Xref: utzoo sci.space:20429 sci.space.shuttle:5625 Path: utzoo!yunexus!ists!helios.physics.utoronto.ca!news-server.csri.toronto.edu!mailrus!uwm.edu!rpi!dali!uakari.primate.wisc.edu!xanth!mcnc!rti!mozart!sasdvp From: sasdvp@unx.sas.com (David V. Phillips) Newsgroups: sci.space,sci.space.shuttle Subject: Re: space news from April 2 AW&ST Message-ID: <1990May31.131126.7884@unx.sas.com> Date: 31 May 90 13:11:26 GMT Article-I.D.: unx.1990May31.131126.7884 References: <1990May14.140220.5895@jarvis.csri.toronto.edu> <1990May15.160005.29719@utzoo.uucp> <21189@boulder.Colorado.EDU> Organization: SAS Institute Inc. Lines: 59 In article <21189@boulder.Colorado.EDU> lhotka@boulder.Colorado.EDU (Doug Lhotka) writes: > >I seem to remember from somewhere (in First Aid Training I think) that >breathing pure O2 is not only dangerous, but can be lethal. When divers >exceed the 'normal' dive limits they are breathing a mixture of helium and >oxygen...is it possible the EVA guys do the same thing? I realize that the >pressure problem is reversed, but even still pure O2? Anyone out there work >with the systems involved who can definitively answer this question? > >Thanks, > Doug There is no need to dilute the O2, given the pressures (about 4 psi?) inside the suits. This low pressure is used to allow the suits to be more flexible (right?) Long explanation follows Hit 'n' now if you don't care I apologize for the delay in responding...our access to news is two weeks behind. Maybe after we get our 9600 modem??? I can speak about O2 toxicity, and HeO2 breathing mixes, since I was a Deep Sea (HeO2) Diving Officer while in the US Navy. Nitrogen under pressure will cause nitrogen narcosis, or 'rapture of the deep'. The rule of thumb is 50 ft of water is equal to 1 drink. You can imagine how hard it is to get good work done at depths in excess of 150 ft! I remember one training dive to 200 ft, breathing air, where I didn't want to surface. Fortunately it was in a wet recompression chamber, so I didn't have much choice! :-) Helium is used to replace the nitrogen to prevent the narcosis. Decompression times are longer with helium, however. High partial pressures of O2 can cause problems, principally convulsions. In general, a partial pressure of 2 atmospheres is the limit, above which the problems occur. This means that breathing pure O2 deeper that 66 feet is rather dangerous, as is breathing regular air ( 79% N2, 21% O2) deeper than 330 feet (10 atmospheres), as is breathing the most common HeO2 mix (16% O2, 84% He2) below 410 feet, etc. The divers who go deeper than this start reducing the percentage of O2 in their breathing mix, to reduce the partial pressure. Humans need a minimum partial pressure of about 0.2 atm of O2 in order to live. Now to the EVA astronauts. If the total pressure inside their suits is 4 psi, they have approx 1/3 of normal pressure around them. Since that atmosphere consists entirely of O2, they have a partial pressure of about 1/3 atmosphere of O2. No problem with toxicity problems. However, if they were to just jump into their suits and start EVA, they would undoubtedly get decompression sickness from the nitrogen coming out of solution in their blood. By breathing pure O2 before EVA, the nitrogen leaves their blood where it is supposed to, in the lungs. BTW, the treatment for decompression sickness is to take the diver down to 60 ft. in a recompression chamber and have him breathe O2. This recompresses the bubbles in the blood, putting the nitrogen back into solution, and allowing it to escape through the lungs. -- David Phillips sasdvp@sas.UUCP ...!mcnc!rti!sas!sasdvp