In scuba, why does the diver’s air get more compressed with depth? Does the tank not protect the air from outside forces?
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The pressure inside a SCUBA tank is much greater than the outside water pressure – even when a couple hundred feet deep.
The air coming from the SCUBA tank is always moving from high pressure to lower pressure, it does not “get more compressed” with depth. It just does not drop in pressure as much as it would if you were breathing at the surface.
The air inside the tank can be over 3,000 psi. The first stage of the regulator (the part that clamps onto the tank) reduces the tank pressure to about 150 psi. That is 150 psi above ambient pressure. The deeper you go the higher the absolute pressure, but the first stage always provides 150 psi above whatever pressure you are in. *150 psi is an example from memory, the exact values may be different.
That means that there is always 150 psi of air in the hose that connects to the second stage (the part that goes in your mouth).
When you are at depth the ambient pressure is greater. When you breathe in air from the 2nd stage regulator at 33 feet depth, the air in your lungs is almost 30 psi (absolute pressure) or about 15 psi greater than when at the surface.
For every 33 feet of depth the pressure increases by about 15 psi.
At some point, when the tank is nearly out of air, the first stage will no longer supply the 150 psi of pressure – because the tank itself is less than 150 psi. You might find that you have to “suck” the air out of the regulator. Of course if you are doing this then you have failed to be a good diver.
Source: I’m a PADI Divemaster and I used to service and repair SCUBA equipment.
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