Sub Pressurization & Decompression Sickness, Serious Replies only please.

Originally posted by defcon5


I am not sure that is correct, because I believe that the first submarine which used compressed air killed its crew with Pressure Sickness. Unless it was not a completely sealed dry hull sub. Or does the volume of the sub have to equal exactly the same CF as the bottle of air? So for like a 80cf tank and 80cf sub?

Right. Putting 80cf of sea level air into a container of less than 80cf will result in greater than sea level pressure inside the container...Boyle's law.

I don't know about the case you mention but perhaps they pressurized the air because; a) they were unaware of the danger and b) by pressurizing the air inside, the hull would not need to be as strong.

(When will I learn to leave well enough alone? I knew my additional "information" would muddle things.)

[edit on 1/15/2009 by Phage]

reply to post by Phage


No… It makes sense.
It would really add to the complexity of making a full pressure hull though, as you obviously have to gauge exactly how much air is being consumed, how much is being added, plus how much of each gas is present, constantly. I can see why military subs have stations that constantly sample the air.

defcon,

I don't know if I can make this simple. I think it was stated by an earlier post...a vessel that is sufficiently strong to withstand the external pressures will likely NOT invoke those pressures on the environment within.

We dive, and realize that our bodies undergo a change, due to the pressures. BUT, safely inside a vessel, there is no change, up to a point where (and I don't know where this is, but I think it's deeper than you're planning) things may change. (This is where the H2 is needed....and that may just be a way to prevent the 'bends', by replacing the N in the bloodstream, and shortening the decompression cycle....just something I think I saw once on the Science Channel).

Again, best.

Tim

I'm open water certified, so here goes.

A submarine that maintains 1 atm inside simply has to be made strong enough to resist being crushed by the water pressure outside it. if you plan on going down 10 meters, you'll need it to resist 14 pounds per square inch of inward pressure. This is pretty much linear all the way down, with every 10 meters or so adding on another 15 pounds per square inch of pressure.

Pure oxygen is toxic at high pressures. Don't even think about using it beyond about 30 feet down. Pressurized air is better, but the nitrogen in it will become dissolved in your blood over time, so deep and or extended dives will require decompression stops. Helium and oxygen mixes are better, because helium comes out of solution in blood faster, doesn't dissolve in tissue as fast, and has far less of a narcotic effect than high pressure nitrogen. Trimix is a common blend used for technical diving, and it's got oxygen, helium, and nitrogen.

Here's the dive table I use to determine dive duration, and how long to stay above the surface before diving again.

External source: PADI Dive table

In a spaceship, or, to a lesser degree, the hull is there to keep the air inside from bursting it and escaping, and is pushed on only by the air inside. In a submarine, the hull is there to keep the pressure of the water from crushing it and entering. it is pushed on from the outside. In both cases, the best shapes are spheres and cigar shapes. Submarines tend to take this more literally than aircraft and spacecraft, because spacecraft at most have 1 atm to hold in, while deep sea submersibles may need to hold out a dozen atm of pressure. Usually, a spacecraft will only be pressurized to 5 psi of pure oxygen, making it require a hull only as strong as a submarine intended to dive to 10 feet down.

Originally posted by defcon5
reply to post by Phage

 

What you are saying makes sense, yet there must be something more to it yet. My tank is also rigid (I have used steel and aluminum tanks), yet my air decreases with each BAR that I descend. There is actually a chart that they use in diving, which shows that 33 feet = 2Bar = 2x density = ½ volume, 66 feet = 3BAR = 3x Density = 1/3 volume, etc.

Unless that all simply happens between the first stage regulator and my lungs, I don’t understand why it would happen at all. Besides this I have actually gone down to 100 feet, and watched the PSI in my tank drop on my guages coming right from the high pressure port on my first stage regulator (which bypasses all the other hoses and such), then increase again on ascent.

So for example: I have 3000PSI at the surface, at 99 feet I have around 1000PSI (of course that is off some cause I would have breathed some of it). Then if I return right back to the surface I would be back up near 1500-2000 PSI (of course it would be lower because I consumed more when the air was denser).

See what I mean?

You're wrong. That doesn't happen. I've never seen anything like what you're talking about. You get less use out of your tank because further you go down, the more air is in each breath you take. The air, if you weren't breathing from it, would remain at a constant pressure through the whole trip. Your consumption of the air increases greatly at depth; you use it up twice as fast at 30 feet as you do at the surface, three times as fast at 60.

once the air is outside the tank, it's at the same pressure as the water around it. If it were any less, it would be instantly crushed down until it was. The regulator lets out gas at ambient pressure.

If you were in a sub at 1 atm, you'd use it at the same rate as you would on the surface.

I have to wonder if, at depth, could you hold your breath for much longer, due to the increased oxygen content in each breath? or would the buildup of CO2 cause a need to exhale? I never tried because I'm trained to always maintain a steady breathing rate to prevent lung overexpansion.

Didn't your instructor cover all this during training? we spent a few days on the science of it all.

Anyway, I'd suggest a diving bell type contraption over a real pressure controlled submarine, if you plan on building it. It would be so much easier.

mdiican seems to have hit upon some of the highlights, the differences between being IN a vessel that protects the inhabitants in two different ways....ONE way is when the vessel is pressurized to provide breathable AIR, because the exterior is less-than-breathable.

THE OTHER extreme is a vessel to protect the inhabitants from being crushed from the incredible under-water pressures.

TWO extremes, two different solutions.....

Originally posted by mdiinican
I have to wonder if, at depth, could you hold your breath for much longer, due to the increased oxygen content in each breath? or would the buildup of CO2 cause a need to exhale? I never tried because I'm trained to always maintain a steady breathing rate to prevent lung overexpansion.

I think a lot of people are getting this part wrong, at least sort of. Yes they are right that a normal breath at 10m uses twice as much air out of your tank as at the surface. So therefore you're using the air twice as fast at 10m down right?

Well the assumptions in that are that you're breathing at the same rate (say number of breaths per minute). And I suppose some people do breathe at the same rate they do at the surface, perhaps out of force of habit, and in that case perhaps they are using air at twice the rate as claimed. But this really doesn't follow from the physics. When you take a breath at 10 meters down, yes it's got the same volume but twice the mass as at the surface, but that also means it has twice the oxygen. Now the lungs ability to absorb oxygen isn't 100% efficient, so you probably do use air at a higher rate than at the surface, but the only reason you'd use it at twice the rate is if you didn't slow down your breathing. Personally I find I can slow my rate of breathing down under water dramatically, and still have plenty of oxygen. I think the slower breathing gives all those extra oxygen molecules in each breath more time to come in contact with the lung surface. But then I may be the exception who breathes more slowly underwater, because when I dive with a group of people all at the same depth, I'm often the last one to hit 500psi, so I don't think other people are slowing their breathing down as much as I am. But I suspect this is from a force of habit in breathing rhythm and not due to oxygen requirements.



[edit on 12-2-2010 by Arbitrageur]