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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18 Comments

  1. Something doesn’t make sense.

    The volume of an 80 ft^3 tank with 3000 PSI doesn’t change regardless of depth. The tank, being inflexible, resists the force of increased outside pressure so no pressure is exerted on the gas inside the tank.

    Since the pressure inside the tank is 3,000 at the surface and remains 3,000 at depth, volume in the tank CAN’T be affected because if there is no change in pressure, there can be no change in volume (the former always affects the latter per Boyle’s law).

    All of the magic happens outside the tank at the regulator in response to pressure/volume changes. A larger volume of air per breath is consumed from the tank at depth than at the surface causing a more rapid decrease in pressure of the tank.

    You could cause the same affect at the surface by increasing the volume of gas consumed proportionately (like two people breathing from the same tank at the surface...all other adjustments accounted for).

    Boyle’s law isn’t any different inside or outside the tank, it’s just that the tank resists external pressure changes (and internal ones or it would grow when you filled it).

  2. The short and simple answer is that volume of gas changes with depth. See Boyle's Law, "Assuming temperature remains constant, pressure and volume are inversely proportionate" (meaning as pressure outside the tank increases, it's volume decreases and vise versa). This is why you're not supposed to hold your breath while ascending on scuba or surface supplied breathing mediums. The formula for this is P1 V1=P2 V2. So as pressure doubles, the volume decreases by half. This is why your tank will never last very long the deeper you dive, as you are consuming more gas due to the volume being compressed at depth. Someone made a comment about a vacuum within the tank...that doesn't happen. the pressure outside the tank is simply exerting more pressure on the tank causing the volume to decrease. This happens with your lungs as well. If you want a good example of this, try 3 experiments...1, take an empty bottle of soda onto an airplane. when you get to altitude, open it...air escapes. Now cap it off...when you land, you'll find that the bottle has collapsed in on itself. this is because at altitude, the pressure is less, therefore the volume of gas in the soda bottle has expanded, and on landing, the pressure of the atmosphere has decreased the volume in the bottle because the pressure is greater on the ground than at 33,000 feet. Experiment 2, take an empty bottle down with you the next time you dive...you'll notice that the bottle has collapsed on itself...like on the airplane, the pressure of the water column is decreasing the volume within the bottle, causing it to collapse. now pretend you have a means of pressurizing the bottle with air at depth...when you get to surface, the bottle will expand, just like it did in the airplane, or will likely explode. The 3rd experiment is simply opening a bottle of soda with soda in it. The Co2 within the bottle is saturated in solution, due to a greater pressure in the bottle. when you release the pressure, the Co2 is freed from the solution forming bubbles. This is why we shouldn't ascend faster than 33fpm, and why we take deco stops after certain times at certain depths.

    So as the atmospheric pressure doubles, the volume within a container decreases. There is about 5 or 6 different gas laws that affect divers. The Combined Gas Law ironically allows you to figure out ALOT of things when it comes to the relationship of gas pressure/volume to the outside world affecting it. The mathematics behind it are far beyond the necessary information required for a scuba diver, however, Gay-Lussac's Gas Law is a good one to know, because it deals with the correlation of pressure-temperature relation...basically, if you have an 80FT^3 cylinder at 3,000psi, the cylinder relief valve will activate if the temperature gets too hot, because the pressure increases as the temperature increases.

    Now, some of the others here, have mentioned the shape of the vessel containing the gas...it does NOT matter what the shape of the container is, the laws apply all the same...for a cylinder, cube, pyramid, etc shaped, as the pressure increases, the volume decreases. PERIOD. The volume of an 80FT^3 cube or spherical tank at 3,000psi, is going to be exactly the same volume of gas as an 80ft^3, 3,000psi cylinder.

    Whoever is teaching that your gas volume always remains constant regardless of depth, needs to stop teaching that, as it can get someone killed.

    Hope this clears the air for you (pun intended lol).

    • I have to disagree. While your experiments are spot on, they don't simulate a scuba tank because the volume of the tank doesn't change i.e. the sides don't expand or contract like those of a plastic bottle.

      If what you say is right and the volume of the CO2 is compressed in a scuba tank, what occupies the space now that the CO2 has been compressed? It can't be nothing or else the CO2 would just expand to fill it.

      Note that I'm not debating whether CO2 expands or contracts under pressure. I'm debating what my PADI instructor was saying that the CO2 inside a tank gets compressed under pressure.

      • Marios,

        You somehow are getting confused with the PSI in the tank and the VOLUME in the tank. The PSI within a tank is not affected by depth. But volume is affected. Boyle's Law doesn't respect the type of container. Indeed, there IS a bit of correlation between the two though, but I've already explained that.

        Now, if you want further proof, there is another experiment you can conduct which involves a balloon and glass jar. I have never conducted it myself, however, one of my commercial diving instructors did this, and reported that the balloon within the glass jar had decreased in size, the deeper he went...If the atmosphere around a rigid container did not affect the air inside it...why did the balloon get smaller.

        https://www.youtube.com/watch?v=fqWL5FsQXRI <<>>>> https://www.youtube.com/watch?v=yBq5uapC-e0

        In reference to the space occupied...The gas within the tank is the same gas as before, just at a lower pressure and volume. Nothing replaces the existing gas except for the gas itself. Lets take a refrigerated substance, such as Liquefied Propane Gas (LPG). 1^3 inch of LPG under pressure, will turn into 277^3 inch of Gaseous Propane when the pressure will release. The purpose of the example is to show, when you have 1^3 inch of LPG in a tank, as the pressure inside releases, the LPG expands into its gaseous state, to fill the void and maintain pressure to keep the LPG from boiling off and causing a really bad day.

        • Ooops. I meant oxygen. I deal with CO2 canisters too and had them on the brain when I was replying :-) I'll give that balloon video a watch and maybe it'll convince me. At least we can agree that PSI isn't affected by depth!

        • So a bunch of searches later and I can't find anything that explicitly says what I'm saying about how the volume of oxygen in a cylinder changes or doesn't change as you descend. However, if the volume of air changed in a sealed container due to outside pressure, wouldn't there be problems for people in submarines and airplanes?

  3. 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.

    • I'm confused. Let's imagine a scenario in which you have a tank filled with breathable air. Let's imagine that when the diver breathes this mix at sea level that they are receiving X number of atoms per ml, and that the tank will provide them with Y ml of gas. Now let's imagine that same tank and diver at 1 atm of pressure. The diver will have .5Y ml of gas, but he will receive 2X atoms per ml. The gas will be compressed, but they will still receive the same number of moles of each gaseous element that they need for cellular respiration...and life. Does it really matter what volume of space those molecules are occupying in your lungs? I can't imagine that the chemical reactions of cellular respiration care. If they have the molecules, then the reaction should proceed.

      • Maybe the volume occupied is important because the oxygen needs to be absorbed by the lungs and so the less volume, the less contact there is with the lung lining.

      • See Matt's response below. I'll add that your lungs have a volume, too, about 6 liters. So you must breathe 6 liters of air every breath (actually less). At the surface, let's say 6 liters of air has 1000 molecules of air. At 1 atm of depth, that 6 liters of air has 2000 molecules of air. So, at depth, you are breathing more air molecules to fill the same volume of gas in your lungs. Since there is a fixed amount of gas molecules compressed in your scuba tank, you use air faster the deeper you are.

        Your cells still respirate at the same rate, so your body is still using the same amount of oxygen, even though you are breathing in twice as much into your lungs. The remaining unused oxygen is exhaled. However, inert gas, such as nitrogen, is absorbed by your tissues at the higher pressure. The rate of off-gassing of nitrogen from your tissues must be calculated with a dive chart or dive computer to prevent decompression sickness. If you ascend too fast, the nitrogen will form bubbles in your blood stream, like opening a soda can, and can kill you.

        • But your lungs don't have to breathe in 6 liters of air every breath. I know that air tanks don't last as long at depth, but I still don't think we've hit the reason why.

  4. 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...

  5. 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.

  6. 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.

    • It's not possible to see an increase in pressure the deeper you go, because the temperature is decreasing. As a gas temperature increases, the pressure will decrease. I don't have it handy at the moment, but there is a gas law that explains this correlation. Probably Charles' Law? That one comes into mind.

  7. 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.

    • Yes, there is. Pressure affects volume exactly the same, whether it is a rigid, or flexible container. As depth (pressure) increases, gas volume decreases. the only difference, is that the rigid container doesn't respond to the pressure differential as the flexible container will. You guys need to get rid of the idea of pressure of the tank has something to so with consumption of air, and use volume of the tank...here's why... at the surface, a 6FT^3 pony bottle at 3000psi contains 6FT^3. Likewise, An 80FT^3 bottle at 3000psi has a volume of 80FT^3. the pony bottle at the surface with a pressure of 1500psi contains 3FT^3, and an 80FT^3 bottle at 1500psi contains 40FT^3. At 33FSW (1ATA), the pony has a volume of 3FT^3 at 3000psi, and the 80FT^3 bottle has 40FT^3 of gas at 3000psi. The PSI of the air has nothing to do with the Volume for the purpose ya'll are trying to use it for...the amount of gas you breathe is based on volume, not PSI. The pressure of the cylinder remains constant at any depth (for the purpose of the statement, assume you have a a separate bottle not used for breathing at 3000psi and 80FT^3), but the volume does not. When you breathe gas at 3 feet, you have a longer bottom time, than if you were breathing at 33, or 60 feet, because the volume is more concentrated.

      • You wrote: "As depth (pressure) increases, gas volume decreases. The only difference, is that the rigid container doesn't respond to the pressure differential as the flexible container will."

        If we assume that's true, can you tell me what is occupying the space that has been vacated by the oxygen that has reduced volume?

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