Question about pressure, submarines and deco.

[RIDIVER501]

Subs have a rigid inner hull called the pressure hull. When sealed it is like a scuba tank...internal pressure stays the same despite external pressure changes.

Subs are better at maintaining thier atmospheric pressure then airplanes becuase they can use materials that are more rigid. this also means the materials weight more. you can only make an airplane so rigid to resist presure changes before the added weight of materails affect it's flight characteristics.

Since the sub is a rigid contain set for one ATM. the people in the sub don;t require deco.

 

[murphdivers286]

The pressure in the tank does not change, but the pressure exerted on the air coming out of the tank does change. For example, you fill a balloon up at the surface to a specific size. (think about the amount of air used from the tank to fill the balloon to that size.) Now, take another balloon down to 100 ft. and fill the balloon to the same size as the one on the surface. You will notice it took about 4 times the amount of air to fill the one balloon at 100 ft and it did for the balloon at the surface.

The ambient pressure at 100 ft. did not affect the air until it was no longer inside the tank. Once the air came outside of the tank, it was force to match the ambient pressure, which compressed the air molecules closer together.

Hope this helps?

 

[RIDIVER501]

That too...
Thanks for the assist murph

 

[Buccaneer]

Okay, Okay... I get it now. I see that the air in the tank must match ambient presure when it leaves the tank. I have already understand could explain most of what has been discussed. However, I was making one mistake, when it comes to the air inside your tank. Something my o/w instructor said has always stuck with me, and i believe that was where the confusion came from. When he gave the well known "take a balloon to depth" demonstration to show the effects of pressure, he also said "this is what happens to your air supply, and why you need more air at depth." I also remember him drawing the chart showing Xata = X depth =X atmospheres and he would draw a little scuda tank next to it, indicating how much air supply available at given depths. I understood the principals, but thought that it meant the air inside the tank was subject to the same effects. I don't know why it didn't dawn on me that your spg would change at depths if that were the case. Thanks for the patience and explanations

 

[RIDIVER501]

The air supply would not last as long at depth not becuase of the design of the tank but becuase of the volume of air you are taking from the tank to fill your lungs.

every breath you take at 99 feet/4atm = 4 breaths at the surface/1atm.

so you go through your air supply 4 time faster or your air supply will last you 1/4 as long as on the surface.

 

[fmansur]

The air supply would not last as long at depth not becuase of the design of the tank but becuase of the volume of air you are taking from the tank to fill your lungs.

every breath you take at 99 feet/4atm = 4 breaths at the surface/1atm.

so you go through your air supply 4 time faster or your air supply will last you 1/4 as long as on the surface.

Just to confirm, this happens because the water pressure over the body pressures our lungs and its volume reduces to 4 times less, correct?

 

[Spectre]

Just to confirm, this happens because the water pressure over the body pressures our lungs and its volume reduces to 4 times less, correct?

No.

You equalize your lungs [by breathing]. It's the same size they are on the surface.

The gas coming out of your second stage is reduced to ambient pressure, which is 4 times that of the surface. Since it's compressed 4 times, it takes 4 times as much gas to fill the same lung space as it does on the surface.

 

[lamont]

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

It is negligible in terms of the pressure change of the air inside and its deco effects on the submariners. It is not negligible in terms of the buoyancy effects it has on the sub, though, and unattended you can get a runaway buoyant ascent just like if you don't dump air out of your BC. This can be an issue in maintaining proper periscope depth.

 

[murphdivers286]

"Just to confirm, this happens because the water pressure over the body pressures our lungs and its volume reduces to 4 times less, correct?"

Here we go again!!!!!

 

[fmansur]

No.

You equalize your lungs [by breathing]. It's the same size they are on the surface.

The gas coming out of your second stage is reduced to ambient pressure, which is 4 times that of the surface. Since it's compressed 4 times, it takes 4 times as much gas to fill the same lung space as it does on the surface.

Ok, got it.