This article takes on the best available information regarding selection of pure oxygen space suit pressure levels, and how they relate to space habitation atmosphere composition and fire dangers. The previous related articles posted on this site all share the same basic methodologies and calculation methods.
The fundamental methodology here is to calculate both atmospheres and design criteria in terms of the wet in-lung partial pressure of oxygen, which in turn is what actually drives the diffusion of oxygen across the lung tissues into the blood.
In the earlier articles, there were unresolved issues with “pre-breathe” (decompression) criteria, and with fire danger criteria, that have since been resolved or sidestepped. This article brings, as a new item, a “leak-down” suit pressure factor, and also brings additional supporting data for the selected suit pressures, which are lower than is typical of NASA practice today. Lower pressures make space suits more comfortable to wear, and easier to design.
Lists of the previous articles follow. The most recent, superseded by this one are:
“A Better Version of the MCP Space Suit?” 11-23-2017
“Suits and Atmospheres for Space” 2-15-2016
Those two in turn superseded these earlier articles:
Space Suit and Habitat Atmospheres” 11-17-2014
“On Orbit Repair and Assembly Facility” 2-11-2014
“Fundamental Design Criteria for Alternative Space Suit Approaches” 1-21-2011
The best way to find any of these is to use the date/title navigation tool at the left. Click on the desired year, then on the desired month. If the article is not top of the list (in view), click on its title.
To view any or all of the figures enlarged, click on any of the figures. You may then scroll through all of them in enlarged format. Once done, you can return to the article by “X-ing-out” of the enlarged figures screen.
Another way is to find one, then click on the “space program” keyword. Then you will see only those articles with that search keyword, which these all share. An alternative keyword is “space suit”, but I’m not sure that all of them share this search keyword. The more recent ones do, for sure.
Wet In-Lung Oxygenation is Not the Oxygen Content of What You Breathe
Atmospheric pressure is easily determined versus altitude using published atmosphere tables. It doesn’t vary much from model to model. The model used here is the US 1962 Standard Day, which for altitudes up to about 65,000 feet, is identical to the ICAO Standard Day.
Air composition is fairly standard as follows. It is oxygen, diluted with mostly nitrogen. The largest trace ingredient is argon. Whether given as fractions or percentages, these compositions are usually given in volume format, which is also molar. True “synthetic air” is the two-gas mix of oxygen and nitrogen, at the same oxygen content as real air. Other ratios are also feasible, for different purposes. The standard air oxygen content used for this article is 0.20946 = 20.946% by volume.
Gas Vol %
Carbon dioxide 0.03 (older figure, has since risen to 0.04)
Trace gases 0.018 or less
These figures are for dry air (no humidity). The presence of water vapor displaces dry air, so that the total of their pressures adds to the atmospheric pressure. The water of interest here is that within the lungs, with liquid moisture present at body temperature. If one assumes the vapor is in equilibrium with the warm liquid, then the vapor pressure in the lungs is the standard steam table value at body temperature:
Pvap = 47.07 mm Hg = 0.061934 atm
at T = 37.0 C = 98.6 F (human body temperature)
The oxygen partial pressure in the dry air is the dry air pressure multiplied by the volume fraction of oxygen. In the atmosphere tables, the pressure ratio to standard sea level pressure is numerically equal to the altitude pressure in atm. Dry air oxygen partial pressure, atm, is thus 0.20946 * (P/PSL).
In Figure 1, oxygen partial pressure in the dry air is plotted versus a wide range of altitudes. To calculate wet in-lung oxygen partial pressure, you reduce the dry air pressure by the water vapor pressure, then apply the oxygen fraction to that reduced value. Both are plotted in Figure 1. The difference between then becomes increasingly significant as altitude increases, because water vapor pressure depends on only body temperature, and is thus an ever-larger portion of the atmospheric pressure as altitude increases.
There are several notes added to the figure. First is that US Navy pilots are required to start using supplemental oxygen when they exceed 5000 feet altitude. Second is that USAF pilots, and FAA civilian pilots, must use supplemental oxygen when above 10,000 feet. In the civilian case, this is coupled with a time limit, so that oxygen is not required if above 10,000 feet, until the time is exceeded. But oxygen is always required if above 14,000 feet.
Also shown on the figure is the usual airliner cabin pressure altitude practice, which is 10,000 to about 15,000 feet equivalent. The 10,000 foot condition is rather close to the elevation of the city of Leadville, Colorado (USA). The 15,000 foot condition is rather close to the elevation of the city of Daocheng, Sichuan (China). La Paz, Bolivia, is not shown, but has an elevation in the middle of the cabin pressure altitude range, at 13,323 feet. These are all cities where people live perfectly normal lives.
Equivalent wet in-lung oxygen partial pressure is also shown in the figure as the arrows A and B for the effects of a (vented) supplemental oxygen mask at 40,000 feet, and at 45,000 feet, respectively. These masks seem quite adequate for long flight times at 40,000 feet, for which wet in-lung oxygen falls in the cabin pressure altitude range at just about 12,000 foot equivalent. They are recommended only for short exposures at 45,000 feet, which seems about equivalent to 20,000 feet. Only a few genetically-adapted herders live and work at this altitude, in the Andes and the Himalayas.
Thus wet in-lung oxygen partial pressures equivalent to 15,000 feet or lower are quite consistent with standard high-altitude flying practices.
The calculation for the two supplemental oxygen mask points was a little different. The calculated curves and some notes are given in Figure 2. The big assumption was that 100% dry oxygen was in the mask, at the altitude atmospheric pressure. Offsetting this down by the vapor pressure gives the wet in-lung oxygen partial pressure, as given in the figure.
The assumption about 100% oxygen inside the mask is probably pretty good at the higher altitudes, and probably not so good at lower altitudes. The pressure drop from the supply to the mask is high enough to ensure choked flow somewhere in the equipment, so that the delivered oxygen massflow is fixed, and thus independent of the delivered density conditions in the mask.
The delivered density is lower at high altitudes, which for the same massflow is larger volume flow. If that volume flow is large enough, it overwhelms the effects of imperfect sealing of the mask to the face, and of the diluting effect of the exhaled gases. At those conditions, the mask is filled with very nearly pure oxygen. This would certainly be the case at the highest altitudes for which the mask is considered effective. Those would be long exposures at 40,000 feet, and short exposures as high as 45,000 feet. Military flying practice requires pressure breathing equipment above those altitudes; effectively, some kind of pressure suit.
It is these wet in-lung oxygen partial pressures from the supplemental oxygen mask at 40,000 and 45,000 feet that was the objective here. Those are the points A and B in Figure 1 above. The possible error at low altitudes is irrelevant to the discussions here.
Figure 2 – Wet In-Lung Oxygen from a Vented Pure-Oxygen Mask, as a Function of Altitude
How to Use Altitude Equivalence for Oxygen Suit Pressure Selection
Figures 3 and 4 show this process for two slightly-different suit design pressures. You start with an assumed design altitude in Earthly air (for which you can also figure its dry oxygen partial pressure if you want, but we don’t use that in this calculation), and offset the ambient pressure down by the water vapor pressure, to the wet in-lung dry air partial pressure. Use the oxygen fraction against the dry air partial pressure to calculate the wet in-lung oxygen partial pressure. Use this wet in-lung oxygen partial pressure as the wet in-lung result to be obtained by your suit. Add to it the water vapor pressure, and that is your dry oxygen suit pressure at design conditions.
Then, ratio-down that suit pressure by your leak-down margin factor (in this case 1.10) to the min tolerable dry sit pressure. Offset that down by the water vapor pressure to obtain the min tolerable wet in-lung oxygen partial pressure. This needs to fall in an acceptable range (generally that defined by the wet in-lung oxygen partial pressure at cabin pressure altitudes, or 10,000-to-15,000 feet equivalent).
Now, divide that min tolerable wet in-lung partial pressure of oxygen by the volume fraction of oxygen in dry air, to obtain the wet in-lung partial pressure of dry air. Add to that the water vapor pressure to obtain the Earthly dry air pressure at altitude. Reverse the table lookup to determine the equivalent altitude for your min tolerable leak-down condition. If you did this right, it will fall in the 10,000 to 15,000 foot range of acceptable cabin pressure altitudes.
Figure 3 does this for an 8700 foot equivalent suit design at 0.2004 atm = 2.945 psia that leaks down by factor 1.10 to a 12,000 foot equivalent design at 0.1822 atm = 2.678 psia. Figure 4 does this for a 10,000 foot equivalent suit design at 0.1930 atm = 2.836 psia that leaks down by 1.10 to an equivalent 13,300 foot design at 0.1755 atm = 2.579 psia. Both fall within the cabin pressure altitude range or lower, for acceptable wet in-lung oxygen partial pressures, considered adequate for pilots. The 13,300 foot condition is also equivalent to the major city of La Paz, Bolivia, to which tourists acclimatize very quickly.
Either design, or an even-higher pressure design, are all quite acceptable for life support and fully-functional human cognition in a space suit. The lower pressures allow easier suit design, and more comfortable suits. So, unless there is an overriding need for higher pressures, these lower pressure designs are to be preferred.
Figure 3 – Relating Design and Leaked-Down Suit Oxygenation to Equivalent Air at Altitude: 8.7/12 kft
Relating Suit Design Pressure to Two-Gas Habitat Atmospheres: Fire Danger and Pre-Breathe Criteria
There are two issues that relate oxygen suit pressure to the pressure and composition of a two-gas habitat atmosphere. One is the “pre-breathe” factor, the other is the enhanced fire danger posed by a too-oxygen-enriched atmosphere.
The pre-breathe factor used by NASA was originally developed for the US Navy, for oxygen-nitrogen two-gas mixtures. If in the dry habitat atmosphere the partial pressure of nitrogen is at or below factor 1.20 times the pure oxygen suit pressure, then no decompression time is needed breathing pure oxygen to blow off the nitrogen in the blood. That decompression time is the “pre-breathe time”.
As an example, for a two-gas oxygen-nitrogen atmosphere at 1 atm pressure and 20.946% oxygen by volume (“synthetic air” at 1 atm), the nitrogen partial pressure is 0.79054 atm. For a pure oxygen suit at 3.8-4.2 psia, the dry oxygen partial pressure is 0.2586-.2858 atm. The ratio of nitrogen to suit oxygen pressures is 3.057-2.766. This range of values far exceeds the 1.20 criterion, so significant hours of pre-breathe time are required. This is pretty much current NASA practice at the ISS (space station).
In the earlier articles, it was unknown to me whether that factor of 1.2 applied to individual dilution gas partial pressures, or to the aggregate sum of their partial pressures. I still do not know, but I sidestepped that issue entirely by only considering two-gas mixtures of oxygen and nitrogen here.
It is also fairly obvious that reducing habitat atmosphere pressure reduces the dilution gas partial pressure, thus reducing its ratio to suit oxygen pressure. It is also fairly obvious that increasing the oxygen fraction of the habitat atmosphere also reduces the ratio. Thus, reduced habitat atmosphere pressures at higher-than-Earthly oxygen content seems to be indicated for lowering or eliminating pre-breathe times.
However, increasing oxygen content runs afoul of enhanced fire danger. I have read of two ways to judge the fire danger. One is that the percent (by volume) oxygen for air pressures near 1 atm should be under 30% at most, and preferably nearer the 20.946% of ordinary air.
Percent oxygen is independent of total pressure, but partial pressure of oxygen is not. The second way to judge the danger is a limit on oxygen partial pressure, limited to about sea level Earth normal.
After thinking about this, I realized that the enhanced fire danger resulting from the enhanced oxygen is really faster chemical reaction rates, leading to very much-accelerated phenomena and enhanced energy release rates. For an overall empirical model of a fuel-air chemical reaction rate, a second-order two-component one-step Arrhenius model is often used:
Rate = k Cf^r Co^(n-r) exp[Ea/RT]
where n ~ 2 and r ~ 1,
with Cf and Co measured as mass/vol
That suggests the real criterion might be the oxygen concentration Co, expressed in mass per unit volume units. If this concentration were no worse than that of Earthly air, then the fire reaction rates should be unaccelerated relative to those seen in Earthly air. Both volume fraction oxygen and atmosphere pressure get into this concentration calculation.
The volume fractions of the two gases, and their molecular weights, give you the molecular weight of the synthetic air mix:
MW-O2 * vol frac O2 + MW-N2 * vol frac N2 = MW-air * 1
The molecular weight ratio and volume fraction of O2 give you the mass fraction of the air that is oxygen:
(MW-O2 / MW-air) * vol frac O2 = mass frac O2
Because the pressure ratio to standard pressure P/Pstd is numerically the pressure in atm, you can use the habitat pressure expressed this way, and its temperature, to correct standard air density to habitat atmosphere conditions. The ignores the difference between the synthetic air and actual air, but that is trivial:
Dens-hab = density-std * (P/Pstd) * (Tstd/Thab)
Multiplying habitat density by the mass fraction of oxygen gives you the oxygen concentration:
C-O2 = dens-hab * mass frac O2 (suggested units kg/cu.m)
For Earthly air at sea level pressure and standard temperature, the density is 1.225 kg/cu.m, and the concentration of oxygen is 0.275 kg/cu.m. If the habitat oxygen concentration is that value or less, the fire reaction speeds and energy release rates should be as slow (or slower) than on Earth.
Now, using exactly the pre-breathe limit factor of 1.20, you want your habitat atmosphere to equal the selected value of suit dry oxygen pressure, and so the habitat nitrogen pressure is 1.2 times that oxygen pressure. That is the inherently-high oxygen volume fraction of 1/(1 + 1.2) = 0.4545, but the atmospheric pressures being considered here are well below sea level.
For a range of suit oxygen pressures from about 0.13 atm up to about 0.24 atm, habitat pressures vary strongly, and so does oxygen concentration. This is shown in Figure 5. The note regarding “synthetic air” refers to a synthetic Earthly air, at 20.946% oxygen, with the remainder all nitrogen. The habitat atmospheres considered here all have more oxygen content and less nitrogen content than a true synthetic Earthly air.
Referring again to Figure 5, the derived habitat atmospheres as a function of oxygen suit pressure reach the Earthly oxygen concentration limit of 0.275 kg/cu.m at a suit pressure of 0.2165 atm, and a habitat atmosphere pressure of 0.4663 atm. That’s your upper limit for fire reaction rates equal to Earthly rates at sea level. It corresponds to a suit pressure lower than current practices, and 45.45% oxygen by volume in the habitat two-gas mix.
Note in Figure 5 that the volume percent-as-fire-criteria is always violated, while in this pressure range, the partial-pressure-of-oxygen criterion is satisfied until you get very close to the concentration criterion limit. Yet, it is these two items working together that actually determine the concentration-driven reaction rates in the fire chemistry. Thus it is oxygen concentration that is the real fire danger criterion, and it should not exceed sea level Earthly values, for fires not to exceed familiar Earthly rates. By this criterion, you may actually have a slightly-higher suit pressure than by the partial pressure criterion. But you may not lower it without triggering pre-breathe time requirements.
Figure 5 – Comparing Fire Danger Criteria from Increased Oxygen Content
In view of that result, what you really want to do is identify a minimum suit pressure design that you want to accommodate, and use it to set your habitat atmosphere. That way, for that suit, and for any higher pressure designs, you will not trigger any pre-breathe time. This is based on the design pressure, not the factor-1.10 leaked-down pressure. This is shown in Figure 6 for two candidate designs: the 8.7 kft equivalent “A”, and the 10 kft equivalent “B”, with the habitat atmosphere “set” by the lower-pressure 10 kft equivalent design. Both the 1.10 leak-down and 1.20 pre-breathe factors were applied.
Doing this produced the results tabulated in the figure: all the pre-breathe factors were at, or under, 1.20, all the way up to (and beyond) the “limit” suit design pressure of 0.2165 atm. There is nothing about this selection which precludes suit pressures as high as current practice!
Note that the factor 1.10 leak-down points are also shown. Decompression down to them is not an issue; you will only be recompressing from them up to habitat pressure.
These were calculated with a spreadsheet, and are given in Figure 7. The habitat atmosphere data is given in the upper part, and the data for the A and B suit designs (design and leaked-down)in the lower part, along with the “limit” suit design (at design only). Bear in mind that the habitat atmosphere is a two-gas oxygen-nitrogen mix set at 45.45% volume percent oxygen, it is fixed. Bear also in mind that still-higher suit pressures, are also compatible with this.
Figure 7 gives suit and habitat pressures in a variety of measurement units for a variety of readers. Note that the wet-in-lung partial pressure of oxygen in the habitat atmosphere is identical to that from the min-pressure design suit (the 10 kft B design). This fell within the cabin pressure altitude range considered adequate for a pilot’s cognition (10,000 feet, actually).
The habitat atmosphere is 0.4242 atm (6.420 psia), and 45.45% oxygen, the rest nitrogen. The lowest compatible (no pre-breathe required) oxygen suit pressure is 0.1930 atm (2.836 psia), substantially lower than current NASA practice (3.8-4.2 psia). Lower-pressure suits might require pre-breathe time, but no higher-pressure suit would require any.
This lowest compatible-pressure suit (at 146.7 mm Hg) is also substantially reduced from the 1968-vintage experiments of Dr. Paul Webb with his mechanical counterpressure (MCP) designs based on stretchable fabrics. His experiments back then used about 170-190 mm Hg as the suit pressure.
Under the conditions proposed here, such MCP designs are far more feasible. And, conventional full pressure suits are far more comfortable, and easier to design.
Finally, the habitat atmosphere calculates to have (at 25 C = 77 F) an oxygen concentration of 0.245 kg/cu.m (per Figure 6 above), which is less that Earthly air at sea level pressure (0.275 kg/c.m). The fire danger in this habitat atmosphere should be no worse than Earthly sea level air, and might actually be slightly reduced, in spite of the high oxygen percentage.
Figure 7 – Results for Recommended Suit Pressures and Recommended Habitat Synthetic Air
What I propose here is a low-pressure habitat atmosphere enriched in oxygen content, yet safe enough in terms of fire danger, while not requiring any pre-breathe time for pure oxygen space suits of suit pressure far lower than current practice. Both the habitat and the min-pressure suit design maintain the wet in-lung oxygen partial pressure of Earthly air at an elevation of 10,000 feet, considered by most authorities as quite adequate for pilot-level cognition. There is no reason that explorer-type astronauts cannot make use of this in vehicles and space stations located anywhere in the solar system.
Colonist-astronauts are different: there are decades of exposure, not just months or years, and there are the inherent (and so far unknown) risks of pregnancy and child development. For that situation, I recommend that we “dance with who brung us”: we evolved in Earthly-air at elevations from sea level to around 15,000 feet.
We are genetically adapted to that. So use it.
I would recommend real synthetic air (20.946% by volume oxygen, the rest nitrogen), at an equivalent pressure altitude not to exceed about 10,000 feet. You will always have pre-breathe time to contend with, when decompressing down to a relatively low-pressure oxygen suit. Recognize that, and just deal with it.
A suggestion for “dealing with it”:
Those parts of the colony where pregnant women and young children might be, should have oxygen-nitrogen at 20.946% oxygen, and no less than the 10.11 psia that is equivalent to 10,000 feet elevation (0.1441 atm partial pressure of oxygen, 0.5437 atm partial pressure of nitrogen). That’s 0.1311 atm wet in-lung partial pressure of oxygen, same as the min-pressure suit design.
Other parts of the colony could use the 45.45% oxygen mix at 6.24 psia (0.1930 atm partial pressure of oxygen, 0.2316 atm partial pressure of nitrogen). People using suits outside could decompress from the 45%/6.24 psia blend without any pre-breathe time. That’s also 0.1311 atm wet in-lung partial pressure of oxygen, same as the min-pressure suit.
Everybody gets the same wet in-lung oxygen partial pressure, whether in the habitat with synthetic air at 10.11 psia, the enriched blend at 6.24 psia, or the min-pressure suit design at 2.84 psia. There’s no pre-breathe time for decompressing to any higher-pressure suit designs, although there would for yet-lower pressure designs.
Whether any pre-breathe decompression time is needed going from the higher-pressure portion of the colony to the lower-pressure portion is something still unknown to me. But the change is rather modest, so any such decompression time should also be modest.
If any readers actually know that answer, please weigh in with your comments!