Does the Air Volume in a Scuba Tank Change As You Descend?

If you read the PADI Scuba Diving Instruction Manual or watch the DVD you may get the impression that the volume of air in a scuba tank decreases as you descend. I'm 99% sure this is wrong i.e. the volume of air doesn't change. I even asked about it during the in-person scuba classes, but the instructor, dive master, and the rest of the students agreed with the manual even though not one of them could explain how air in a rigid, sealed tank could be affected by outside air pressure.

I knew something was amiss when I heard these descriptions of what was happening:

  • Osmosis was in effect allowing the pressure from the outside of the tank to seep into the interior of the tank. The problem with this is that osmosis would require the aluminum or steel sides of the tank to be permeable. They aren't.
  • Since the volume of the air decreases, the remaining space in the tank is a vacuum. The problem with this is that a vacuum is a low pressure are so how could the high pressure air and the low pressure vacuum exist next to each other?

It took me a while to come up with a good way to describe the way I was thinking about the problem, but now I've got one.

  1. Imaging you're pressing your finger against someone's forehead. You are, in effect, applying pressure in the same way as the air (or water) around us does. Of course, the pressure you're applying is focused on a limited area.
  2. If you put a sponge in between your finger and the person's forehead, the person will still feel pressure. This is because your finger is applying pressure to the sponge and the sponge is applying pressure to the person.
  3. If instead you have the person stand on one side of a wall and you stand on the other side, you can press all you want against the wall and the person won't feel a thing. This is because the wall is rigid. All of the pressure you're applying with your finger is absorbed by the wall and distributed along its structure. This is how a scuba tank works. The water pressure isn't great enough to deform the tank walls and so the pressure doesn't “get in” i.e. the volume of the air inside stays constant (until you start sucking air out of it).

I'd love to discuss this more with other divers, but I refrained from doing so during the class. I didn't want to be one of those guys. Perhaps after a dive one day when the food is plenty, drinks are flowing, and everyone is in a good mood.

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5 Comments

  1. You are correct. The volume of a scuba tank is fixed. A gas will fill the volume of a container. So, the volume of air in a scuba tank is fixed. As a scuba tank is filled with air, more gas gets compressed into the fixed volume increasing the pressure. As a diver breathes the air, gas is released decreasing the pressure. Therefore, scuba tank pressure is directly correlated to the mass of air inside the tank. So a diver knows how much air mass is in the fixed volume of his tank based on the pressure. This is why a pressure gauge tells the diver how much breathable air is remaining.

    As a diver breathes underwater, air comes out the regulator at a fixed pressure (about 150 psi). However, immediately upon release from the regulator, the air is affected by ambient pressure. So, at 1 atm of depth, where it is double the pressure, the air is compressed in half as it comes out the regulator. So, the diver must get twice as much air out of the tank compared to the surface, since the air is immediately compressed in half. In other words, the diver must inhale twice as much air to fill the same volume of his lungs where the ambient pressure is double, since the air is immediately pressurized by the depth as it comes out the regulator. This is why divers use more air at deeper depths.

  2. Thanks for the article. I was in the middle of reading my PADI dive manual and I was confused because I thought they were implying that the volume inside of the tank was shrinking...

  3. I was searching this topic again today and re-discovered this discussion. I had one more thought to add.

    Think of the air in your tank as a fixed quantity of oxygen molecules rather than a volume or a pressure because those are difficult concepts to visualize.

    Say you've got a million molecules of air in your tank (in reality it's vastly more than that) and you use about 1000 per breath at sea level.

    The reason you use 1000 molecules per breath here is because that's the same air pressure that's surrounding you. About 14 pounds per square inch. That's actually quite a bit of pressure, but you don't even notice it because a) most of your body is liquid or solid and not very compressible and b) the air pockets in your body are at exactly the same pressure as your surroundings.

    At double that pressure you'd have air trying to force its way into your ears, nose and mouth to equalize with the pressure of the surrounding air. It would feel like a tight bear hug around your chest. You could probably seal your mouth tight enough to not let air in, but the longer you hold it the more it's goign to hurt. So you breath in, pop your ears and it feels better. But now you've got 2000 air molecules in your chest instead of just 1000.

    The air in your lungs wasn't compressed to fill only half the available capacity, but higher pressure air was trying to sneak in at any vulnerable place to equalize.

    The same thing happens under water, except obviously it's water trying to force its way in, not air, so you have to bring some compressed air along with you.

    I've got 1 million molecules in my tank. That's 1000 breaths at the surface. But if I go down to 33 feet now I have to breath in 2000 molecules just to keep the water from bear hugging my chest cavity or rupture my ear drums.

    Don't worry about the pressure or volume of the tank. The pressure is constantly decreasing with every breath but the volume is remaining the same. All that matters is the molecule count. Go down to 66 feet and now you need 3000 molecules to equalize with the water pressure.

    The outside water pressure is having absolutely no effect on the volume or pressure within your tank. It's a sealed system and it's at a WAY higher pressure than the water around you. If anything your tank is trying to bulge outwards, not shrink inward. Your regulator is designed to release air at exactly the same pressure as the surroundings. But the quantity of air in the tank is not finite. Every breath you take decreases the mass of air inside. The deeper you are the more you're drawing out with every breath.

  4. It doesn't change*, this is nonsense. A scuba tank is nothing more than a pressure vessel. Seen those big LPG tanks outside of motels or at camping grounds? Same idea. That's why it's a round cylinder - a sphere would be best but they're rather impractical. Submarines and airplanes are typically cylinders for exactly the same reason.

    I suspect what they meant (at some point) is that at pressure you use more and more air since you're equalized with the pressure of the water and obviously the air has to be at that pressure.. which would make it denser.. etc - that's the whole point of scuba after all. My guess is this somehow got corrupted into meaning the tank air is getting compressed? Utter nonsense.

    *: If you want to be incredibly anal, technically it would be getting compressed a teensy amount as the tank's shape would distort slightly as you went deeper into the water (and/or as you used up air since that would have the same effect of changing the relative pressures). This is so small that it'd be very difficult to measure. You'd need a very precise pressure gauge and would probably need to lower a tank down pretty deep to get a reading. At some point you would notice an increase in pressure but for all practical scuba purposes this is completely irrelevant.

  5. I had exactly the same thought when I was doing my PADI dive training. There's simply no way that external water pressure is "squeezing" the air in your tank the same way it physically squeezes the air in your BCD's air bladders.

    And they've got it backwards anyway....the pressure inside your tank is still HIGHER than the water pressure around it. Your tank is at 3000psi. Many times greater than the water pressure even at 100 feet (around 40 or 50psi I think)

    You use air faster at lower depths not because the water pressure is squeezing it into a smaller volume, but because your regulator doesn't reduce the air pressure as much, meaning you use a higher percentage of the air in the tank with every breath. Your regulator is designed to deliver air at the same pressure as the surroundings, so if you're in 40psi water, it only delivers 40psi of air. If you're in shallower water it delivers lower pressure air. The volume remains contant because your lung volume isn't changing.

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