Why is there an upward or downward trend in the data when measuring with a spirometer?

A spirometer measures the rate of air flow in liters per second. The total volume of air that has moved through the spirometer can be calculated by integrating the air flow rate as a function of time. Use the function Analyze/Process > Integral for this.
When the air flow data is converted to volume you may find an upward or downward trend in the volume data. You may apply a drift correction to the collected data in one of the following ways:

• before you start your measurement set the signal measured by the sensor to zero by right clicking the sensor icon on the screen panel and choosing ‘Set to zero’, or
• correct the data after the measurement is finished: determine a straight line through the drift by using the Function fit option and then use a formula to subtract the drift data from the measured sensor data