Pneumotachographs measure the flow according to the Venturi effect. The Venturi effect is the phenomenon that occurs when a flowing fluid is forced through a narrow section, resulting in a pressure decrease and a velocity increase.

There are 2 types of pneumotachographs: Fleish and Lilly.

The Lilly type measures the difference in pressure over before and after a membrane with known resistance.

Fleisch types use a series of parallel capillaries. It includes a tube for connection on CPAP/BiPAP devices to analyse the quantitative breathing and leakage flow.