Expired volume is greater than inspired volume under most atmospheric conditions.  The increase, due to warming and humidification, is typically 5-10%.  For this reason there will be “breath-dependent drift” of the volume trace, even when the Spirometer is correctly zeroed.  We try to minimize this within the software by performing the volume correction, after the pod/spirometer has warmed up, and at the beginning of the experiment as it can vary per person, per day, and per equipment.