If you're correctly weighted, you should be neutrally buoyant—able to make a safety stop at 15 feet with 500 psi and no air in your BC.

The Ultimate Buoyancy Check

Most of us don't know how much the air in a particular tank weighs, nor do we want to get in and out of the water to readjust our weight before diving. Therefore we estimate how much weight we need to be slightly negative at the beginning of a dive. This makes it possible to control ascents and make a safety stop at the end of a dive when the air from the tank has been consumed.

This has led to the "ultimate buoyancy check": Fully equipped, you make your dive and breathe the tank down to 500 psi. Now go to 10 or 15 feet and vent all air from your BC. If your buoyancy is correct, you'll be neutral, so when you inhale you tend to rise and when you exhale you tend to sink.

Depending on the results of this test, either add or subtract weight before your next dive. At the end of your next dive, double-check your buoyancy.