From: knapp@spot.Colorado.EDU (David Knapp)
Subject: The story on Oxygen Was Re: Lunar "colony" reality check
Organization: University of Colorado, Boulder
Date: Thu, 12 Nov 1992 01:16:47 GMT

In article <1992Nov11.222130.8652@infodev.cam.ac.uk> sl25@cus.cam.ac.uk (Steve Linton) writes:
>In article <1992Nov11.143433.18514@news.weeg.uiowa.edu>, jboggs@umaxc.weeg.uiowa.edu (John D. Boggs) writes:
>|> From article <1992Nov11.005151.15358@jpl-devvax.jpl.nasa.gov>, by jenkins@fritz (Steve Jenkins):
>|> > 
>|> > Oxygen, like many gases, has narcotic effects at very high pressures,
>|> > such as in deep-sea diving.  It can cause blindness in newborns
>|> > 
>|> 
>|> Yes to blindness in newborns, but it is the *nitrogen* that has the narcotic
>|> effect in deep sea diving -- hence the use of helium for the really really
>|> deep dives.
>
>Oxygen causes drunkenness-like symptoms above around 2atm. Any inert gas (except
>helium, which I'll come back to) causes narcosis (anaesthesia) at sufficient
>partial pressure. Roughly, the larger the molecule, the lower the pressure
>needed. For nitrogen it's about 10atm, for ether or chloroform, less than 1.
>Helium, presumably because of its very small molecule, causes the opposite effect
>at very high pressure (> 50atm), causing trembling and over-excitation  of the
>nerves. Hence divers at great depths breathe  mostly helium, with a few %
>nitrogen to balance the effect of the helium, and less than 1% oxygen. Hydrogen
>is also used, but I'm not sure of the details.


Oxygen Poisoning:

Oxygen is essential for life, so it may seem surprising that it can have 
poisonous effects; but this is important for divers to understand. The normal
partial pressure of oxygen in air is 0.21 atm, equivalent to 21 percent oxygen
at sea level.  Above about 1/2 atmosphere (e.g., 50% oxygen at surface
or 25% oxygen at 33ft), lung damage will develp within hours or days depending
on the oxygen "dose".  This is a recognized problem in treating hospital
patients who need extra oxygen, but it seldom affects divers except in 
prolonged periods at depth in saturation diving or in extended recompression
with oxygen breathing.

More important for most divers is the fact that oxygen can cause convulsions,
like th efits or seizures of grand mal epilepsy. Drowning is the usual result
of an underwater convulsion, but the risk of convulsion selfom arises unless
the partial pressure of oxygen approaches 2 atmospheres. Such a dose is
reached with pure oxygen at 33 ft, 50% at 99ft, 20 % at 297 ft, etc.
Convulsions may occur at lower pressures with excess carbon  dioxide either
inhaled or retained.

There shoud be no possibility of oxygen poisoning in the use of open-circuit
scugba unless charged with something other than compressed air. In most
closed-circuit scuba, only pure oxygen can be used; and the depth of safe
diving is severely limited. Special closed- and semiclosed- ciruit 
apparatus designed for gas mixtures must be used with great care to avoid
oxygen poinsonin. The severity of the limits is illustrated by U.S. Nave
rules which normally restrict the use of 100 % oxygen for working dives
to no more that *25 ft* (the emphasis mine), with a 75 
minute time limit at that depth. Limits permitted for unusual circumstances
extend to 40 ft, where only 10 minutes are allowed.

Taken from  The New Science of Skin and Scuba Diving, by the Council
for National Cooperation in Aquatics, New Century Publishers, 1980.


Oxygen Poisoning:

...an excess of oxygen is just as bad as too little. If you breathe an excess
of oxygen for an extended period of time, the outcome can be harmful. Excess
oxygen can injure lung tissues and can adversely affect the central 
nervous system.

Parial Pressures:

How much oxygen is too much? This depends on three things: the amount, its
pressure, and the exposure time. Ordinarily, we breathe a mixture of 78%
nitrogen and 21 %g oxygen at atmospheric pressure (14.7 psi). Dalton's
Law states that the total presssure of a gas mixture equals the sum of the
partial pressures that make up the mixture. In other words, the sea level
partial pressure of nitrogen equals 78 % of 14.7 psi, or 11.6 psi. The
parital pressure of oxygen is 21 % of 14.7 psi or 3.09 psi. 

Now, oxygen poisoning can occur when the partial pressure of pure oxygen
equals two atmospheres absolute, or 29.4 psi (two times 14.7 psi). This
could happen if you were breathing pure oxygen (not air) at a depth of
33 feet. It could also happen by breathing ordinary air at a depth of 297
feet (this is where He-Ox mixes are used, with lower partial pressures
of oxygen -d.k.), or 10 atmosphere (10 time 3.09 psi).

Taken from Open Water Certification Sport Diver Manual by Jeppeson Co
(used by PADI diver training)

The bottome line is that we don't dive much beyond 100 ft on air. If 
we're going to go deeper, we get life insurance and use a helium oxygen
mix with much lower fraction of oxygen. Helium is used as a buffer
because it is not chemically reactive when it dissolves in the blood.

I'm not sure if it dissolves less into blood than N2 but I know that 
decompression is still an issue using heliox mixes.

Note that at Steve's upper limit of 50 atm, most people would have
been long dead if they were using pure oxygen, so there's no need to worry 
about convulsions.



All spelling errors are mine.





-- 
David Knapp                                   University of Colorado, Boulder
Perpetual Student                                 knapp@spot.colorado.edu
Looks like hate *is* a family value after all and Colorado families are for it.

Xref: sparky sci.space:15734 alt.sci.planetary:291