Sub Pressurization & Decompression Sickness, Serious Replies only please.

I intend this to be a serious thread for obvious reason, and I am seeking REAL information, not rumor, theory, or information based on science fiction, the reason should be apparent quickly....

I need some help in tracking down some information, and I feel this is the place to find that information. Here in the Military forum, I believe that we have people who have actually worked with similar equipment, in military service, and maybe I can finally find my answer. I have tried PADI, NAUI, and the guys at several SCUBA Shops, and none of them seem to be able to answer this question adequately. I have searched the Internet, and still nothing specific enough in my opinion. I am certified to dive up to advanced level, Nitrox, and deep diving.

I am thinking of making personal sub, just for the fun of it, and there are lots of plans for doing so on the Internet. All the ones I have found so far, work in a similar manner to regular scuba gear, working off of compressed tanks to supply air. Obviously you would run on your normal Dive computer to avoid decompression sickness, CO2 is blown off, and you make safety stops to shed excess nitrogen as you would on a normal deep dive. In a rig like this, with a canopy and an air pocket, wouldn't you get a false reading on your dive computer due to the rig taking pressure beyond what your body actually is? If I rig a dive computer to the inside of the cockpit of the sub, then exit on my own dive computer, I would have two computers to justify to each other causing a nightmare of paperwork on the multi-level dive tables between dives. On top of this the safety alarms on either unit would never have all the information to be trusted. So that is my first problem.

To my understanding, when you dive the air in the tank condenses, thus with every breath you are taking in extra nitrogen in accordance with how many BAR you have descended. Therefore, you have to do safety stops along the way to shed that extra nitrogen. Now in my aviation experience I know that aircraft, which pressurize, do so by adding extra air into the fuselage in order to maintain 1BAR even though they ascend to much lower pressures, which causes air to expand. Modern Subs also pressurize to maintain 1BAR, but how do they do it? Do they take air out of the Fuselage to maintain pressure, then add it again as they ascend? It seems to me that no matter the volume of air in the fuselage, it should still compress? For example, how is a sub half full of air any different then if I blew out half the air in my tank on a dive?


How about surface fed dive bells, or hookah diving? Do you run on your normal computer or tables in those situations as well? I have been in surface fed bells a couple of times, but I really don't understand how you are supposed to calculate those into your dive profile, and have yet to hear any explanation as to how your supposed to. If the air is surface fed, but through a compressor and tube, is it subjected to the same level of compression as it descends into the Bell, sub, or regulator?

Many folks died in the first submarine experiments, and even in the first deep helmet dives, due to not understanding this exact situation. You would think that this would be easy information to find, yet I am having a heck of a time tracking it down for some reason. Any help would be fantastic.

The Merck Manual of Diagnosis and Therapy (16th or 17th edition) has a couple of pages (packed with technical info) on "Depth and Pressure" that should get you started. You really need to know and understand first some basics about gases, i.e. the ideal gas law, PV=nRT, especially Boyle's law which tells you how much gas will compress under pressure at constant temperature, and second what the effects of the various gases (such as bends, nitrogen narcosis, and pulmonary oxygen toxicity) will be at the partial pressures you will be experiencing.

I am sure you can find a lot of the basic physics on Wikipedia, too. You also need a good understanding of the math behind partial pressures and to be able to figure out in your head beforehand everything your dive computer is going to tell you.

In an open bell, the pressure in the entire airspace is equivalent to the depth of the level the water is pushed down to. If your sub is going to support any of that pressure it needs to be really, really strong --- it's about a ton per square foot to support the pressure of a depth of 30 feet.

Modern Subs also pressurize to maintain 1BAR, but how do they do it? Do they take air out of the Fuselage to maintain pressure, then add it again as they ascend? It seems to me that no matter the volume of air in the fuselage, it should still compress?

I think you may be making this more complicated than it needs to be. The only reason SCUBA gear needs to deliver pressurized air to you is to keep your lungs from collapsing due to water pressure. If you simply build the hull of your submarine sturdy enough to withstand the pressure, you should be able to forego any need to pressurize the cabin, as well eliminate as any need to slowly surface to give your body time to eliminate the nitrogen.

thus with every breath you are taking in extra nitrogen in accordance
with how many BAR you have descended.

Yes, but the nitrogen is just to prevent oxygen poisoning from breathing pressurized air. If you don't pressurize the air, there's no need to add nitrogen.

It seems to me that no matter the volume of air in the fuselage,
it should still compress?

...one of the two of us doesn't understand your question. The volume of air in the cabin will remain constant, whether or not it is "pressurized." If, however, you pump more air into the cabin, you will be "pressurizing" it, even though its volume remains constant. But this should be pretty much irrelevant for what you want to do. Like you yourself point out, modern submarines maintain one atmosphere of pressure. Air pressure in a submarine at the bottom of the ocean is the same as the air pressure in a submarine sitting on the surface. The cabin is not artifically pressurized. They simply build the hull out of a solid piece of metal that can withstand the greater water pressure around it. Whatever air pressure is on the surface, they simply close the hatch and submerge, maintaining that same internal air pressure at all depths.

For example, how is a sub half full of air any different then if
I blew out half the air in my tank on a dive?

...again, one of the two of us doesn't understand your question. How can a sub be "half full of air?" As to your divetank, when you "blow out half the air" the air pressure in your tank will be half, even though the "volume" of the tank remains the same.

There may be some confusion here simply because US dive shops tend to use cubic feet to describe air pressure, which is somewhat misleading.

lots of plans for doing so on the Internet. All the ones I have found so far,
work in a similar manner to regular scuba gear

...if the plans you're working from involve a pressurized cabin...then what I've said above would not apply...but if so, I would expect them to also include the information you need. If they don't, well...you might consider not using those plans.

Nitrogen bubbles coming out of your body are not fun.


[edit on 13-1-2009 by LordBucket]

[edit on 13-1-2009 by LordBucket]

Thank you for the reply.

Originally posted by LordBucket
I think you may be making this more complicated than it needs to be. The only reason SCUBA gear needs to deliver pressurized air to you is to keep your lungs from collapsing due to water pressure. If you simply build the hull of your submarine sturdy enough to withstand the pressure, you should be able to forego any need to pressurize the cabin, as well eliminate as any need to slowly surface to give your body time to eliminate the nitrogen.

These types of subs are more like a dive bell with a bottom that is either open to the water, or a cockpit that is partially filled with water. They are all going to be strong enough to go to moderate depths, without needing air pressure to support structure. I figured that as long as I was looking into the subject I would like to understand how the pressurized ones work as well. I have heard where some folks have actually made pressurized ones out of old jet fighters, which is neat, but heavy, expensive, and hard to maintain due to saltwater exposure.

You are correct that if you made a bell that ran solely off surface air in a pocket that you took down with you, it would eliminate the problems with pressurized air, but you create new ones. CO2 retention, and limited air supply being the first two that come to mind. As you do not have cycling air in that situation, you would slowly poison yourself to death with each exhaled breath.

Originally posted by LordBucket
Yes, but the nitrogen is just to prevent oxygen poisoning from breathing pressurized air. If you don't pressurize the air, there's no need to add nitrogen.

That is not the way it was explained to me. From my understanding at 2 BAR your air decreases by half (becomes twice as dense), each breath using twice as much air as at the surface, and your air supply lasts half as long. At each BAR after 2, everything decreases by 1/10th, and continues to do so the deeper you go. The tanks are filled with normal compressed air, they are not adding extra nitrogen. The reason for the nitrogen build up is that you are breathing twice (or more) the density of air with each breath. Your body cannot naturally remove that amount of Nitrogen, so it stores it in body tissue and releases it over time during and after ascent. Changing BAR too rapidly causes the nitrogen to fizz out into the blood stream causing the Bends. You only have to worry about O2 toxicity if you are running enriched O2 mix (Nitrox). When you want to go deep for extended periods, then you can displace some of the nitrogen with helium, which is called Tri-Mix. Professional divers displace nitrogen 100% with Helium and use a Helium/Oxygen mix called Helox, which just so happens to be what was used on the first rescue submarine Squalus…

Originally posted by LordBucket
...one of the two of us doesn't understand your question. The volume of air in the cabin will remain constant, whether or not it is "pressurized."

Unless I do not understand what you mean by “volume”, this is not true. If I take a cup, flip it over and take an air pocket down with me to 33 feet, the cup (which was full of air at the surface) will only be half full at 2 BAR. If I return to the surface with the cup, when I get to the top, the cup will again be full of air. If I took the cup back to the bottom, then when I get there I add more air off my compressed tank, and resurface again, extra air will spill out from the cup on ascent.

Similarly, if you have ever seen them launch a high altitude balloon, you will notice that they only half fill it at ground level. Once it gets to high altitude the balloon becomes full due to air expansion. As a matter of fact, air expansion is the main reason for it being colder at high altitude.

In a pressurized cabin that does not happen for some reason that remains a mystery to me. This is really the main question, because an aircraft, a sub, and the tank I wear on my back should all react the same way IMHO to pressure changes, yet they do not. It makes sense to me that you would add air to and aircraft, increasing PSI, to maintain 1 BAR at high altitude/Low pressure, but for some reason I cannot think of how they do it when you go into Higher pressure. I guess they must remove air, but it would seem to me that the air which remains would still be denser then it should be. That still does not make sense to me because if it were true, then I should be able to decrease the PSI in my tank by half at 2 BAR, and never have to worry about the bends, yet that is not the case.

Originally posted by LordBucket
Air pressure in a submarine at the bottom of the ocean is the same as the air pressure in a submarine sitting on the surface. The cabin is not artifically pressurized. They simply build the hull out of a solid piece of metal that can withstand the greater water pressure around it. Whatever air pressure is on the surface, they simply close the hatch and submerge, maintaining that same internal air pressure at all depths.

There most be something else to this though, because as I stated, when you dive the air becomes denser and takes up less volume. With each breath you are breathing in denser air, thus extra oxygen and nitrogen. A sub has to replace its air with something, modern military subs can operate for months without surfacing. Yet after all that down time, a modern sub can make an emergency surface, without harming its occupants or giving them the bends.

Originally posted by LordBucket
...again, one of the two of us doesn't understand your question. How can a sub be "half full of air?" As to your divetank, when you "blow out half the air" the air pressure in your tank will be half, even though the "volume" of the tank remains the same.

Half the PSI as normal.

Originally posted by LordBucket
There may be some confusion here simply because US dive shops tend to use cubic feet to describe air pressure, which is somewhat misleading.

Could be, I have noticed that they sometimes seem to explain things in ways that are easy to understand and remember, but may not be the actual scientific reason.

Originally posted by LordBucket
...if the plans you're working from involve a pressurized cabin...then what I've said above would not apply...but if so, I would expect them to also include the information you need. If they don't, well...you might consider not using those plans.

I have seen sets of unpressurized ones, though I have heard of making pressurized kits out of old aircraft fuselages. The whole thing just got me thinking about how it works, and why certain things work the way that they do. Some of it certainly does not seem to make sense, but some of that might just be the way that they standardize and simplify things for safety reasons.

reply to post by defcon5


Hi, defcon.

Quite an undertaking you're trying to accomplish, there!

Coming from a layman's view (I'm only a sport diver, no NITROX!!) I believe that UP TO a certain depth, depending on the structure of the vessel, a regular 14.7PSI N/O2 blend can provide a normal 'shirt-sleeve' environment...but I suppose that assumes a self-contained air supply.

I've seen some past documentaries about very DEEP diving vessels that use an O2/H2 mixture...possibly because at extreme depths the water exerts so much pressure that there is a danger of Nitrogen forming.

( I think even in WWII subs could dive to 300-400 feet, and re-surface, with no harm to the crew... ) Of course, modern cababilites would be secret.

From an opposite standpoint, the early Space Program decided on a 100% O2 approach, at about 3-4 PSI (with CO2 scrubbers) in order to save weight in the pressurized portions of the vehicles. PLUS, it works well in the EVA suits, as well...easier to make the joints bend. The Shuttle Crew Compartment is pressurized to Sea Level (14.7), so any EVA takes time for breathing of pure O2, and purging N, whilst decreasing the pressure, because even modern EVA suits operate much the same as forty years ago...although with better materials.

I'm trying to recall the physics of a vessel under water pressure, and whether ANY of the pressure will affect the gases contained in the vessel...a rigid vessel should not convey the exterior pressure, unless under extreme conditions, I believe.

Perhaps a real physicist could help?

A pressure hull, in an aircraft or a submarine is holds a fixed volume because it is for all intents and purposes rigid. In aircraft, the cabin pressure is maintained at about the equivalent of 5000' (as I recall). When the plane ascends beyond that altitude the skin of the plane keeps the gas from expanding as it would in a balloon. Only by the air expanding within will the pressure in the aircraft decrease.

In a submarine the same process happens in reverse. The pressure hull prevents the volume of air from decreasing because of its rigidity. Because the volume of air does not decrease the pressure does not change. The pressure within the sub is maintained at about 1atm no matter the depth. CO2 scrubbers and supplemental O2 are provided to keep the air breathable. Since the interior remains at 1atm, there is no decompression when the sub ascends.

reply to post by Phage


Yep, it all depends on the pressures you tend to run the sub. How deep are you going to go and how much pressure can the sub take. If you say you are going to 100 feet, but inside the sub it was 10 feet of pressure there is no issue.

Just like an airplane. If you are flying at 35,000 feet and the cabin is 2000 more or less, then your body is at 2000. In a sub if it was 100 feet, but inside it was at 10 feet then your body is still at only 10 feet of pressure.

Originally posted by weedwhacker
Quite an undertaking you're trying to accomplish, there!

I don’t know if I am going to build one or not, I was simply looking at them and trying to figure out how a pressurized one works as opposed to a non-pressurized one.

This started out, simply enough, with looking at building a DPV (Divers Tug), but then I came across all kinds of interesting stuff:
Scuba Tow
Delta Wing
Silent Runner
SportsSub2
Deep Flight
Extream Sub
It seems like there are a pretty fair number of companies that either make subs, or provide the plans for making them.

Originally posted by weedwhacker
Coming from a layman's view (I'm only a sport diver, no NITROX!!) I believe that UP TO a certain depth, depending on the structure of the vessel, a regular 14.7PSI N/O2 blend can provide a normal 'shirt-sleeve' environment...but I suppose that assumes a self-contained air supply.

I am not too worried about the gas mixture ATM, more so about how they retain 1BAR in a higher pressure lower altitude environment.

Originally posted by weedwhacker
From an opposite standpoint, the early Space Program decided on a 100% O2 approach,

You know I always wondered about that as well. In the medical field we never place someone on pure O2, over 3 LPM, for fear of overriding their Hypoxic Drive, and making them stop breathing. How did they put those guys in pure O2 and not knock out the chemical reaction that makes you take a breath?

Originally posted by weedwhacker
a rigid vessel should not convey the exterior pressure.

See that is the bit that gets me. My SCUBA tank is a rigid vessel, yet my air decreases in volume according to the pressure that I am under. As you know, you only get ½ PSI at 33 feet, 1/3 at 66, ¼ at 99… etc… Yet as my tank is basically a rigid cylinder, same as a sub, why does it not get effected the same way?

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?

reply to post by defcon5


I think you need to take another look. The pressure gauge will always read true. When my gauge says 500psi it's time to wrap things up and ascend if I'm at 90 feet, or 75, feet, or 50 feet. When I get to the surface my gauge will read 500psi less the amount I consume on the way up (the deeper I am, the longer it will take and the less will remain when I reach the surface). The pressure inside the tank remains the same regardless of depth.

Of course, the deeper you dive, the faster you'll consume those 80cf of air because it is being fed to your lungs at higher pressure. At 33 feet you will consume twice the air that you do at the surface. At 33 feet that 80cf at surface pressure is now 40cf.
Boyle's law: PV=k, with k being constant. Volume is directly proportional to pressure.

reply to post by Phage


You could be right. It been a few months since I have been in, maybe I am just remembering it wrong. I use a Oceanic Pro-plus, and I tend to watch the minutes remaining more so then the actual PSI.

At 33 feet that 80cf at surface pressure is now 40cf

Then shouldn’t the PSI drop by half accordingly?

So are you saying that the PSI is not changing, it’s the expansion or compression of the air as it goes through the first stage reg yet it remains constant in the tank? Increased flow maybe?

Hi defcon.

Well it's lovely to see a diving related post on ATS, and from what I see it's a first! So thanks for bringing it up as I'm what you might call an experienced diver

You've got pretty confused at some point during your training, both on terminology and theory. I'll try and explain the scenario you talk about with your bottle in laymans terms (no offence intended)

Imagine you have a bottle in the boat and you open the valves letting all air out of the bottle until it stops hissing, you end up with a pressure in the bottle of one atm.
Now close the valve, the bottle is now essentially the same as a submarine pressure hull.

If you then dropped the bottle to the sea bed the pressure and volume of air inside the bottle would not change because the bottle structure stops this happening.
At some depth the external pressure acting on the empty bottle would eventually crush or implode the bottle, known as collapse or crush depth. This occurs when the structure of the bottle is no longer strong enough to resist the external pressure acting on it.
This is how a pressure hull works, this is how all large military submarines operate, with CO2 scrubbers and compressed oxygen stored within the pressure hull to maintain a breathable atmosphere. The pressure and air volume inside the hull is the same at sea level as it is at depth.

Your statements about your bottle pressure gauge reading different at the surface to what they are at depth are incorrect too. The bottle pressure and volume only change as you breath. You are correct that you will use your supply of air twice as quickly at 33ft than at 0, but this is nothing to do with external pressures acting on the bottle. It's because at 33ft, (or more precisely 10m) you are at two atmospheres, therefore you require twice the air pressure to inflate your lungs and allow you to breath.
The volume of air inside your bottle does not change therefore you effectively only have half the amount of air to breath due requiring twice the pressure to inflate your lungs.
If you were reading your computer and it said you had 60 mins of air at the surface, it would tell you that you have 30 mins at 10m.

In a diving bell you are not in a pressure hull, the air pressure inside the bell is governed by the depth you are at. A bell at the surface with no external air supply will be full of air. As you dive the volume of the air inside the bell will drop, so at 10m or two atmospheres the pressure inside the bell will be double what it was at the surface. This means that the volume of air inside the bell will half, and you'll be getting wet

Most modern bells/chambers are pressurised, as would your open bottomed sub be (same principle). So here, we have a bell at 10m, air is pumped at 2atm pressure into the bell and forces the water back down to the bottom of the bell. Keeping everyone nice and dry! However, you are still breathing air at pressure, and the same rules apply regarding decompression sickness, partial pressures etc as when you are diving on a bottle!

So the long and short of it is, if you want to dive without the complications of decompression, heliox etc then you need a pressure hull....a closed hull system with life support systems for breathing.
If you are happy with dealing with compression issues an open bottomed craft is fine. These vehicles are much easier and cheaper to build due to not needing an ultra strong hull capable of resisting 1000's of psi. But they do usually have a surface support vessel holding pumps etc.

Cheers

Robbie

Submarines are not balloons, nor are they like your lungs. In submarines, the hull takes the pressure load, not the air inside. Therefore, there is no need to change the pressure and amount of the air, it should be one bar anyway, as there is no additional pressure compressing the air.

The only reason SCUBA gear needs to deliver pressurized air to you is to keep your lungs from collapsing due to water pressure.

If the air was not pressurized it would be impossible to breath it in; similar to a really long snorkel.

See that is the bit that gets me. My SCUBA tank is a rigid vessel, yet my air decreases in volume according to the pressure that I am under. As you know, you only get ½ PSI at 33 feet, 1/3 at 66, ¼ at 99… etc… Yet as my tank is basically a rigid cylinder, same as a sub, why does it not get effected the same way?

Because your lungs are not rigid. They are under pressure. Thus at five atmospheres of pressure, you'll need to take out five times the amount of air as you would on the surface, to fully inflate your lungs.

[edit on 14/1/2009 by C0bzz]

Originally posted by defcon5
reply to post by Phage

 

At 33 feet that 80cf at surface pressure is now 40cf

Then shouldn’t the PSI drop by half accordingly?

So are you saying that the PSI is not changing, it’s the expansion or compression of the air as it goes through the first stage reg yet it remains constant in the tank? Increased flow maybe?

No, I worded that badly (probably should have left it out entirely). If you fill a balloon from an aluminum tank at sea level it would fill a volume of 80cf. If you did the same 33ft below the surface it would fill a volume of 40cf because it is under twice the pressure. In the balloon, the internal pressure equalizes with the external (the skin of the balloon will cause a bit more internal pressure).

One more (I hope this doesn't confuse things more); if you have an empty (a vacuum) submarine which had a volume of 80cf, take it to any depth and release the tank into it, the internal pressure will be 1atm.

[edit on 1/14/2009 by Phage]

reply to post by Phage


Phage, my friend....you're probably helping defcon more than me, but just a clarification for you, and for everybody, about AIRPLANE pressurization:

(This is the opposite of what defcon is asking, but it may help...)

In modern pressurized air transportation the point is to keep the cabin 'altitude' below 10,000' ASL. THIS is the 'magic' number in aviation that defines whether supplemental O2 is required. (For pilots, one can exceed 10,000', for no more than 30 minutes.)

On a typical Boeing the PSID limit is about 8.5 --- meaning, the PSI differential --- so at, say 39,000' cruising altitude the cabin will be at about 8,700' --- airliner pressurization systems and parameters vary according to airplane....but suffice to say, when an airplane has a 'Maximum Altitude' Limitation, it is either due to engine/weight performance, or cabin pressurization restrictions.

BUT, all of this preceeding is the opposite, as noted before, of diving into an environment that is, in some ways, more deadly than high altitudes and thin air.

A modern airliner is designed ONLY to be positively pressurized. In fact, there exist negative pressure relief valves built in to ensure that the interior pressure is never less than the exterior...(That's why I laugh at one of those "Airport" movies, with the B747 at the bottom of the ocean...but, I digress...)

The Human body can adapt, fairly easily, to these minor pressure changes. Since water is far more dense, and hence heavier, as we know the situation is very different.

defcon, depending on how deep you wish your submersible to go, and as long as you had a self-contained air supply, it seems to me that since there is no surrounding pressure, from the depth of the water, acting directly on your body, that up to a point it is only a matter of the structural integrity of your vessel.

Again, my basic understanding of physics tells me that a rigid container will NOT be affected by the surrounding pressure...one person used the ballon example, and of COURSE a balloon is not rigid.

I remember seeing another example of a styrofoam object being taken down, and showing how it was compressed....but, of course, styrofoam is mostly air, so of course! It would be compressed...just as human tissue would be....

Hope this helps!!

reply to post by stratsys-sws


Thank you, stratsys...

I jumped in by request, and used my (not so) extensive knowledge of diving, but extrapolated my more extensive knowledge of aviation, in order to help (with a pinch of very old, rusty, physics knowledge).

Great post!

edit, for 'Robbie'....you've quite well vindicated my original 'layman's opinion regarding a pressure vessel, self-contained, undersea. In fact, you went one further by mentioning the CO2 scrubbers. It fairly well is similar to a spacecraft...whether we're talking Apollo and the 4 PSI cabin, or the Shuttle and a 14.7PSI cabin, CO2 scrubbers are always necessary...

[edit on 1/14/0909 by weedwhacker]

reply to post by stratsys-sws


Nice post!

That really explains it well in layman’s terms, and made me think about it from a whole different angle. Just to make sure I have this understood correctly, let me throw this out and make sure its right…

The air in the tank, is already under pressure (3000PSI), and if opened at the surface it would expand to fill a volume of 80cf. At depth, where you are under pressure, the air does not expand all the way to the 80cf that it would at 1BAR, and if it were opened to a room at 2BAR would only fill it to 40cf. In other words its not that the air in the tank compresses at depth, because its already compressed in the tank, its that it does not fully expand. Kind of the exact opposite of how I understood it in class.

I think that in simplifying things in class, they just never really explained it that well. It was more like, “here are the rules, and a simple explanation of why they work, now follow them”… You know they get into all the examples showing what happens if you take a plastic jug of water under pressure, how the air compresses, and I assumed that it was the same thing happening inside the tank.

So, the same would hold true for a surface air compressor breathing through a line then? You still have to dive your computer/tables, and all the same rules apply then.

Originally posted by Phage
No, I worded that badly (probably should have left it out entirely). If you fill a balloon from an aluminum tank at sea level it would fill a volume of 80cf. If you did the same 33ft below the surface it would fill a volume of 40cf because it is under twice the pressure. In the balloon, the internal pressure equalizes with the external (the skin of the balloon will cause a bit more internal pressure).

I get it now....
Thanks

Originally posted by Phage
One more (I hope this doesn't confuse things more); if you have an empty (a vacuum) submarine which had a volume of 80cf, take it to any depth and release the tank into it, the internal pressure will be 1atm.

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?

reply to post by C0bzz


Good points.
Thank you

reply to post by weedwhacker


Good points about the aircraft.
Though as I stated above, I have heard of making a sub out of a fighter fuselage. I guess they must weld over the pressure release areas you mention.

I think that the reason for the Human body being able to take it so well, is because its 80% water, which only compresses so far and is basically a rigid structure in that sense. That is why you only have to worry about your internal airspaces when diving (lungs, sinus, ears, etc).