deco out in space...

[Dr Deco]

Hello Readers:

Prebreathe Before EVA

The material on the MIT link (referenced by reefraff) seems a bit out of date. I have seen other web pages from MIT by Dava Newman, PhD, and Michael Barratt, MD, both of whom I have worked with and contain more contemporary material. The prebreathe system used today was initially developed by me. Its salient points are:

• Exercise is employed to shift the “slow tissues” to “fast” ones. This is accomplished by exercise and increasing the heart rate, activating the muscle pump, and local vasodilation (from carbon dioxide.
• Vigorous exercise was not employed for the prebreathe washout phase.
• Nuclei generation was reduced by not walking five hours prior to ground tests and during the depress itself.

Depress R-value

The depressurization step is indeed small because the individuals are saturated with nitrogen. The step is however bigger than a jump from under the sea for a saturated diver to the surface.

Dr Deco :doctor:

 

[MikeC]

To counter decompression sickness, the crewmembers, prior to EVA, must pre-breathe 100% oxygen.

I seem to remember reading about either U-2 or SR-71 pilots having to pre-breathe O2 as well to prevent the bends. I'm not sure what "mix" they used during flight.

 

[Dr Deco]

Hello MikeC:

It is correct that U-2 pilots must prebreathe oxygen prior to their high ascents. To my knowledge, the breathing mix in the plane is pure oxygen. However, there may be some cases where there is a use of an on-board oxygen generation system. When this is so, then there is some nitrogen or argon that is also in the breathing mix.

Dr Deco :doctor:

 

[ScubaJorgen]

ScubaJorgen...
why would they be fully saturated with N2 in the shuttle? They are breathing pure 02 to get rid of the N2, no?
The article doesn't mention if, nor do I think that, they breathe 100% O2 during the EVA, so that would automatically mean that N2 start building up (or at least - inside my head it does, which is a scarey thought, but would belong in another thread...), add to that the strenous activity, possibly some anxiety - I mean, outside the shuttle they have a pretty big place - wouldn't the N2 levels rise?

and as a comment on your extrapolation...but they don't breathe pure O2 for 6 hours... only 40 min albeit at a lower pressure...

Don't worry if my questions don't make sense or if I don't understand your answers.. physics isn't my cup of tea....

Saturation
According to the article, our Space wariors breathe at 101 kPa. I guess they just breathe air (Oxygen would be dangerous: oxtox would burn up the space wariors and the shuttle might burn up easily). Their bodies are saturated with N2.
Prebreathing only gets rid of part of the N2, mainly in the fastest tissues.
Decompressing the diver on EVA would only reduce the N2 further in slowest tissue and might increase N2 in fastest tissues that have been depleted during O2. For these fastest tissues the EVA compares to compression

6 hours
The article mentions 6 hours ox breathing is required to get the body in the right shape for EVA (which is to long indeed)

 

[DepartureDiver]

After 6 hours breathing pure oxygen, the N2 level in the 635 min tissue is:

Pt = ...
= 0.8 - 0.8 [1 - e^(-0.39)] = 0.26 bar

I just noticed you forgot to subtract the .26 from the 0.8. Everything is right, just the little oops. So the end result is .54 bar (a 32.5% reduction) After a full 635 minutes of O2 breathing, it would naturally be reduced one-half from 0.8 to 0.4 bar.

 

[ScubaDadMiami]

In the mid '90s, I attended a conference where an astronaut was speaking on this subject. I was looking for the materials from that conference but can't seem to locate them anymore. So I no longer can provide the references. Beware, I am working on recollection here.:jump3:

From what I recall, the standard procedure before spacewalks is to suit up and breathe oxygen for a given period, which I recall being 20 minutes.

One of the other interesting points of the lecture dealt with the differences between the Russian space suits versus the American design. Each is pressurized differently, resulting in differences in things like manual dexterity in gloves, etc.

This material may now be out of date.