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#1 2015-12-15 22:29:33

IanM
Moderator
From: Chicago
Registered: 2015-12-14
Posts: 276

Pressurized Air in Suits

I was wondering how much oxygen would be needed for the suits colonists would wear for activity outside of the colony, once the colony is set up. Further, I am looking at to what extent such oxygen would be pressurized above normal, and whether a colonist would thus need any sort of precaution from the bends on his return.

NASA has a spacesuit wherein the oxygen is put at one third of the normal atmospheric pressure (=0.333 atm) and that allows, with a carbon dioxide removal unit, up to 6 to 8.5 hours of EVA. However, such suit is, as NASA admits, clunky and unergonomical (http://spaceflight.nasa.gov/shuttle/ref … q/eva.html), and although the volume is not given, per Boyle's Law it would be three times the volume of the same amount of oxygen on earth - not a very practical option for Martian expeditions, as we'll see later. Due to the same space considerations as NASA, I shall suppose pure oxygen is used here.

According to http://members.shaw.ca/tfrisen/how_much … person.htm, a person needs around 6 L/minute of oxygen at rest, and 50 L/minute after a bout of hard exercise. In the absence of anything to the contrary, I assume this is at standard temperature and pressure (25.00 C (298.15 K) and 1.000 atm, respectively).

I'm assuming that most of a colonist's time would be in a sort of pressurized habitat at exactly 1.000 atm; whether this is a dome, or inflatable covered with regolith, or anything else is irrelevant. As such, the amount of Oxygen needed in a suit would be much less than what is noted above would imply. I further assume that a typical out-of-colony expedition would, depending on its purpose, vary in length anywhere from 30 minutes to 8 hours. This results in a range of

30 min * 6 L/min =180 L oxygen,
30 min * 50 L/min = 1,500 L oxygen
8 hr * 60 min/hr * 6 L/min = 2,880 L oxygen
8 hr * 60 min/hr * 50 L/min = 24,000 L oxygen

180 to 2,880 L for someone always at rest, and 1,500 to 24,000 L for someone always exercising hard. This is a really wide range, and in any case an unacceptably large amount of idle space for someone to carry for such an extended period of time. Therefore, the air will have to pressurized in order to accommodate it, much like scuba diving.

The rest of these calculations will involve the van der Waals equation of state for gasses, which expands the ideal gas law:
PV=nRT
into
(P+a(n^2/V^2)) (V-nb) =nRT
which in turn gives the pressure thus:
P=((nRT)/V-nb) - a (n^2/v^2)

For all of these:
-P is the pressure of the gas, in atm
-V is the volume of the gas, in L
-T is the temperature of the gas, in K, which will always be 25.00C (298.15 K) for these calculations
-n is the number of moles of the gas
-R is the ideal gas constant, here 0.0820578 atmL/Kmol
-a is a constant related to attractive forces between the molecules, for oxygen 1.360 atmL^2/mol^2
-b is a constant related to repulsive forces between the molecules, for oxygen 0.032 L/atm

The number of moles of oxygen calculated from this ranges from 7.348 mol to 753,255.346 mol.

There is a maximum pressure in practice; rental scuba tanks are rated up to 3,000 psi (204.138 atm). Using the latter, rounded down to 200.000 atm , as the absolute maximum, the minimum volume will range from 0.810 L to 83,082.450 L, the latter of which is actually higher than that at STP. Given that the absolute maximum only applies with 8 straight hours of hard exercise, it is unlikely, and the value of roughly 30 minutes of hard exercise, 1,500 L at STP, is more likely, especially combined with the 8 hours of rest, which would give 4,380 L at STP; this in turn gives a volume at the maximum pressure of 14.714 L, which is a far more manageable, if still a bit large, number.

So, it seems like 15 L of oxygen compressed at 200 atm would be a good fit for a suit. However, as assumed earlier, the colony would be pressurized at 1.000 atm, a difference by a factor of 200, quite a lot. In fact, 200 atm is the pressure roughly 2 km deep in the ocean (http://www.calctool.org/CALC/other/games/depth_press). Erring on the side of caution per the NOAA (http://divetables.info), no ascents should ever be faster than 9 meters/minute, equivalent to a decrease in pressure of 0.900 atm/minute. Given that there is a 199 atm pressure difference, that would require at least 221.111 minutes of decompression, a whopping 2 hours in addition to the time of the journey.

The volume of the human body is 66.4 L (http://www.wolframalpha.com/input/?i=volume+human+body). Taking advantage of lower Martian gravity, let us use 75 L as the maximum desired volume. This, with the maximum number of moles, would result in a pressure of 30.249 atm, with a resulting decompression time of slightly more than 32.5 minutes, far more manageable, even with a slightly clunkier. Therefore, 75 L of pure oxygen compressed at 30.249 atm should be used for suits.


The Earth is the cradle of the mind, but one cannot live in a cradle forever. -Paraphrased from Tsiolkovsky

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#2 2015-12-15 22:45:21

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 17,890

Re: Pressurized Air in Suits

Welcome to Newmars IanM, I have minimal experience with eva suits but what I think is a couple of scuba size tanks filled with air at 5,000 psi weigh 16 pounds and will last approximately 8 hours for the average person. The unit has a manifold that lowers the presure and flow rate for use as designed.

If you use google advance search of this forum you with find other related suit topics....

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#3 2015-12-15 23:18:49

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 5,890
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Re: Pressurized Air in Suits

Welcome. We've had many discussions about suits. Several of us advocate the Mechanical Counter Pressure suit, first developed by Dr. Paul Webb. You can read about it here:
The Space Activity Suit: An Elastic Leotard for Extravehicular Activity - submitted for publication December 1967, published in the Journal of Aerospace Medicine, April 1968 issue.
Development of a Space Activity Suit - NASA contractor report, November 1971

A couple fundamentals: partial pressure oxygen is necessary for breathing. Earth at sea level is 1.0 atmospheres pressure = 1.01325 bar = 1013.25 millibar = 101325 Pascal = 14.69595 psi. Earth's atmosphere consists of 20.946% oxygen, 78.084% nitrogen, 0.9340% argon, 0.039% carbon dioxide, plus trace gasses. So that means partial pressure O2 at sea level = 14.69595 * 20.946% = 3.07821 psi. For Skylab, NASA used 3.0 psi partial pressure O2 + 2.0 psi partial pressure N2 = 5.0 psi total. Exact balance changed a bit due to astronauts metabolizing O2 and expelling CO2. For Apollo suits they used 3.3 psi pure oxygen; the rationale was if a suit developed a leak, it could lose 10% pressure and the astronaut would still breathe the oxygen he was used to. That was the original design, they elevated pressure to 3.7 psi by the time Apollo flew. Shuttle used the same pressure as the launch site at KSC, and ISS was designed to use the same pressure as Shuttle. Russians used pressure at their launch site in Kazakhstan, there was debate about pressure but NASA insisted on doing it their way.

There are a couple advantages with lower pressure: lower stress on the hull, lower launch mass, but most importantly you don't have to worry about the bends when decompressing to go outside in a suit. When the Shuttle flew, astronauts would spend hours pre-breathing pure oxygen to expel nitrogen from their blood before putting on a suit and going through the airlock. Still, the EMU suit had elevated pressure to reduce pre-breathe time: 4.3 psi. That requires some hard joints, EMU is a hybrid between hard and soft. Astronauts report their hands are sore and tired by the time they finish doing work in a suit. Gloves are inflated synthetic rubber bags in the shape of gloves, with a durable fabric covering for protection. They don't have convolute joints for fingers. So every time you bend a finger, you compress the air bag of your suit.

Whether you use a conventional air bag suit, or MCP, life support is closed loop. That means oxygen is not expelled. Human metabolism consumes O2, expels CO2. Suits used to use lithium hydroxide to absorb CO2, because that's the lightest material to do the job. But it can't be reused. EMU suits on ISS now use silver oxide. That can be baked out in an oven. On ISS they use an electro-resistive oven (electric oven), but I have a paper from the 1990s detailing experiments with silver oxide granules regenerated in a microwave oven. It's more energy efficient because it heats the CO2, not the oven or canister. The same reason any microwave oven is more efficient. It requires granules instead of silver oxide plates, so the canister has to be changed. For some reason they're not using it. But the point is using silver oxide to remove CO2 means you carry expelled CO2 back to the habitat/station/spacecraft. The life support system there can recycle CO2 back to O2.

::Edit:: Humans can breathe partial pressure O2 lower than sea level; after all humans often live in cities at higher elevations. Boulder, Colorado is at 5,430 feet (1,655 m) so pressure is 84 kPa = 12.183 psi. That means partial pressure O2 = 12.183 psi * 20.946% = 2.55 psi. Studies by the US Air Force showed pilots can endure 2.0 psi partial pressure O2, as long as total pressure was sufficient, and if they were strong healthy pilots, and went through high altitude training. But could only endure 2.5 psi total pressure pure oxygen. They could breathe 2.0 psi pure oxygen for a number of minutes, up to 30 minutes for the strongest pilots, but eventually would black out.

Last edited by RobertDyck (2015-12-16 14:37:26)

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#4 2015-12-16 01:15:31

IanM
Moderator
From: Chicago
Registered: 2015-12-14
Posts: 276

Re: Pressurized Air in Suits

Thank you all for the welcome. The Mechanical Counterpressure Suit looks like it would be the most feasible, in my opinion. It seems like it is the most efficient and, as is the intent, the most flexible. I thought it could be used, sans life support, as underwear within the colony, much like a union suit or long johns, continuously to avoid having to go through the trouble of donning, but circulation problems seem to put a damper on that, and it would still have to be donned daily due to showering, etc. Nonetheless, outside the colony, it would indeed help with longer expeditions, being limited only by the oxygen available.


The Earth is the cradle of the mind, but one cannot live in a cradle forever. -Paraphrased from Tsiolkovsky

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#5 2015-12-16 18:35:05

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,890

Re: Pressurized Air in Suits

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#6 2016-02-03 18:59:30

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 17,890

Re: Pressurized Air in Suits

Looking at whats needed and how much one can lift for long periods of time we are limited to what we can carry. That would appear to be one of the reasons for only having the capability of just 8 hours. Another factor may be the batteries that would power the electronics within the suit as well.

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