Question about pressure, submarines and deco.

[Buccaneer]

Here is my question:
Why is it that people in submarines do not need decompression? Now I realize the easy answer is because the sub maintains 1 ATA at all times, but how? I don't know much about subs, but to me it seems like it would be just like any other canister underwater, and that the air inside would be subject to pressure changes. Especially when you consider the subs of WWI and WWII when not that much was known about decompression tables. I thought about an airplane that is pressurized, but you still experience some degree of pressure change, and that doesn't even compare to the 20, 30 ATA and greater a sub is exposed to.

 

[Otter]

The short answer is that because subs are RIGID, it is possible to maintain 1 ATA inside at all times. The pressure exerted on the hull is not transferred to the air inside unlike a flexible container (like our bodies).

Realistically, there is probably some flex in the sub walls that slightly increases the air pressure inside, but its got to be negligible.

As for airplanes, they are pressurized to 8,000 feet or so, so they are less than 1 ATA. I am guessing they could pressurize it to 1 ATA but with less air pressure outside, the fuselage would expand outwards and the constant expansion and contraction would likely cause increased metal fatigue....not unlike what the Aloha (or Hawaiian) 737 experienced a few years ago where the fuselage basically 'burst'.

 

[Buccaneer]

The short answer is that because subs are RIGID, it is possible to maintain 1 ATA inside at all times. The pressure exerted on the hull is not transferred to the air inside unlike a flexible container (like our bodies).

Well that was my thinking. But Scuba tanks are rigid too. The tanks themselves don't compress enough to actually compress the air inside. And what about all those tunnel workers along time ago? They are in a dry environment, fully encased yet they still got the bends. That still does't quite seem like the answer to me, especially when you consider how deep a sub reallly gets. Admittedly i have no reason other than my own thinking to dispute your answer. I'm picturing say a mouse inside a sealed scuba tank w/ one ATA pressure inside. Take him down to depth and the air inside is subject to the same pressure- I still don't get why it doesn't apply to a sub

 

[Albion]

The short answer is that because subs are RIGID, it is possible to maintain 1 ATA inside at all times. The pressure exerted on the hull is not transferred to the air inside unlike a flexible container (like our bodies).

Well that was my thinking. But Scuba tanks are rigid too. The tanks themselves don't compress enough to actually compress the air inside. And what about all those tunnel workers along time ago? They are in a dry environment, fully encased yet they still got the bends. That still does't quite seem like the answer to me, especially when you consider how deep a sub reallly gets. Admittedly i have no reason other than my own thinking to dispute your answer. I'm picturing say a mouse inside a sealed scuba tank w/ one ATA pressure inside. Take him down to depth and the air inside is subject to the same pressure- I still don't get why it doesn't apply to a sub

Dive master candidate and you dont understand this problem = failed.

The tunnel workers were in a pressurised tunnel. The tunnels were presurised to keep the water out, it would not be a dry environment if not for the pressure.

Scuba tanks are rigid but air is compressed into them by a compressor (hence name). mouse inside scub tank at 1ata take it down to 20 m still one ata unless you open the tank valve.

subs are huge and rigid there is no meaningfull pressure transfer between the inside and the outside as they descend. The have a lot of volume and any pressure increase is minimal

 

[paulwlee]

Albion explained it all very well.

I think you are a little confused with respect to SCUBA tanks.
The pressure inside the tank remains constant whether it's at the surface or at 200ft.

It's the regulator 1st stage that brings down the pressure to ambient + ~140psi, so the IP changes according to depth.

 

[Buccaneer]

Perhaps I am confused on the tank thing.. What i am thinking about is for example when you dive, the air inside your scuba tank becomes compressed in relation to the outside pressure. Forexample if you are at 40m or 5 ata the air in your tank is compressed to a density 5 times normal atmospheric pressure, that being the case am I wrong to assume that a rat inside is also experiencing 5 ata of pressure the same as outside the tank? Therefore if the tank was down past safe deco limits wouldn't the rat inside be past those limits as well?

If you are saying that is correct but because the sub is so large it doesn't apply then okay, i guess i get it. But then what about smaller one man subs?

 

[Buccaneer]

Dive master candidate and you dont understand this problem = failed.e.

Scuba tanks are rigid but air is compressed into them by a compressor (hence name). mouse inside scub tank at 1ata take it down to 20 m still one ata unless you open the tank valve.

Not correct.
While 2400psi is 2400psi at any depth, the air inside the tank is subject to the same ATA as outside the tank. That is why you run out of air 5 times faster at 100 ft than on the surface

 

[paulwlee]

Forexample if you are at 40m or 5 ata the air in your tank is compressed to a density 5 times normal atmospheric pressure,

No, the pressure in your tank stays the same as what you had before you took it there.

Say you take a AL80 tank that is full, at 3000psi. That is
about 200ata. You take that tank to 40m, it's still at 200ata.

Now as you breath down the tank, it will eventually get down close to 5ata, at which point it will get really difficult to get any air out of it as it's about the same as the ambient pressure. If you ascend to the surface, you will be able to get more air out of it, or 5-1=4ata worth.

If the air in your tank were subject to the same pressure as the ambient pressure, it will have the volume of a telephone booth at the surface. Think of a balloon the size of a telephone booth.
Since the tank is rigid, you can compress it into a smaller volume at high pressure, and when the air is in a rigid container, it is minimally affected by ambient pressure.

 

[paulwlee]

Not correct.
While 2400psi is 2400psi at any depth, the air inside the tank is subject to the same ATA as outside the tank. That is why you run out of air 5 times faster at 100 ft than on the surface

No, the reason you run out of air 5 times faster is that every breath you take takes 5 times more air molecules out of the tank as on the surface. (5 times the pressure at the same volume is ideally 5 times as many gas molecules.)

Since you are a DMC, you need to completely understand this before you go on. Reread the physics section (and especially Boyles Law) in your manuals.

 

[Buccaneer]

Okay.. had to dig out my books here. each ata is equal 14.7 psi so you are right in that the inside of the tank is about 204 ATA, so I conceed i was wrong in that it changes, but where i am not getting the connection is that the air inside obviously compresses at depth, because it becomes more dense in porportion to your depth. So if it is not increasing ATA that makes the air more dense (giving you less breathing time) what is it?

No, the pressure in your tank stays the same as what you had before you took it there.

Say you take a AL80 tank that is full, at 3000psi. That is
about 200ata. You take that tank to 40m, it's still at 200ata.

Now as you breath down the tank, it will eventually get down close to 5ata, at which point it will get really difficult to get any air out of it as it's about the same as the ambient pressure. If you ascend to the surface, you will be able to get more air out of it, or 5-1=4ata worth.

If the air in your tank were subject to the same pressure as the ambient pressure, it will have the volume of a telephone booth at the surface. Think of a baloon the size of a telephone booth.
Since the tank is rigid, you can compress it into a smaller volume at high pressure, and then the volume is minimally affected by ambient pressure.