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.
- 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.
- 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.
- 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.