I think it applies to both, but mostly to settlements.10-12psi would be a pretty conservative compromise.
1-2psi if you have a pure oxygen mask could be done once you're acclimated,
Quote from: Robotbeat on 01/29/2017 04:26 am1-2psi 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.
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?
Quote from: Robotbeat on 01/29/2017 04:26 am1-2psi 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.
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.
kPa isn't too bad. 101kPa is 1 atmosphere. So kPa is roughly atmosphere in percent. Pretty easy to remember.
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.
Indeed, it's partial pressure that matters
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.
Quote from: Rei on 01/29/2017 11:38 amNow, what I'm wondering: why would people use PSI?It would be easier to discuss pressure in atmospheres.
Now, what I'm wondering: why would people use PSI?
I usually prefer SI, but lots of reference material for space suits, etc, is in psi.
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.
[...]
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.
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.
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.
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.
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.
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.
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.
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.
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].
Quote from: AegeanBlue on 02/02/2017 11:49 pmI 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 constrictedAndre & 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 @101kPaHe 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).
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.
Quote from: Rei on 02/04/2017 12:32 amAs 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?
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.
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.
TBH I was thinking more in terms of terraforming than food, where a fairly light cover would provide a few pounds over pressure...
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).
Quote from: john smith 19 on 02/07/2017 10:43 pmTBH 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)
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.
Quote from: Dalhousie on 02/05/2017 08:34 amQuote from: Rei on 02/04/2017 12:32 amAs 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?
Quote from: Nomadd on 02/08/2017 04:22 pmAs 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.
Quote from: Comga on 01/30/2017 01:09 amThe 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.
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.
Wouldn't Argon and Nitrogen be pretty swappable in biological terms?
Quote from: Bob Shaw on 02/18/2017 11:21 pmWouldn'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.
Quote from: clongton on 02/19/2017 12:24 pmQuote from: Bob Shaw on 02/18/2017 11:21 pmWouldn'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?
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.