I thought pure oxygen was essentially poisonous?
Brit 'naut Peake gears up for spacewalk
European Space Agency astronaut Tim Peake is gearing up to exit the International Space Station (ISS) on Friday on his first spacewalk. Tim Peake tries his spacesuit on for size Flying the flag: Tim Peake tries his spacesuit on for size. Pic: Tim Peake He'll be joined by the orbiting outpost's other Tim - NASA 'naut Kopra …
COMMENTS
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Wednesday 13th January 2016 11:55 GMT My-Handle
So did I. A short bit of research shows that breathing pure oxygen at atmospheric pressure causes oxygen toxicity, where excess oxygen attacks the lungs. It appears that this can be countered by breathing pure oxygen at lower pressures. The levels of oxygen in the lungs fall below the maximum rate at which the blood can absorb it, so no excess remains to do damage.
This is from memory, but I have heard that breathing oxygen mixed with an inert gas like helium at normal pressure can also be used to ward off the bends, as the helium displaces the nitrogen. I'm not sure how accurate this is though.
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Wednesday 13th January 2016 12:18 GMT graeme leggett
It's a subject that I remember from diving theory lessons.
In a mixture of gases, the partial pressure of O2 below a certain value is no good, too high and you suffer differently.
For the bends, its the different way helium and nitrogen dissolve into body tissue that is the solution to the issue.
https://en.wikipedia.org/wiki/Partial_pressure
https://en.wikipedia.org/wiki/Breathing_gas
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Wednesday 13th January 2016 14:21 GMT TitterYeNot
"This is from memory, but I have heard that breathing oxygen mixed with an inert gas like helium at normal pressure can also be used to ward off the bends, as the helium displaces the nitrogen. I'm not sure how accurate this is though."
Perfectly accurate - heliox mixtures are often used for commercial & technical dives, which can be far deeper than those performed by recreational divers using compressed air (40m depth is the recommended absolute limit for recreational dives in order to avoid nitrogen narcosis and/or decompression sickness, AKA the bends.)
Using low pressure oxygen has another advantage - a space suit inflated to a pressure of one atmosphere can become extremely rigid in a vacuum, so a lower internal pressure makes movement easier. The first ever human EVA almost ended fatally as the cosmonaut's space suit became so stiff that he couldn't get back into the Voskhod airlock. He had to let much of the air out of his suit via a release valve just to be able to get through the hatch. I'd hate to find out what the inside of his suit smelled like after that experience...
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Thursday 14th January 2016 13:52 GMT Vic
"I have heard that breathing oxygen mixed with an inert gas like helium at normal pressure can also be used to ward off the bends, as the helium displaces the nitrogen. I'm not sure how accurate this is though."
Perfectly accurate
Not at all accurate.
Helium is used in diving because nitrogen at high partial pressures is narcotic - you end up acting like you are drunk. This is obviously not a good thing for work at depth.
Helium is essentially non-narcotic, so you can tolerate high ppHe with no loss of thinking capability[1]. But as it is a smaller molecule than N2, it transfers into the body tissues rather more rapidly, and as such, constitutes a much higher risk in terms of decompression sickness. A small whiff of helium means your ascent schedule is quite a bit longer...
Breathing normobaric heliox before a dive simply loads the body with helium - so the total inert gas load goes up rapidly as you get onto the dive. This is a Bad Thing(tm).
Vic.
[1] It does, however lead to something called "high pressure neurological syndrome", which can mean involuntary twitching and the like. Deep divers usually add a little nitrogen to the mix, which seems to counteract HPNS completely.
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Thursday 14th January 2016 13:58 GMT Vic
I thought pure oxygen was essentially poisonous?
No. Pure O2 is a very nice thing :-)
There are two major effects that exposure to O2 causes - the Paul Bert effect (acute O2 toxicity) and the Lorrain Smitth effect (pulmonary O2 toxicity). The Paul Bert effect has a number of symptoms, the most dangerous one being seizures. Although the seizures themselves aren't that much of a problem, they invariably leave you unable to get to safety. But the Paul Bert effect only happens at high partial pressures - normobaric O2 won't cause it, no matter how long the exposure.
The Lorrain Smith effect occurs at lower pressures, and will be caused by normobaric 100% O2. The symptoms involve a degradation of the lung surface, leading to fluid build-up in the lung. This can, of course, be fatal - but it takes a while for any of that to occur. Richard Pyle has a story of when he suffered form that after being on O2 for about three days.
TL;DR: Oxygen is perfectly safe to breathe as long as you control the pressure and duration of the exposure according to well-understood principles.
Vic.
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Wednesday 13th January 2016 18:36 GMT Camilla Smythe
Spare Parts...
The job should be straightforward, since the duff unit is held in place by just one bolt. Once it's swapped, the two Tims will "lay cables in advance of new docking ports and reinstall a valve that was removed for the relocation of the Leonardo module last year".
Presumably the new unit will be held in place with zero bolts and someone has just worked out what the left over valve was meant for whilst the spare bolt resulting from this EVA will be consigned to the 'bag of leftover bits' for the next game of 'Where the fuck did that come from?'.
.. maybe the 'duff unit' would not be 'duff' if they would have fixed it in place with the required two bolts rather than just one. I suppose the other one is in the 'bag of spare bits'.