The following is an evaluation of oxygenation issues for
space habitations and oxygen suits, without
using any models for in-lung oxygen partial pressure. Only what is in the breathing gas to be
inhaled is considered here! That
way, the issue is not clouded with the
effects of breathing gas displacement by in-lung water vapor, or by in-lung carbon dioxide, or by in-lung dead-end volume effects. Not everyone agrees on the efficacy of those
models.
Shown in Figure 1 is a list of selected air pressures versus altitude from a US 1962 Standard Day atmosphere model, which is identical to the ICAO Standard Day model up to about 65,000 feet. The breathing gas to be inhaled is either Earthly air at 20.9 volume percent oxygen, or 100% oxygen in a vented face mask, with delivery at the altitude’s pressure. The corresponding oxygen partial pressures were computed and included, along with a description of the circumstances, and an indication of the duration of the exposure.
Figure 1 – Oxygenation Limits Short and Long Term
Partial pressure of oxygen is important, because it is related to the partial pressure
of oxygen in-lung. The difference
between in-lung partial pressure and that in the blood, is what drives the diffusion of oxygen across
the lung membranes into the blood.
The duration of the exposure is very important to
determining what levels of oxygen partial pressure are suitable. What I tried to identify as short-term
criteria relate to the military and civil altitude requirements for going
on oxygen in unpressurized airplane cockpits.
Use of such oxygen can be for many hours exposure, and it makes sure the pilots are fully
cognitive, up to the upper altitude
limits for such vented masks.
Those upper altitude limits for vented oxygen face masks are
“fuzzy”. Different people quote
different values, usually close to
40,000 or 45,000 feet. Below that upper
altitude “limit”, you are “good” for
hours. Above it, exposure time with high cognition is limited
to only minutes or even seconds. These
are typically very short zoom-climb experiences, peaking at 50-some thousand feet
altitude. One needs a pressure suit to
stay up there.
For long-term criteria, the experiences of populations living at high
elevations are very informative. There
are two things known that are relevant:
chronic hypoxia effects that manifest as “chronic mountain
sickness”, and an increase in pregnancy
and birthing difficulties above the low-elevation rates, presumably also due to chronic hypoxia. Both start above around 2500 m
elevation, and get worse as elevation
increases. Below 2500 m, there seem to be no detectable chronic
mountain sickness symptoms, and the rates
of difficulties with pregnancy and birthing seem indistinguishable from those
at sea level.
Maximum oxygen partial pressures are limited by enhanced
fire dangers above 21 volume percent near sea level, and seem to be “OK” up to about 0.83 atm
partial pressure for several hours flight time,
using the Navy criterion for going on oxygen at 5000 feet. The ultimate “fatal exposure” limit (1+ atm)
derives from experiences with oxygen in diving.
All of these things are shown in the figure, and highlighted in different colors. The overall conclusions I drew from this are
in a small table at the bottom of the figure.
I chose partial pressure limits for long-term exposure suitable for
habitations, and short-term exposure
limits suitable for hours-long work shifts in pure oxygen space suits. The space suit criteria further divide into
full cognition, versus mere survival
(with presumed cognitive impairment if longer than a few minutes).
Turning to the space suit issue, Figure 2 shows the suit pressures in a
variety of units of measure,
corresponding to the oxygen partial pressure criteria already identified
above, and also as corrected upward to
compensate for a 10% pressure leak-down during a long shift. These would obtain, if there were no other effects to
consider, but there are!
Experience also shows that breathing oxygen at low pressures
causes a loss of water from the tissues in the lungs and respiratory
tract. This drying-out of tissues can
cause bleeding, which is a very serious
problem indeed!
That same experience suggests that there is a minimum suit
pressure below which there is a tissue-drying problem, and above which there is no problem. This is a bit “fuzzy”, but the value most quoted is 3.00 psia
(corresponding to 0.2041 atm). This is
the value to which the 10% leak-down factor needs to be applied for a higher
design suit pressure, in order to avoid
tissue dry-out during a long work shift,
even with leakage.
The result is the final table at the bottom of the
figure, showing the “min suit design”
pressure, the 10% leak-down pressure for
long work shifts, and the short term
survival criteria with full cognition,
and with impaired cognition, after only minutes.
Figure 2 – Oxygenation Issues for Pure Oxygen Space Suits
Returning to the two-gas mixture habitat atmosphere issue, we have a good minimum partial pressure
criterion for very long-term exposures: near
0.15 atm, per the discussion above. However,
there are three other things to worry about when setting the habitat
breathing gas mixture, presumed to be
oxygen and nitrogen. These are: (1) a pressure leak-down over time, to be compensated when detected, (2) the enhanced fire danger of higher
percentage oxygen, but which is offset
by lower total pressures, and (3)
avoiding pre-breathe time, if
possible, when going from the two-gas
mix to the pure oxygen suit.
I picked the partial pressure of oxygen in the standard
atmosphere table at 8200 feet (2500 m) as the “exact” long-term criterion. That partial pressure of 0.1551 atm is
indicated in the first calculation with green highlighting, near the top of Figure 3. Just below it, I ran the compositions of air as it was known
in the 1960’s, air as it is today with
increased carbon dioxide, and a
synthetic air (2-gas mix) at the same oxygen content as today’s air. This includes evaluations of molecular
weight, gas constant, densities at 1 atm and 59 F (15 C) and also
at 1 atm and “room temperature” 77 F (25 C).
From those densities and the mass fraction of oxygen, I computed the oxygen concentrations in kg/m3, at 1 atm and 77 F (25 C) as “room temperature
air at sea level”. That value is 0.2738
kg/m3. That would be the max
oxygen concentration allowable in the habitat breathing gas, to keep the fire danger as no more hazardous
than that of “room temperature sea level air”.
It is based on an overall Arrhenius reaction rate model.
I also checked the oxygen concentration of 40% hospital
oxygen at sea level and room temperature,
known to be a severe fire hazard.
It is about twice the concentration of oxygen in ordinary air at sea
level and room temperature. That is near the bottom of the figure, result highlighted in blue. One would expect those fires to propagate
twice as fast.
The habitat atmosphere calculations start with design values
for gas composition and total pressure,
and include a leaked-down set of values.
These are on the right of the figure,
with much blue highlighting, and
user inputs highlighted yellow. Based
on prior work, I chose to
investigate what I call a “rule of 43” 2-gas mix atmosphere of oxygen and
nitrogen. The oxygen is 43 volume
percent of the mix (leaving 57% nitrogen),
and the total pressure is 43% of 1 standard atmosphere. The resulting oxygen partial pressure (0.1849
atm) exceeds the criterion 0.1551 atm,
which is a lower limit, and the
calculated oxygen concentration (0.2418 kg/m3) is less than the
criterion value, which is an upper
limit.
Just above that calculation is an estimate of the minimum
suit pressure, based on the nitrogen
partial pressure of 0.2451 atm, divided
by the “no pre-breathe” criterion of factor 1.2. That produces a 0.2043 atm min suit
pressure, which meets the “no tissue
dry-out” criterion of 3 psia, and also
far exceeds the cognition limits. That
would be the minimum suit pressure you can use with no pre-breathe interval to
blow off nitrogen: you can just don the
suit and go right out of the airlock.
Any higher suit pressure also qualifies.
Down in the lower right corner of the figure is the habitat
leak-down analysis, set by the min
partial pressure of oxygen. It says we
can leak down 19% in pressure at the same oxygen percentage, and still meet all criteria for safety. Even the fire danger is OK: the oxygen concentration reduces as pressure
reduces, depending as it does on
density.
Figure 4 below summarizes these results in one
place. The “rule of 43” habitat
atmosphere allows the use of rather low-pressure space suits without any
pre-breathe requirements, and provides
plenty of leak-down margin, while at the
same time keeping the fire spread danger similar to that in sea level room
temperature air. The recommended suit
pressure still meets the 3 psia tissue dry-out criterion as leaked-down 10%
after a long work shift.
Figure 3 – Oxygenation Issues for Space Habitats
Figure 4 – Results for Combined Habitats and Space Suits
Final Remarks
There are other habitat 2-gas mixtures and pressures that
would qualify. Not all of these produce
space suit designs with min pressures as low as the one found here.
My selection of the “rule of 43” habitat atmosphere is based
on previous work I did trying to meet all these criteria while getting as low a
suit pressure as possible. It is also an
easy specification to remember.
Further, that lowered suit
pressure is important for 2 very compelling reasons:
First, a
higher suit pressure not only is more difficult to design, it also stiffens like a sports ball at higher
pressures, greatly reducing the mobility
available to its wearer.
Second, higher
suit pressures pretty much rule out suit designs based on mechanical
counter-pressure (MCP), since the
materials and design practices for MCP are unavailable in any form that might
be donned and doffed with reasonable ease at higher pressures.
References:
All these listed references are prior studies posted at
http://exrocketman.blogspot.com. For
rapid access, there is an archive tool on
the left side of that page. All you need
is the posting date and the title. Click
on the year, then the month, then the title if need be (such as if other
articles were posted that same month).
1-2-22
Refining Proposed Suit and
Habitat Atmospheres best case and easiest-to-remember cases, plus an
independent estimate of the utter min suit pressures feasible
1-1-22
Habitat Atmospheres and
Long-Term Health adds a long term hypoxia
criterion for the habitat in addition to short term criteria for the min-P suit
3-16-18
Suit and Habitat Atmospheres 2018
11-23-17
A Better Version of the MCP Spacesuit?
2-15-16
Suits and Atmospheres for Space
1-15-16
Astronaut Facing Drowning Points Out Need for Better Space Suit
11-17-14
Space Suit and Habitat Atmospheres
2-11-14
On-Orbit Repair and Assembly Facility
12-13-13
Mars Mission Study 2013
1-21-11
Fundamental Design Criteria for Alternative Space Suit Approaches
