How can air volume shrink as you go deeper, when the tank itself doesn't shrink?

[RXTdiver]

In the picture below, at 50 meters the gas volume is only 17% of what it was at the surface. At that depth, surely the tank itself isn't 17% of what it was at the surface. Although the tank is exposed to much greater pressure at 50 meters than at the surface, how does the air in the tank actually get compressed into such a small volume? The tank is sealed with no openings so it's not like water can get in there to compress the air that is in the tank.

Here's another question. If you were to visualize the arrangement of the air particles when the air is compressed at any given depth, what would that arrangement of air particles look like? Would all of the air be concentrated at the bottom of the tank (due to gravity), concentrated at the top of the tank, spread out evenly throughout the tank, something else?

 

[L13]

That graphic is for a non-rigid bubble of gas, such as in your BCD, lungs, or dry suit. The gas in your tank is unaffected.

The air molecules are evenly distributed.

 

[Jeepster]

A SCUBA regulator is designed to provide an air mixture at the ambient pressure of the current depth. The air in the tank is compressed at up to 3000 psi and the breathing gas exits the second stage of the regulator at ambient pressure. Molecules of the air mixture are decompressed to that pressure when they leave the second stage, so the volume of a lungful of air at depth contains more molecules than at the surface.

 

[RXTdiver]

That graphic is for a non-rigid bubble of gas, such as in your BCD, lungs, or dry suit. The gas in your tank is unaffected.

The air molecules are evenly distributed.

Well this really changes things. Since the air in the tank is an air space, I thought that the air in the tank would get compressed as you go down deeper.

 

[Jcp2]

There’s a reason why the Beatles used “yellow” instead of “tiny”.

 

[admikar]

Well this really changes things. Since the air in the tank is an air space, I thought that the air in the tank would get compressed as you go down deeper.

You are technically correct, but that graphic relates to air at atmospheric pressure.
You can't compress something until outside pressure is higher than your own.

 

[ofg-1]

Oops.

 

[ofg-1]

delete my computer is going nuts

 

[CT-Rich]

The relative pressure in the tank would change with depth, but it is less than 15 psi for every 33’/ 10m (less than 0.5%). Changes in temperature will cause a more notable change in tank pressure.

 

[Gone for diving]

Usable volume in the tank changes with depth.
For normal dives it's not not noticeable and irrelevant..

But I 800 ft deep a certain amount is locked in the tank unusable until you ascend....

That is why if you run out of air at depth, you will get a breath or 2 on the way up.