Question about pressure, submarines and deco.

[paulwlee]

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.

The air inside the tank does not compress further. It is already much more highly compressed. The regulator takes out that compressed air and makes it 'less compressed' so that you can breath it. So when you breath one breath at the surface, you are breathing in X amount of air. When you are at 40m, 5ata, you are breathing in 5*X amount of air because the air you breath in has to be at 5 ata for you to be able to breath in. But both of that amount of air comes out of the tank that is compressed to much higher pressure (unless you are almost out of air), and it is the regulator that brings down that high pressure to the pressure you are breathing.

So the pressure in the tank remains the same regardless of depth, it's the pressure of what you actually breath in, that has already been 'processed' to a lower pressure by your regulator, that changes depending on your depth.

Hope I'm not just confusing you further with my long-winded explanation..

 

[Albion]

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

I am going to have to take your rescue card off you as well as throw you out of dive master training

To make it easy will do all in Atmospheric not Guage.
For example you have 200BarA in your Scuba tank irrespective of Depth. Your First stage regulator breaks this down to an intermediate pressure (IP) i dont know exact numbers but for example lets assume 10BarA

When at 10M your ambient pressure is 2 BarA. The second stage regulator breaks down the pressure from 10BarA to 2BarA

If you were down at 30M the ambient pressure would be 4BarA
and the 2nd stage breaks it down to this pressure from 10 BarA.

The pressure in the tank never changes the IP never changes the only thing that change sis the pressure at which it comes out the second stage. And so it is the density (to use your term) in which it comes to you which matters.

Would suggest you re read rescue theory book to see how a regulator works before going any further this is basic stuff at that level.

 

[paulwlee]

the IP never changes

The IP changes. It is some set pressure (usually around 125~150psi, or 8.5~10bars) above ambient pressure.

Thus, in your example, if IP is set at 10 bars, at 30m the IP will be 13 bars. If it were constant, the regulator wouldn't work any more when you get down to 90m, where the ambient pressure is about 10 bars. As you know, this is not the case.

Now this is all the ideal case, as the IP varies depending on remaining pressure and a couple other factors. Some manufacturers actually design the first stage to increase the IP more as pressure increases, so as to counteract the increased breathing effort due to denser gas at depth.

 

[The Kraken]

Bucaneer wrote:
"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."

I'm not a DM candidate, but I do know that the air INSIDE your tank is not compressed as you go deeper. It is compressed when it comes OUTSIDE of the tank/regulator. If the the tank itself is not being compressed, which I'm sure you will agree that it isn't, how can the air inside become more compressed?

 

[miketsp]

I think the point Bucaneer is missing is the amount of air needed to fill the lungs at different ambient pressures.
Lets just say that a full breath is 4 litres so at the surface you need to take out of the tank 4 litres at 1ATA. Now at 30m you still need to take the same 4 litres but at 4ATA so at 30m you need to take 4 times more air out of the tank for each breath.
The air coming out of your 2nd stage at 30m is at 4ATA, which is why it's denser and harder to move around ie breathe.

 

[Spectre]

If the pressure in your tanks changed with depth; you would see a change in your pressure guage as you decended.

If you climbed -into- your tank, you would remain at 1 ATA.

You have to equalize your body's air spaces to the ambient pressure [eqalizing your mask, your ears] because the force from the pressure results in pain on your soft tissues. steel doesn't feel pain, so it doesn't need to equalized.

When you hear about 'crush depth', that is the depth where the object can not withstand the difference between ambient and internal pressure. For example, if you have a tank that has a 4500psi burst disk, You'll start to have problems once that tank gets down to about 11,000 feet.

Now if you go searching for "newt suit" or other related machines; you'll learn about actually diving inside your scuba tank

 

[miketsp]

Bucaneer wrote:
"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."

I'm not a DM candidate, but I do know that the air INSIDE your tank is not compressed as you go deeper. It is compressed when it comes OUTSIDE of the tank/regulator. If the the tank itself is not being compressed, which I'm sure you will agree that it isn't, how can the air inside become more compressed?

Strictly speaking it is decompressed to ambient pressure.
So obviously the more you can decompress it, the more useful volume you get.

 

[The Kraken]

Oops !!!! Screwed up, didn't I ? ? ?
Well, I was headed in the right direction anyway. Sorry I got the guy killed !!!

 

[redacted]

If you would like to demonstrate what is happening, you could perform an experiment. Take 2 small balloons and a sturdy jar with a well sealed lid. A canning jar or a mayonese jar should work. Inflate one balloon so it fits inside but almost fills the jar. Infalte the other balloon to the same size as the first. Take them down to about 30 ft and observe what happens. You probably should wear some good gloves just in case you exceed the "crush depth" of the jar. This demo will not work if the jar leaks.

While it may be tempting, the animal rights advocates would probably prefer you not include the mouse in the experiment.

 

[Soggy]

Dude. The air on the inside of your tank does not compress at depth. It's really simple. The tank is rigid, thus external pressure does not affect the contents of the tank.

If you stick a mouse inside a rigid container, assuming there is enough air in the container for it to survive any length of time, drop him down to the bottom of the Marianas Trench and bring him up, the mouse will be fine.

How else do you think we sent people down to the bottom of the ocean at 30+ thousand feet deep?