A kind of strange Underwater Air Bubble question

[CS_SJ]

Hello,

Something I’ve wondered about is those amazing air bubbles that are sometimes found in underwater caves. In particular, I was wondering how the depth of the water does or does not affects them.

In my Figure 1 example, the water is 50 feet deep and air bubble in the cave has 3 feet of air from the top of the cave to the water, and the air pressure in the bubble is the same as sea level.

Well, what would happen if the same cave was 1000 feet deep? (see Figure 2.) Would the bubble still have 3 feet of air from the top of cave to the water? Or would the tremendous weight of the water make the bubble much smaller like just 3 inches tall or something? And what about the air pressure? Would the air be super compressed?

If this is the wrong place to ask this question, I apologize in advance, and if you could direct me where I could get an answer for this, I would appreciate it.

Thank you,

- SJ

 

[BvU]

Hello @CS_SJ ,

$\qquad$ !
​

There is a law of communicating vessels having to do with static fluid pressure. At 50 feet below sea level (the bottom arrow in the left picture) the pressure is sea level pressure + $\rho g h \ \approx$ 2.5 times sea level pressure (100000 + 1000 * 10 * 15). Going horizontally to the left the pressure is the same, so NOT 14.7 "psi" but two and a half times as much.

With 300 m depth the same calculation gives about 31 times atmospheric pressure !

$\$

 

[BillTre]

The air pressure in a bubble 50 feet down below the water's surface will be greater than air pressure at sea level. If it weren't, the water pressure would push in the bubble surface (making a smaller volume) until the pressures are equal.
The water pressure at that depth will add to the surface air pressure to give you the pressure in the bubble at depth.

At greater depth, the bubble would be reduced in size, proportional to the amount of pressure the water is putting on the bubble.
The bubble size will be reduced until the air pressure in the bubble equals the water pressure.

However, gasses can dissolve in water. This can change the size of the bubble. Some gasses (CO₂) dissolve in water quite readily. This could affect how much bubble is left, and for how long, in some of these conditions.
It might be that, at really high pressures (which 1,000 feet of water seems like to me), the gas may just very quickly dissolve (or be forced) into the water.

For similar reasons, this is why, when SCUBA diving, your air lasts longer closer to the surface. Less pressure, fewer air molecules per breath (per volume).

Here's another example: a bubble released at some great depth, will increase in volume as it raises in the water column. It might not be easy to see this, because big bubble raising in a column water to tends to break up into many smaller bubbles.
However, if you invert a calibrated cylinder, trapping some air in it, and move it down deeper into the water column, the volume of air in the cylinder will decrease as the water pressure goes up. You could easily measure that.

 

[DaveC426913]

Well, what would happen if the same cave was 1000 feet deep? (see Figure 2.) Would the bubble still have 3 feet of air from the top of cave to the water? Or would the tremendous weight of the water make the bubble much smaller like just 3 inches tall or something? And what about the air pressure? Would the air be super compressed?

That air bubble at 1000 feet will be at 30 atmospheres. If you captured a bit of it in a bag and brought it to the surface, it would expand by thirty times.

Yes, you could swim into that cave and breathe that air, but it still acts like the compressed in your tank. You must observe proper ascent techniques to avoid the bends or worse.

 

[CS_SJ]

Thank you for your reply!

 

[CS_SJ]

Hello @CS_SJ ,

$\qquad$ !
​

There is a law of communicating vessels having to do with static fluid pressure. At 50 feet below sea level (the bottom arrow in the left picture) the pressure is sea level pressure + $\rho g h \ \approx$ 2.5 times sea level pressure (100000 + 1000 * 10 * 15). Going horizontally to the left the pressure is the same, so NOT 14.7 "psi" but two and a half times as much.

With 300 m depth the same calculation gives about 31 times atmospheric pressure !

$\$

Thank you!

 

[DaveE]

Been there, done that while diving. When the waves (longer period ocean swells) pass over head you can easily feel the air pressure change in your ears. Water is quite heavy compared to air. In one cave you could see fog form in the air pocket whenever the pressure dropped.