[breathing pressure limits]: forming composit space station skin in situ
From: Kent Paul Dolan (xanthian_at_well.com)
Date: 02/09/05
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To: sci-space-tech@moderators.isc.org Date: Wed, 9 Feb 2005 04:40:20 +0000 (UTC)
"Ian Stirling" <root@mauve.demon.co.uk> wrote:
> I'd argue slightly about the 3PSI, you can go a
> fair bit lower if your goal is "won't die in 5
> minutes without exertion".
No, the situation under discussion was normal
breathing pressure for work, not "emergency
requirements".
> At the lung wall at 37C is 47 torr (780 torr = 1
> atmosphere = 14.7 PSI) of water vapour.
> This can't be reduced, and is a hard limit
> (barring hypothermia).
> About 15 torr of CO2, at normal metabolic rates.
> People (nutters) have climbed everest without
> supplemental oxygen.
These cannot be used as a standard. Mountain
climbers spend a significant fraction of their lives
"at altitude", so gain some of the same benefits as
the South American natives living normally at
altitude, and their lungs are arguably (and
documented to be) more efficient as a result of long
practice breathing thin air.
> At the top, you're looking at 276 torr, of which
> about 1/5 is O2.
> So, at the lung wall, we have 276-62 torr = 214
> torr of atmosphere, or 43 torr of oxygen.
> So, for pure O2, 43+62 = 125 torr
> or 2.35 PSI will get you the same oxygen
> saturation as on the top of everest.
No, if I understand what you're claiming, you can't
do the math that way, you _still_ have to overcome
the displacement effect within the lungs of the
water vapor and CO2 enhancements with a "higher than
final desired lung interior level" external O2
pressure.
> <snip>
>> I think you're missing the point that if you put
>> 3 psi of oxygen _inside_ the human body, you have
>> to put 3 psi _outside_ the human body as well, or
>> the person just explodes.
> Err, no.
> If you try to hold your breath, your lungs
> rupture. If you don't, you'r fine until you die
> from lack of oxygen (about a minute until you need
> more than CPR).
Again, you are confusing the "emergency" situation
with the "working day" situation. So far as I can
determine from a very muddled description, the OP
was claiming one could, during the working day,
breath 3PSI oxygen in the suit while supplying only
1-2 PSI counterpressure, from inert gases, in the
containment vessel, and do that using a "flight
suit", which in its normal incarnation supplies
extra pressure to the legs to counteract G forces
and keep blood in the brain, but does not somehow
magically supply an overall external counterforce to
the internal overpressure. The worker would indeed
"explode", just as warned. Human skin is not strong
enough to withstand pressure differentials of such
sizes, nor is it strongly enough attached to the
underlying flesh to stay attached when "inflated",
thus the common technique of skinning animals with
an air pump.
HTH
xanthian.
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