Tell your friends that you're learning to Bali scuba dive, and at least one will ask you how long you can stay underwater with a scuba tank. A polite answer is, "Oh, around an hour, give or take," but as you'll see, the technically correct answer is, "It depends:" That is, it depends on how deep you Bali scuba dive (as well as your breathing rate).

Scuba gear supplies air equal to the sur-rounding pressure. So apply what you learned earlier about pressure and an air volume, and you'll see that you consume your air faster as you go deeper. For exam-ple, the pressure at 20 metres/66 feet equals three bar/ata, so for each breath you need three times the number of air molecules to fill your lungs to the same volume. Therefore, all other factors equal, your air supply lasts only one third as long at 20 m/66 ft as it does at the surface.

Likewise as you've learned, the deeper you descend, the denser the air becomes. Dense air is harder to inhale and exhale than air at normal surface pressure and density, with the effort increasing exponentially the faster you try to breathe it. That is, it takes about four times the effort to breathe twice as fast. So, you want to take deep, slow breaths while breathing dense air while Bali scuba diving. For maximum air conserva-tion, save energy and don't over exert yourself. Pace yourself so that you breathe normally through your entire Bali scuba dive.