The volume trace can be separated into inspired and expired components. The subject breaths in a fixed volume of air which is at room temperature. When the air enters the lungs, it becomes warmed and humidified, which increases the volume of the air. Subsequently the volume of the expired portion of breath is greater than the volume of the inspired portion. The result is a drifting volume trace. This can be resolved by using a heated flow head (such as the Heated Pneumotach 800L with Heater Controller (MLT3813H-V)) which ensures the temperature of both the inspired and expired air is the same. Alternatively, the Spirometry extension in LabChart eliminates drift by applying a correction factor to reduce the expired volume.