Re: Why Pure O2 at 5 PSI?

In article <1133779523.848453.212690@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Proponent!!!N0_SP@M!!!@gmx.net says...
>
>Jeff Findley wrote:
>> I'm sure this doesn't completely answer your question, but you may want to
>> read this:
>>
>> http://www.hq.nasa.gov/office/pao/History/SP-4003/ch6-4.htm
>
>That's interesting. It contains the following summary of reasons for
>choosing 5 PSI:
>
>"This pressure level was chosen as the best compromise to provide (1)
>necessary oxygen partial pressure, (2) efficient use of supply for
>emergency modes of operation, (3) a pressure offering small
>differential change during cabin decompression emergencies, and (4) a
>level for which decompression sickness would be minimal."
>
>Item (1) requires a pressure of at least 3 PSI, but does not favor
>higher pressures.

Actually, it does. The ideal partial pressure of oxygen in a breathing
mix may be 3 psi, but that's partial pressure *in the lungs*. The air
in your lungs consists of ~1 psi carbon dioxide and water vapor being
exhaled, and the balance whatever you were inhaling.

So, at sea level, 1 psi CO2/H2O and 13.7 psi air, giving 2.87 psi oxygen.

Breathing 3 psi O2, your lungs see 1 psi CO2/H2O and 2 psi oxygen. That's
about what you get on a 10,000 foot mountain, and where people start running
into problems.

4 psi oxygen roughly duplicates sea-level air in that respect, and 5 psi
gives you a safety margin.


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.

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