A portable anemometer primarily measures the speed of an airflow, but can also be used to calculate the volumetric flow rate in a ventilation duct, or at a supply or exhaust vent. Today, these instruments often incorporate a temperature sensor to measure the temperature of the airflow and its velocity at the same time, which is why they are called thermo-anemometers. 

Measuring the volumetric flow rate of supply and exhaust air not only ensures that an aeraulic system is working properly, but is also necessary for assessing the indoor air change rate (IARR) and mixing rate, which are key parameters in indoor air quality (IAQ) and pressure cascades within a building.