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]

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.

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 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?