Determination of various lung volumes and capacities by Spirometry PRINCIPLE  Dry spirometer is a hand-held spirometer, on which an indicator moves as the air is  exhaled, and only expired air volumes can be measured directly. . .  Wet spirometer consists of a plastic or metal bell within a rectangularor Cylindrical tank, so that air can be added or removed from it. The outer tank contains water and has a tube running through it. It can carry the air above the water level. The floating bottomless bell is inverted over the water containing tank and is connected to an indicator which represents the volume of air.