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

[OMyMyOHellYes]

To the OP's point - the amount (volume - and number of molecules of N2 and O2 (and all those pesky trace gases) of air in a closed cylinder will not change until you submerge the tank past it's pressure failure point. Then the air will be compressed into a tiny tiny tiny space. Then as it ascends that tiny tiny bubble will beet bigger bigger bigger.

But no, the air inside a tank does not get compressed into a smaller volume with depth. That stays the same.

 

[Angelo Farina]

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.

The air in the tank has a much higher pressure than ambient. You see the current pressure in the tank watching at your pressure gauge.
You should watch it very often: initially it will be above 200 bars, then it will slowly reduce as you are using some of air for breathing.
When you reach a pre-defined safety treshold (100 bars, usually) it is time to end the dive and ascend.
Most operators are unhappy if you have less than 50 bars left when back on the boat...
During a dive within recreational depth, the ambient pressure usually never exceeds 5 bars (40 m).
The regulator has the task of reducing the pressure from the high pressure inside the tank down to ambient pressure.
So you always breath air at the current pressure defined by your depth, wathever is the air pressure in your tank.

 

[RXTdiver]

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.

L13 said that the gas in your tank is unaffected. If that's the case, then how can the pressure in the tank change with depth?

 

[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.

I think you're reply caused a light bulb to go off in my head. Near the beginning of the PADI Open Water online course they were talking about how air supply and breathing as you go deeper. There is this:

1. "Your air supply lasts ½ as long at 10 metres/33 feet (2 bar/ata) than at the surface (1 bar/ata)."
"Your air supply lasts 1/3 as long at 20 metres/66 feet (3 bar/ata) than at the surface (1 bar/ata)"

2. "the air gets denser as you go deeper; denser air is harder to breathe than air at normal surface pressure. The deeper you are, the more
energy you use to breathe."

So now that I know that the air molecules in the tank are unaffected, how do you explain the statements above?

As you go deeper, your lung volume goes down. Because of this, you have to breathe more faster and more frequently in order to get the same amount of air you would breathe in at the surface. Because you are breathing more frequently, your air supply doesn't last as long the deeper you go. Do I have this right? Does this explain point #1 above?

Because you're breathing faster, this requires more energy. Does this explain point #2 above?

 

[L13]

L13 said that the gas in your tank is unaffected. If that's the case, then how can the pressure in the tank change with depth?

The pressure in your tank only changes when you remove some of it by breathing.

 

[tursiops]

1. "Your air supply lasts ½ as long at 10 metres/33 feet (2 bar/ata) than at the surface (1 bar/ata)."
"Your air supply lasts 1/3 as long at 20 metres/66 feet (3 bar/ata) than at the surface (1 bar/ata)"

2. "the air gets denser as you go deeper; denser air is harder to breathe than air at normal surface pressure. The deeper you are, the more
energy you use to breathe."

As you go deeper, your lung volume goes down. Because of this, you have to breathe more faster and more frequently in order to get the same amount of air you would breathe in at the surface.

Not so. You breathe at the same rate (breaths per minute) -- approximately -- no matter your depth. What matter is is number of air molecules you are breathing...and the molecules are packed more closely together at depth, so a "smaller" breath has just as many molecules in it. In fact, there because of the compression of the air there are MORE molecules than you need so you are using up the air faster than you need to.

Because you are breathing more frequently, your air supply doesn't last as long the deeper you go. Do I have this right? Does this explain point #1 above?

You are not breathing more frequently. You are just using up more aiar bcasue the air is compressed and is denser than at the surface.

Because you're breathing faster, this requires more energy. Does this explain point #2 above?

No, you are not breathing faster. The air is compressed at depth (more compressed at 20m than at 10m, say), so is denser, and it takes more work to move the denser gas into your lungs.

 

[L13]

I think you're reply caused a light bulb to go off in my head. Near the beginning of the PADI Open Water online course they were talking about how air supply and breathing as you go deeper. There is this:

1. "Your air supply lasts ½ as long at 10 metres/33 feet (2 bar/ata) than at the surface (1 bar/ata)."
"Your air supply lasts 1/3 as long at 20 metres/66 feet (3 bar/ata) than at the surface (1 bar/ata)"

2. "the air gets denser as you go deeper; denser air is harder to breathe than air at normal surface pressure. The deeper you are, the more
energy you use to breathe."

So now that I know that the air molecules in the tank are unaffected, how do you explain the statements above?

As you go deeper, your lung volume goes down. Because of this, you have to breathe more faster and more frequently in order to get the same amount of air you would breathe in at the surface. Because you are breathing more frequently, your air supply doesn't last as long the deeper you go. Do I have this right? Does this explain point #1 above?

Because you're breathing faster, this requires more energy. Does this explain point #2 above?

In your tank, they are very densely packed. And stay at the density until you breath. Your regulators reduce the pressure and the density on the air to match the pressure and resulting density at the depth you are at as described by PADI. If you are deep, the air from your tank is reduced to a higher pressure and density and lasts for a shorter time than if you are shallow.

You breathe the exact same rate whether you are deep or shallow (or should), but different amounts of the air molecules are needed from the tank to fill you lungs depending on the pressure/depth, so the air in the tank lasts a different amount of time.

 

[tursiops]

The Big Apple: “I can explain it to you, but I can’t understand it for you”

 

[L13]

Here is an example:

Say you have a tank with a volume of 1/2 cubic foot (cuft) at surface pressure. If we compress air into it to a typical operating pressure of ~200 atmospheres of pressure, it would now have the molecules of 100cuft of surface air in it (in the US, this tank would be labeled as having a size of 100cuft). On the surface it would last a typical person a couple hours.

Now we go down to 33 feet deep. the pressure of the water is is about 2x what it was on the surface. When you take a breath, it takes twice as many molecules to fill your lungs as it did on the surface, so the tank lasts half as long, and the air you are breathing is twice as dense.

Now go to 100ft, the pressure of the water is about 4x what it was on the surface, and it takes 4x as many molecules of air to fill your lungs, so the tank lasts 1/4 as long.

 

[davehicks]

I think you're reply caused a light bulb to go off in my head. Near the beginning of the PADI Open Water online course they were talking about how air supply and breathing as you go deeper. There is this:

1. "Your air supply lasts ½ as long at 10 metres/33 feet (2 bar/ata) than at the surface (1 bar/ata)."
"Your air supply lasts 1/3 as long at 20 metres/66 feet (3 bar/ata) than at the surface (1 bar/ata)"

2. "the air gets denser as you go deeper; denser air is harder to breathe than air at normal surface pressure. The deeper you are, the more
energy you use to breathe."

So now that I know that the air molecules in the tank are unaffected, how do you explain the statements above?

As you go deeper, your lung volume goes down. Because of this, you have to breathe more faster and more frequently in order to get the same amount of air you would breathe in at the surface. Because you are breathing more frequently, your air supply doesn't last as long the deeper you go. Do I have this right? Does this explain point #1 above?

Because you're breathing faster, this requires more energy. Does this explain point #2 above?

FWIIW, I applaud you for asking these questions and attempting to better understand the theory and physics of scuba diving. If you absorb this information, you will become a much better and safer diver.

Cheers!