To determine a person's actual vital capacity, a test is given to determine how much air [in liters] can be moved in or out of the lungs in a single breath. These tests are given on medical equipment such as a respirometer or spirometer. In 1982, Jacobs introduced to the music world the Voldyne®, an inexpensive medical device that can give an approximate reading up to five liters.
There are two chambers--the larger [right] is to measure the air volume and the smaller [left] for air pressure. To use, place the tube between the teeth over the tongue so as not to obstruct the air passageway. Inhale with a fast breath, keeping the ball in the pressure chamber as close to the top as possible. Watch the main chamber for the amount of air inhaled indicated by the top of the disk. There is a marker to manually mark the amount of air previously inhaled.