How is air in a scuba tank which is fixed and rigid get affected by the pressure of the water outside of the tank? Shouldn’t the volume of the air in the tank remain constant once as the tank does not get deformed (squished)?
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You are correct that the tank itself does not change. It doesn’t get noticeably compressed, nor have a different volume of air inside it, regardless of where you’re diving.
What does change is the pressure of the surrounding ocean, at the depth you choose to dive to. And the way the regulator works is that it delivers air to your face at (approximately) the same pressure as submergence pressure for your depth. And density is proportional to pressure, for all intents and purposes here.
Ergo, if you dive twice as deep, when you pull a lungful of air, it’s twice as high in pressure, twice as dense, and depletes your tank twice as fast.
A standard tank is pressurized to *approximately* 20 atmospheres. At the surface (aka one standard atmosphere of pressure), if it were the size of your lungs, you could inhale 20 times. But for every ten meters (33 feet) below the surface you dive, you add another atmosphere of pressure. So ten meters down (2 atmospheres), you could only take 10 lungfuls. Thirty meters down (4 atmospheres), only 5 lungfuls.
Make sense?
You’re correct the volume of air in the tank doesn’t change at depth. however to use it, the air must leave the rigid tank and go into the hose, the squishy person and their lungs. So it needs to push against the forces in the tube and then in the person and fill up their lungs, which have the same volume but now need more gas to equalize the pressure on the body.
The volume of the tank isn’t the issue. The tank itself is rigid and at way higher pressure than the surrounding water at depth, otherwise the air wouldn’t come out.
Instead the issue is that the air is trying to fill the diver’s lungs and the diver themselves are somewhat squishy. In order for them to be able to inhale against ambient water pressure the air supplied must be of a matching pressure, which means a higher relative surface volume in each breath at depth.
The air decompresses to the pressure of the environment it is being released into. Decompression = expansion. The deeper you are, the higher the pressure in the environment, so the less the air has to decompress to match it. This means, to fill your lungs (which are always the same size), it takes more air, i.e. air that has not expanded as much as it would at the surface. Or you could say it takes denser air. More air molecules packed into the same space.
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