A novice diver usually requires more belt weights than an experienced diver of like displacement. The beginner, apprehensive, involuntarily inflates his lungs too much and needs more ballast to be equilibrated. After a few dives he breathes normally and finds that he is overweighted. Then he learns the wide possibilities of adjusting air ballast by his own breathing discipline, a factor that has a range of 6 to 12 pounds on his displacement. At the beginning of the dive the air cylinder weighs about 3 pounds, as it is consumed the diver weighs less with each breath. When the air is gone the cylinder exerts a 3 pound ascension force. The perfectly adjusted diver begins slightly overweighted,( in order to sink) and returns at the end of the dive slightly underweighted in order to ascend. From “the silent world” by Jacques Cousteau
