How Everything Works
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Question 781

If air in a rigid 80 cubic foot scuba tank is pressurized to 3000 psi, giving the diver a certain amount of breathing time, then why does bottom time decrease with depth? I know about external pressure, but how does the pressure affect air inside the tank? - RJ
The deeper a scuba diver goes, the greater the water pressure and the more the water presses in on the diver's chest. To be able to breathe, the air in the diver's mouth must have roughly the same pressure as the water around the diver's chest. That way, the diver will be able to use chest muscles to breathe the air into the diver's lungs. But the pressure of the air in the diver's mouth is proportional to its density and thus to the number of air molecules contained in each liter of air. At great depths, the diver must breathe dense, high-pressure air and this air contains a great many air molecules per liter. Since the scuba tank contains only so many air molecules, these molecules are consumed more rapidly at great depths than they are at shallow depths. The scuba regulator automatically controls the density of air entering the diver's mouth so that the air pressure is equal to the surrounding water pressure. That way, the air is easy to breathe. The deeper the diver goes, the more air molecules the regulator releases into each of the diver's breaths and the faster the air in the scuba tank is consumed.

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