059 ATA. Pretty close to a vacuum for all intents and purposes. Therefore we'd want to design the LSS of the hab with the absolute minimum pressure we can in order to reduce the pressure delta between Mars atmosphere (outside) and hab atmosphere (inside).

10 psi for the hab is a good starting point, though perhaps it could iterate down to a lower pressure.
That's 0.68 ATA. Of that we'll use 0.21 ATA for oxygen. You don't want to go much lower because it's more healthy for us mammals when we're not hypoxic. And you definitely don't want to go much higher because higher PP O2 can lead to pulmonary toxicity over time.

So we have to backfill our breathing gas with "something". In the world of rebreather diving this something is called diluent. Diluent can be any non-metabolized gas mixture - inert.

Dalton's law says we can simply add up the partial pressures of the constituent gasses to get the total pressure. We have 0.47 ATA to play with.

Let's start with CO2. Plants like it, we don't. Let's not mess around with it. We'll maintain it at what's in air. 0.004 ATA. So we need to come up with 0.466 ATA.

It's a good point about nitrogen with regards to plants and it's also a good point about the thermal benefit of higher densities, but higher densities, as mentioned, is a problem for transition to Mars atmosphere (or for transitioning to a suit designed to operate in the Martian atmosphere). I had mentioned argon upthread because argon is an excellent insulator, it has much lower thermoconductivity than nitrogen. If both gasses can be extracted from the CO2 rich atmosphere then perhaps we should look at a 50/50 split. Reason being is perhaps a bit too off topic for this thread, but when going to an EVA suit that operates at around 4 psi of pure O2 you need to worry about isobaric counterdiffusion and argon is more forgiving than nitrogen in that respect. Argon is more narcotic at higher pressures, but we are never leaving with pressures > 1ATA regardless, so that's a non issue.

So in conclusion I'd suggest designing the LSS to operate at 10psi, or 0.68 ATA, with 0.21 ATA oxygen, 0.23 ATA nitrogen, 0.23 ATA argon, and allow 0.004 ATA carbon dioxide.
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Offline Paul451

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Re: The case for lower pressure Mars habitats
« Reply #14 on: 01/29/2017 06:42 pm »
Indeed, it's partial pressure that matters

Only for biology. For fire management, percentage of buffer gas is more critical.

As a side note, plants in general seem indifferent to or appreciative of the removal of nitrogen. They can also tolerate significantly lower total pressures than humans can, albeit with some negative consequences once you start dropping the O2 too far, and obviously they can no more tolerate life below the Armstrong limit than we can.

When you lower the total pressure, the optimal range of levels of CO₂, humidity, even oxygen, narrows. Making it harder to keep the plants growing at the target rate or production.

For eg, at normal atmospheric pressure, plants can tolerate a range of CO₂ levels. Obviously as low as Earth's atmosphere, 3-400pa before growth stops, up through 1-3 kpa before water in cells acidifies and growth stops. (Many plants actually prefer higher levels of CO₂, but the optimum level varies wildly between species. Wheat grows best at levels between 1 & 2 kpa, and tolerate levels up to 3kpa before growth rate falls below Earth-normal. Corn prefers between 600-800pa, and growth rate drop back below Earth-normal if levels go above 1kpa. (800pa seems to be the butter zone for greenhouse crops.))

But when you drop the pressure, that optimal range narrows. While the optimum peak is in the same place, the amount of variation the plant can tolerate is much less. And a full greenhouse can use up all the available CO₂ inside the structure within a couple of hours, then the plants will stop growing. Reducing your margin for growth makes plant management much fiddlier.

(At least that's the impression I got from the Scaling Agriculture On Mars thread, and from playing with hydroponics some years ago.)

[Edit: Very low pressure causes most plants to enter a permanent "drought response" until they die, regardless of how much water is available, due to a breakdown in the transpiration mechanism. On the flip side, some plants prefer reduced oxygen levels. A number of papers on low pressure greenhouses on Mars assume some kind of bioengineering of the crops.]

Aside: This is also an issue for humans. If you reduce the pp of O₂ to the lowest safe level, then you have much less margin for sub-optimal conditions. So, for example, you need to increase the air circulation rate everywhere to prevent O₂ levels dropping too far in any one location, say because a bunch of people randomly crowded into one room. You can no longer rely on average consumption rates and the large margin between normal levels and unsafe levels, you have to micromanage levels in every room, every cupboard, etc.

CO₂ goes the other way, of course. If you allow the default levels to be as high as humans can tolerate, it gives you much less margin for crowded spaces or low-circulation air pockets.

Now, what I'm wondering: why would people use PSI?
It would be easier to discuss pressure in atmospheres.

Except, when discussing a pressurised object on another planet, that leads to statements like "Even if the air pressure is just 1/3 atmospheres, it's still nearly 50 times the atmospheric pressure", "at that altitude, the atmospheric pressure climbs above 1% of an atmosphere", etc, unless you add qualifiers on each term.

I usually prefer SI, but lots of reference material for space suits, etc, is in psi.

Or worse, in inches of mercury. (Or bizarrely, in mm of mercury.)
« Last Edit: 01/29/2017 09:41 pm by Paul451 »

Offline Paul451

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Re: The case for lower pressure Mars habitats
« Reply #15 on: 01/29/2017 07:02 pm »
The difficulty of working in spacesuits in low or zero pressure is only because they are pressurized, which makes them less flexible. There are solutions to this being worked on in various places that employ a hard suit where pressure against the skin is maintained by a close-fitting shell with flexible joints where the body needs them. the only part of the suit that is actually pressurized is the helmet.

You seem to be jumbling hard-suits and mechanical counter-pressure suits.

Hard-suits are gas-pressure suits, where the entire suit is pressurised by gas. The advantage of the hard-shell is that the suit can tolerate a higher internal pressure without balloon-stiffening. The down-side is that they are bulky, heavy and cumbersome. Many space-suits are hybrids of hard-suit and soft-suit components.

Mechanical counter-pressure suits are soft-suits where the elasticity of the suit replaces the required force from gas pressure everywhere except the helmet. MCP suits are low pressure suits, but vastly lighter and easier to move around in.

Wikipedia might help.

I'm not aware of anyone who has proposed a hard-shell mechanical counter-pressure suit. I'm not even sure how that would work.



[...]

Once again, argon is a poor choice for buffer gas. Because of its low thermal capacity (not conductivity), it performs worse than nitrogen at reducing flammability. Hence for any partial pressure of oxygen, you need a higher pp of argon to match the performance of a lower pp of nitrogen, giving you a higher total pressure for no advantage.

Offline Vultur

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Re: The case for lower pressure Mars habitats
« Reply #16 on: 01/29/2017 08:39 pm »
Aside: This is also an issue for humans. If you reduce the pp of O₂ to the lowest safe level, then you have much less margin for sub-optimal conditions. So, for example, you need to increase the air circulation rate everywhere to prevent O₂ levels dropping too far in any one location, say because a bunch of people randomly crowded into one room. You can no longer rely on average consumption rates and the large margin between normal levels and unsafe levels, you have to micromanage levels in every room, every cupboard, etc.

I dunno. There's a significant difference between what's safe indefinitely and what's safe for short-term exposure. Visiting Pikes Peak (~14000 ft, ~60kPa/0.6 atm) briefly isn't that bad, but most sea-level natives couldn't live at that altitude indefinitely without serious problems.

EDIT: Similarly for Everest. It is possible (though very dangerous) to climb Everest without supplemental oxygen, but people who do that are quite literally "living on borrowed time". Long-term adaptation to altitude that high is not physiologically possible, even for people who grew up at 14000-15000ft or so (Tibetans and some Andeans).

Denver to Mexico City levels of oxygen partial pressure really shouldn't cause any issues.
« Last Edit: 01/29/2017 08:41 pm by Vultur »

Offline Bob Shaw

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Re: The case for lower pressure Mars habitats
« Reply #17 on: 01/29/2017 09:13 pm »
Just to throw a spanner in the works, is there a case for *enhanced* pressure in some parts of a Mars habitat?

I'm thinking of agriculture, aquaculture, sewage processing and so forth - some of which might also benefit from a low-oxygen air mix, though earlier discussions regarding the problems near airlocks resulting from invasive atmospheric CO2 might even make 'air levees' worthwhile.

Online Lee Jay

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Re: The case for lower pressure Mars habitats
« Reply #18 on: 01/29/2017 09:37 pm »
I'm from the Denver area (5,340, in my case) but I was at 9,600 for the last two days with no noticeable differences.  Most airliners are pressurized to about 8,000 feet and almost everyone from sea-level is fine with that for hours.  I've spent days camping and even waterskiing in Leadville, Colorado (10,000+ feet) with no ill effects.  The town of Leadville is above 10,500 feet and lots of people visit and stay there from many other places.  Altitude sickness is a very, very serious thing but it's easy to avoid entirely if you follow a few very simple rules (don't exercise until you acclimate, stay hydrated, don't drink alcohol while you acclimate, acclimate in steps like sea-level to Denver to high altitude if possible).

My point is, for healthy people, even those who grew up at sea level, living at an equivalent pressure above 10,000 feet should be easy within a couple of days.  Higher oxygen concentrations and lower pressures come with lousy flammability limits.

Offline Rei

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Re: The case for lower pressure Mars habitats
« Reply #19 on: 01/30/2017 12:02 am »
Just to throw a spanner in the works, is there a case for *enhanced* pressure in some parts of a Mars habitat?

I'm thinking of agriculture, aquaculture, sewage processing and so forth - some of which might also benefit from a low-oxygen air mix, though earlier discussions regarding the problems near airlocks resulting from invasive atmospheric CO2 might even make 'air levees' worthwhile.

Plants generally benefit from a reduction in atmospheric nitrogen content, not an increase in one.  Theories vary as to why, but they involve things like more efficient transpiration, faster removal of gases like ethylene (the "death hormone" for above-ground parts of the plant), etc.

Offline Comga

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Re: The case for lower pressure Mars habitats
« Reply #20 on: 01/30/2017 01:09 am »
I'm from the Denver area (5,340, in my case) but I was at 9,600 for the last two days with no noticeable differences.  Most airliners are pressurized to about 8,000 feet and almost everyone from sea-level is fine with that for hours.  I've spent days camping and even waterskiing in Leadville, Colorado (10,000+ feet) with no ill effects.  The town of Leadville is above 10,500 feet and lots of people visit and stay there from many other places.  Altitude sickness is a very, very serious thing but it's easy to avoid entirely if you follow a few very simple rules (don't exercise until you acclimate, stay hydrated, don't drink alcohol while you acclimate, acclimate in steps like sea-level to Denver to high altitude if possible).

My point is, for healthy people, even those who grew up at sea level, living at an equivalent pressure above 10,000 feet should be easy within a couple of days.  Higher oxygen concentrations and lower pressures come with lousy flammability limits.

Living near 2 km altitude and spending a lot of time near and above 3 km altitude I used to make the same argument about oxygen content.  "If they had put the Air Force base in Houston and Manned Spaceflight Control in Colorado Springs, the astronauts would have acclimatized and spacecraft and space stations could be so much lighter."

However, in addition to the problems mentioned above is flammability, Lee Jay's last point.  If you keep the oxygen content constant and reduce the nitrogen things burn easier, and fire in spacecraft is something NASA works hard to avoid.    The Apollo 1 fire, now just over a half century ago, is the extreme example, but it doesn't have to be 100% oxygen or 1 Atm to dramatically increase the risk. 

Now where are they going to get all that nitrogen for Martian cities?

edit:  The martian atmosphere is 1.9% nitrogen, and about 1% of Earth sea level pressure so all one has to do is extract all the nitrogen from 3500 times the volume of your air supply and you have your nitrogen.  3000 if you keep the pressure equivalent to Lee Jay's Denver.  ;)
« Last Edit: 01/30/2017 04:55 am by Comga »
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Offline guckyfan

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Re: The case for lower pressure Mars habitats
« Reply #21 on: 01/30/2017 07:29 am »
The martian atmosphere is 1.9% nitrogen, and about 1% of Earth sea level pressure so all one has to do is extract all the nitrogen from 3500 times the volume of your air supply and you have your nitrogen.  3000 if you keep the pressure equivalent to Lee Jay's Denver.  ;)

Extracting the CO2 needed for fuel ISRU will yield all the nitrogen needed. They may or may not have to separate it from the argon which is present at the same procent value.

Offline AegeanBlue

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Re: The case for lower pressure Mars habitats
« Reply #22 on: 02/02/2017 11:49 pm »
In an ecosystem on Earth animals are less than 1% of biomass by weight. Simple mass fraction calculations show that, assuming that food is grown locally, human biomass will not be the majority of the biomass in the Martian habitat, it will be the plant biomass. I think we ought to optimize for the plants rather than the humans and plants do not like lower pressure, it interferes with plant water flows leading to water stress. Plants also do not like if the atmosphere contains too little or much CO2. On Mars we have an atmosphere which we can use as a base to set up the habitat atmosphere. If we grow food in the habitat we will be pumping CO2 and water in the greenhouse which gets converted to O2 and biomass. We will need to remove the O2, MarsOne dies from Oxygen toxicity as the MIT study found. Bars Landorp mentioned an Oxygen concentrator, that can work. Over time if we pump Mars atmosphere we remove both CO2 and O2 (through the concentrator) leading to enrichment of the minor components of the Martian atmosphere. There are issues with this approach, most importantly CO which is toxic to animals but not plants, plants will oxidize it to CO2 but we need to make sure that concentration does not get toxic to the living animals of the habitat, most importantly humans. I am more inclined for variable pressure and composition, we start with barely livable for humans and plants in terms of pressure and composition and then as we establish plants and people we move to closer to earth surface or whatever is optimum for both.

Offline Robotbeat

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Re: The case for lower pressure Mars habitats
« Reply #23 on: 02/03/2017 12:42 am »
I HIGHLY doubt we will copy Earth's biosphere for the interior of the habitat. It'd have to be so enormous (and so expensive) that even a large colony could only support a tiny human population.

And we'll probably develop plants that can survive straight on Mars's surface, if not right away then immediately after terraforming starts to slightly thicken the atmosphere.
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Offline guckyfan

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Re: The case for lower pressure Mars habitats
« Reply #24 on: 02/03/2017 07:49 am »
I think we ought to optimize for the plants rather than the humans and plants do not like lower pressure, it interferes with plant water flows leading to water stress. Plants also do not like if the atmosphere contains too little or much CO2.

Large greenhouses will be optimized to different parameters than human habitats. I remember that issue with water stress. It became apparent at pressures of 3, maybe 2 psi. I would like to see a pressure in greenhouses that would allow to work with no more than oxygen masks or rebreathers.

Offline john smith 19

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Re: The case for lower pressure Mars habitats
« Reply #25 on: 02/03/2017 10:04 am »
I HIGHLY doubt we will copy Earth's biosphere for the interior of the habitat. It'd have to be so enormous (and so expensive) that even a large colony could only support a tiny human population.

And we'll probably develop plants that can survive straight on Mars's surface, if not right away then immediately after terraforming starts to slightly thicken the atmosphere.
It looks like they already can

http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html/

So a minimal green house which supplied some liquid water and a fairly low air pressure would help a lot.
it also suggests there is a fairly solid basis from which to genetically engineer further plants.

Carbon capture is viewed as a key process for turning a CO2 atmosphere (created by releasing the contents of the South Pole ice cap) into one that's breathable.

BTW in all this there seems to be no analysis wheather raising the atmospheric pressure would supress  those large scale dust storms that are seen, or wheather it would turn them into phenomenally destructive months long super tornadoes, making them immensely more destructive. 
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Offline Rei

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Re: The case for lower pressure Mars habitats
« Reply #26 on: 02/03/2017 04:37 pm »
I think we ought to optimize for the plants rather than the humans and plants do not like lower pressure, it interferes with plant water flows leading to water stress.

Quite to the contrary, plants generally like low nitrogen partial pressures, so long as the O2 and CO2 partial pressures remain the same.  Not always, but most often.  And they can survive much lower pressures than humans.

Here's some random studies from the last time I checked (need to translate my notes from Icelandic...).  Since you guys normally seem to work in PSI, I'll note that 1 atmosphere is 101,325 pascals.

Mansell et al, 1968: No negative effects in Brassica rapa (bok choy / turbip) at 50kPa, just more water loss.
Rule and Staby, 1981: Tomatoes @17 kPa constricted;  @33kPa were stronger that tomatoes grown at @100kPa, but not bigger.
Daunicht and Brinkjans, 1992: tomatoes @40kPa and @70kPa constricted
Andre & Richaux, 1986: barley grows better @3kPa in a nitrogenless atmosphere than a conventional O2/N2 one.
Gale, 1972: CO2 is easier to take up when the pressure is lower.
Smith & Donahue, 1991: At 50kPa+,  CO2 uptake is inversely proportional to pressure.
Andre & Massimino, 1992: Wheat can sprout @10 kPa and grows better at @20kPa than @100kPa if N2 is low  (O2@14 kPa, N2@ 3,4kPa, CO2@ 3,4 kPa). Appears that lowering N2 in general helps.
Musgrave et al, 1988: Mung beans grow independent of  O2 partial pressure, and general pressure reduction is negative (tested @21kPa)
Goto et al, 2002: Rice can grow @25kPa and @50kPa; having O2 partial pressure at least.10 kPa prevents damage.
Spanarkel & Drew, 2002: Lettuce @ 70kPa grows similar or better than @101kPa
He et al, 2003:  Plants in general grow similar or better at @ 30kPa vs. @1atm because of better removal of ethylene.
Wheeler et al, 2001: Corey et al, 2002: Plants grow similar or better at 30kPa
Ferl et al, 2002: In addition to plants already doing well at low pressures, there's significant potential for genetic improvement to increase it (aka avoiding the drought response).

Offline Rei

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Re: The case for lower pressure Mars habitats
« Reply #27 on: 02/03/2017 04:40 pm »
However, in addition to the problems mentioned above is flammability, Lee Jay's last point.  If you keep the oxygen content constant and reduce the nitrogen things burn easier, and fire in spacecraft is something NASA works hard to avoid.    The Apollo 1 fire, now just over a half century ago, is the extreme example, but it doesn't have to be 100% oxygen or 1 Atm to dramatically increase the risk. 

Reference(s)?  The Apollo capsule was 100% O2 at *above* atmospheric pressure - 6x the normal O2 partial pressure.

Offline AegeanBlue

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Re: The case for lower pressure Mars habitats
« Reply #28 on: 02/03/2017 04:52 pm »
Well, whether lichens qualify as plants is subject to debate. When I was an undergrad we considered them plants, according to Wikipedia today we do not. They are a symbiosis of cyanobacteria and fungus and none of them are edible by people, at least most of the time. Some are edible by some animals but if you read the article it is debatable just how much lichen agriculture is viable: We would need to have a lichen that somehow thrives in Martian conditions (survive and thrive are different things), harvest them and according to the article they were growing sheltered in cracks rather than in the open and then feed them to say reindeer living inside the habitat with humans. Also pure lichen is not a balanced reindeer diet. From

http://animals.howstuffworks.com/mammals/reindeer-find-food1.htm

Quote
However, lichen isn't a particularly hearty food, and for that reason, reindeer will eat 4 to 11 pounds (1.8 to 4.9 kilograms) of reindeer moss each day [source: Dieterich and Morton]. That's why reindeer pack on the pounds in the warmer months when there's more to choose from. In fact, these animals gradually lose weight starting in the fall and continuing to March [source: University of Alaska Fairbanks].

I am not claiming to redo a natural terran biosphere, I am trying to show how we need to put our emphasis on viability elsewhere than just people. The average American eats a little under 1 metric ton of food every year. We have mentioned the ISS astronaut in the forum, I do not remember if it is 500 kgs or 800 kgs per year but even if we use the 500 kgs food/yr value producing it requires another 1000 kgs of plant biomass, above and below ground. But let us assume that for simplicity's sake this food is harvested for times a year and that we fully recycle the biomass of one crop to the next (impossible). So for every 75 kg astronaut there are 1500/4 = 375 kgs of plant biomass providing food. As Napoleon said, armies march on their stomach. So do astronauts.

I think we ought to optimize for the plants rather than the humans and plants do not like lower pressure, it interferes with plant water flows leading to water stress.

Quite to the contrary, plants generally like low nitrogen partial pressures, so long as the O2 and CO2 partial pressures remain the same.  Not always, but most often.  And they can survive much lower pressures than humans.

Here's some random studies from the last time I checked (need to translate my notes from Icelandic...).  Since you guys normally seem to work in PSI, I'll note that 1 atmosphere is 101,325 pascals.

Mansell et al, 1968: No negative effects in Brassica rapa (bok choy / turbip) at 50kPa, just more water loss.
Rule and Staby, 1981: Tomatoes @17 kPa constricted;  @33kPa were stronger that tomatoes grown at @100kPa, but not bigger.
Daunicht and Brinkjans, 1992: tomatoes @40kPa and @70kPa constricted
Andre & Richaux, 1986: barley grows better @3kPa in a nitrogenless atmosphere than a conventional O2/N2 one.
Gale, 1972: CO2 is easier to take up when the pressure is lower.
Smith & Donahue, 1991: At 50kPa+,  CO2 uptake is inversely proportional to pressure.
Andre & Massimino, 1992: Wheat can sprout @10 kPa and grows better at @20kPa than @100kPa if N2 is low  (O2@14 kPa, N2@ 3,4kPa, CO2@ 3,4 kPa). Appears that lowering N2 in general helps.
Musgrave et al, 1988: Mung beans grow independent of  O2 partial pressure, and general pressure reduction is negative (tested @21kPa)
Goto et al, 2002: Rice can grow @25kPa and @50kPa; having O2 partial pressure at least.10 kPa prevents damage.
Spanarkel & Drew, 2002: Lettuce @ 70kPa grows similar or better than @101kPa
He et al, 2003:  Plants in general grow similar or better at @ 30kPa vs. @1atm because of better removal of ethylene.
Wheeler et al, 2001: Corey et al, 2002: Plants grow similar or better at 30kPa
Ferl et al, 2002: In addition to plants already doing well at low pressures, there's significant potential for genetic improvement to increase it (aka avoiding the drought response).


Thanks for the literature but I want to mention again, survive and thrive are different.

Offline Rei

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Re: The case for lower pressure Mars habitats
« Reply #29 on: 02/04/2017 12:32 am »
As you'll note, most were "same or better" - without any selective breeding / genetic engineering.  Plants uptake CO2 and remove waste products better when the nitrogen partial pressure is reduced.  Given that reducing pressure also reduces system mass, there's no reason not to do it.

Offline Dalhousie

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Re: The case for lower pressure Mars habitats
« Reply #30 on: 02/05/2017 08:34 am »
As you'll note, most were "same or better" - without any selective breeding / genetic engineering. Plants uptake CO2 and remove waste products better when the nitrogen partial pressure is reduced.  Given that reducing pressure also reduces system mass, there's no reason not to do it.

This has been stated several times in this thread but I can find little evidence for it. Do you have a reference?
Apologies in advance for any lack of civility - it's unintended

Offline Rei

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Re: The case for lower pressure Mars habitats
« Reply #31 on: 02/06/2017 01:53 am »
As you'll note, most were "same or better" - without any selective breeding / genetic engineering. Plants uptake CO2 and remove waste products better when the nitrogen partial pressure is reduced.  Given that reducing pressure also reduces system mass, there's no reason not to do it.

This has been stated several times in this thread but I can find little evidence for it. Do you have a reference?

Did I not just post a giant list of references?  Do you need full APA-format cites to look them up?
« Last Edit: 02/06/2017 01:53 am by Rei »

Offline Robotbeat

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Re: The case for lower pressure Mars habitats
« Reply #32 on: 02/06/2017 02:39 am »
Actually, yeah. You shouldn't expect people to spend an entire day picking through a long list of references for a single fact you mentioned.
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Offline Rei

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Re: The case for lower pressure Mars habitats
« Reply #33 on: 02/06/2017 09:03 am »
Actually, yeah. You shouldn't expect people to spend an entire day picking through a long list of references for a single fact you mentioned.

Except that I wrote what's in each of them, so you don't have to.  :Ţ

Offline john smith 19

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Re: The case for lower pressure Mars habitats
« Reply #34 on: 02/07/2017 10:43 pm »
I am not claiming to redo a natural terran biosphere, I am trying to show how we need to put our emphasis on viability elsewhere than just people. The average American eats a little under 1 metric ton of food every year. We have mentioned the ISS astronaut in the forum, I do not remember if it is 500 kgs or 800 kgs per year but even if we use the 500 kgs food/yr value producing it requires another 1000 kgs of plant biomass, above and below ground. But let us assume that for simplicity's sake this food is harvested for times a year and that we fully recycle the biomass of one crop to the next (impossible). So for every 75 kg astronaut there are 1500/4 = 375 kgs of plant biomass providing food. As Napoleon said, armies march on their stomach. So do astronauts.

Well the NASA standard for consumables is 5Kg/day. That's food, water and oxygen. That's 1825Kg/Year/person.  A good reason to start working on something more like a farm than a chemical plant.

TBH I was thinking more in  terms of terraforming than food, where a fairly light cover would provide a few pounds over pressure and keep wind and sand off. The real benefits of lichen is that peat bogs are more effective carbon capture systems than trees or any mechanical systems. They would let you turn the CO2 from the South Polar (dry) ice cap into Oxygen.

For human crops I think plants can tolerate surprisingly low pressures, less than half SL pressure. the question is what can the humans tolerate routinely? The MIT "biosuit" can exert 30%of full SL pressure so if people can handle that (with appropriate adjustments for CO2 and N2 and O2 levels) people can just put one on to do an EVA.
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Offline Rei

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Re: The case for lower pressure Mars habitats
« Reply #35 on: 02/08/2017 01:36 pm »
TBH I was thinking more in  terms of terraforming than food, where a fairly light cover would provide a few pounds over pressure...

This statement got me off and thinking (I was going to doubling the radius of a dome doubles tensile requirements in the envelope, so such Mars domes would either have to be kept proportionally small, or have an elaborate reinforcement system).  But the side thought:Ignoring all the issues of difficulty in producing such a thing:has anyone ever proposed a system of terraforming a planet involving weighing down the whole atmosphere?  I mean a literal planetwide floating row cover, transparent polymer withstanding 1ATM, loads transferred to fibre reinforcement to catenary curtains to cables, with a net loading on them of ~20 tonnes per square meter (something in that ballpark).

I'm not saying I find it a realistic option.  But when we're talking crazy megaengineering plans....  ;)  I mean, you could do that sort of thing on any body, even barren moons, and simultaneously reduce gas escape.  The only difference is you need a lot more mass to weigh it down when gravity is lower (although the stresses on the materials remain the same)
« Last Edit: 02/08/2017 01:39 pm by Rei »

Offline Jim Davis

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Re: The case for lower pressure Mars habitats
« Reply #36 on: 02/08/2017 04:04 pm »
I mean a literal planetwide floating row cover, transparent polymer withstanding 1ATM, loads transferred to fibre reinforcement to catenary curtains to cables, with a net loading on them of ~20 tonnes per square meter (something in that ballpark).

If you can do that why bother with the planet at all?

Online lamontagne

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Re: The case for lower pressure Mars habitats
« Reply #37 on: 02/08/2017 04:19 pm »
TBH I was thinking more in  terms of terraforming than food, where a fairly light cover would provide a few pounds over pressure...

This statement got me off and thinking (I was going to doubling the radius of a dome doubles tensile requirements in the envelope, so such Mars domes would either have to be kept proportionally small, or have an elaborate reinforcement system).  But the side thought:Ignoring all the issues of difficulty in producing such a thing:has anyone ever proposed a system of terraforming a planet involving weighing down the whole atmosphere?  I mean a literal planetwide floating row cover, transparent polymer withstanding 1ATM, loads transferred to fibre reinforcement to catenary curtains to cables, with a net loading on them of ~20 tonnes per square meter (something in that ballpark).

I'm not saying I find it a realistic option.  But when we're talking crazy megaengineering plans....  ;)  I mean, you could do that sort of thing on any body, even barren moons, and simultaneously reduce gas escape.  The only difference is you need a lot more mass to weigh it down when gravity is lower (although the stresses on the materials remain the same)
Yet, it's called a shell world, and was proposed a few years ago by Kennedy & all in the JBIS.  See joined paper.
« Last Edit: 02/08/2017 04:21 pm by lamontagne »

Offline Nomadd

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Re: The case for lower pressure Mars habitats
« Reply #38 on: 02/08/2017 04:22 pm »
As I mentioned in another thread, I spent a week in Cusco, and few people seemed to have any trouble at 9.4 psi after the first day. And the city is all walking up and down steep hills to get around.
« Last Edit: 02/08/2017 04:22 pm by Nomadd »
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Offline john smith 19

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Re: The case for lower pressure Mars habitats
« Reply #39 on: 02/08/2017 06:49 pm »
As I mentioned in another thread, I spent a week in Cusco, and few people seemed to have any trouble at 9.4 psi after the first day. And the city is all walking up and down steep hills to get around.
However to be able to slip into an MIT Biosuit you need to go down to more like 4.4psi.

I don't say it can't be done but I think at that level there will be other consequences.
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Offline Dalhousie

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Re: The case for lower pressure Mars habitats
« Reply #40 on: 02/08/2017 09:04 pm »
As you'll note, most were "same or better" - without any selective breeding / genetic engineering. Plants uptake CO2 and remove waste products better when the nitrogen partial pressure is reduced.  Given that reducing pressure also reduces system mass, there's no reason not to do it.

This has been stated several times in this thread but I can find little evidence for it. Do you have a reference?

Did I not just post a giant list of references?  Do you need full APA-format cites to look them up?

Yes, you should, it's called scholarship.  Links would be helpful as well.
Apologies in advance for any lack of civility - it's unintended

Offline Robotbeat

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Re: The case for lower pressure Mars habitats
« Reply #41 on: 02/10/2017 12:00 pm »
As I mentioned in another thread, I spent a week in Cusco, and few people seemed to have any trouble at 9.4 psi after the first day. And the city is all walking up and down steep hills to get around.
However to be able to slip into an MIT Biosuit you need to go down to more like 4.4psi.

I don't say it can't be done but I think at that level there will be other consequences.
Yet even a slight reduction in pressure makes a big difference in prebreathing times. 9.4psi would drastically cut prebreathing.
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Offline Bob Shaw

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Re: The case for lower pressure Mars habitats
« Reply #42 on: 02/18/2017 11:21 pm »
The martian atmosphere is 1.9% nitrogen, and about 1% of Earth sea level pressure so all one has to do is extract all the nitrogen from 3500 times the volume of your air supply and you have your nitrogen.  3000 if you keep the pressure equivalent to Lee Jay's Denver.  ;)

Extracting the CO2 needed for fuel ISRU will yield all the nitrogen needed. They may or may not have to separate it from the argon which is present at the same procent value.

Wouldn't Argon and Nitrogen be pretty swappable in biological terms?

Offline Bob Shaw

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Re: The case for lower pressure Mars habitats
« Reply #43 on: 02/18/2017 11:22 pm »
In an ecosystem on Earth animals are less than 1% of biomass by weight. Simple mass fraction calculations show that, assuming that food is grown locally, human biomass will not be the majority of the biomass in the Martian habitat, it will be the plant biomass. I think we ought to optimize for the plants rather than the humans and plants do not like lower pressure, it interferes with plant water flows leading to water stress. Plants also do not like if the atmosphere contains too little or much CO2. On Mars we have an atmosphere which we can use as a base to set up the habitat atmosphere. If we grow food in the habitat we will be pumping CO2 and water in the greenhouse which gets converted to O2 and biomass. We will need to remove the O2, MarsOne dies from Oxygen toxicity as the MIT study found. Bars Landorp mentioned an Oxygen concentrator, that can work. Over time if we pump Mars atmosphere we remove both CO2 and O2 (through the concentrator) leading to enrichment of the minor components of the Martian atmosphere. There are issues with this approach, most importantly CO which is toxic to animals but not plants, plants will oxidize it to CO2 but we need to make sure that concentration does not get toxic to the living animals of the habitat, most importantly humans. I am more inclined for variable pressure and composition, we start with barely livable for humans and plants in terms of pressure and composition and then as we establish plants and people we move to closer to earth surface or whatever is optimum for both.

Again, I wonder about enhanced pressure values...

Offline guckyfan

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Re: The case for lower pressure Mars habitats
« Reply #44 on: 02/19/2017 07:15 am »
Wouldn't Argon and Nitrogen be pretty swappable in biological terms?

For a human breathing mix yes. But there was the argument that Argon is less good than nitrogen in keeping fire temperatures down. I don't know how much that would be and if it would be relevant. Maybe in a low pressure environment it would be relevant.

Offline clongton

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Re: The case for lower pressure Mars habitats
« Reply #45 on: 02/19/2017 12:24 pm »
Wouldn't Argon and Nitrogen be pretty swappable in biological terms?

For humans yes, but not for the plant life that humans depend on for life.
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Offline guckyfan

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Re: The case for lower pressure Mars habitats
« Reply #46 on: 02/19/2017 12:32 pm »
Wouldn't Argon and Nitrogen be pretty swappable in biological terms?

For humans yes, but not for the plant life that humans depend on for life.

Plants need Nitrogen. But not nearly as much as is in the atmosphere, even at lower pressure. Also most of the plants need Nitrogen in the form of nitrates which need to be provided. They can't even use atmospheric Nitrogen. It will have to be tested out for individual plants though.

Offline clongton

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Re: The case for lower pressure Mars habitats
« Reply #47 on: 02/19/2017 01:09 pm »
Wouldn't Argon and Nitrogen be pretty swappable in biological terms?

For humans yes, but not for the plant life that humans depend on for life.

Plants need Nitrogen. But not nearly as much as is in the atmosphere, even at lower pressure. Also most of the plants need Nitrogen in the form of nitrates which need to be provided. They can't even use atmospheric Nitrogen. It will have to be tested out for individual plants though.

Ture enough by why go to the complexity of providing 2 different biospheres?
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Offline guckyfan

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Re: The case for lower pressure Mars habitats
« Reply #48 on: 02/19/2017 03:34 pm »
Ture enough by why go to the complexity of providing 2 different biospheres?

Not sure what you mean. There will be the habitat for humans. There may be some greenhouses that double as recreational areas for humans that will have full habitat pressure.

Given that large greenhouse spaces will be needed I think it is a safe assumption that they will be optimized for plants. Optimized in the sense of maximum production for a given construction effort. That may be quite low pressure for some plants and higher pressure for others that need it. Even oxygen and nitrogen content may be optimized differently for different crops. That would happen later, when the settlement expands. Initially they will probably go for one greenhouse habitat type. That may or may not be the same mixture and pressure as the human habitat. Same mixture will make air circulation between human and plant habitats easier.

Offline Russel

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Re: The case for lower pressure Mars habitats
« Reply #49 on: 02/21/2017 11:00 pm »
I'm not sure if this has been covered in this thread, but its an issue that has popped up on earlier threads on space vehicle design.

There is a potential lower limit to pressure set by flammabilty and oxygen concentration.

IIRC the bottom line was about 7psi.

Offline Robotbeat

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Re: The case for lower pressure Mars habitats
« Reply #50 on: 02/22/2017 04:29 am »
I'm not sure if this has been covered in this thread, but its an issue that has popped up on earlier threads on space vehicle design.

There is a potential lower limit to pressure set by flammabilty and oxygen concentration.

IIRC the bottom line was about 7psi.
That's not a limit. It's a conservative level that keeps things similar to here on Earth. Apollo exceeded this. Skylab did, too. So again, definitely not a "limit." Perhaps a good compromise, however.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Russel

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Re: The case for lower pressure Mars habitats
« Reply #51 on: 02/24/2017 07:41 am »
It depends on materials and how conservative tou want to be about flame behaviour. One thing is certain though and that is sooner or later someone is going to set fire to something.

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