The warming-up of room air in the pneumotachograph during inspiration phase and the different components of gas and water vapour in the expired air compared to the inspired air are the most important influences on the measured flow volume in routine pneumotachographic measurements. In a physical and mathematical analysis these influences were calculated in order to estimate whether there is an important or negligible difference between the inspired and expired flow volumes. (1). During inspiration the temperature effect of the heated pneumotachograph on volume extension is compensated close to 50% by the temperature effect on gas viscosity. (2). During expiration the effect of body temperature on the total viscosity (in comparison to viscosity at room temperature) is mostly compensated by the different O2 and CO2 compositions of the expired air including the effect of the water vapour. Comparing the corrected flow equation for the inspiration and the expiration phases, the actual difference between the inspired and expired flow volumes will be of 1.2% in routine measurements. This result matched very well that of Barres and Gauge  obtained with a lung model.