The volume flow is one of the determining variables of ventilation technology in the field of building automation. The stationary measurement of the volume flow at partial points of the ventilation duct enables the permanent control of normative limit values, which, for example, define a specific air exchange rate or minimum flow rates. In the case of intelligent control of ventilation and air-conditioning systems, the volume flow measurement in combination with the measurement of various room air parameters, such as temperature, humidity or CO2 load, serves to ensure energy-efficient and demand-oriented control. This means that only as much fresh air is delivered as is necessary to ensure hygienic and comfortable room air quality.

The measurement of the volume flow is mostly relevant for medium to large room air conditioning systems. The measurement is carried out directly in or on the duct, whereby a duct network calculation is used to specify which diffusers should "receive" which air volumes. The measuring points can be planned, for example, according to DIN EN12599. As a general rule, the measuring points should be as far away from sources of interference as possible. Sources of interference such as bends or dampers affect the flow pattern and can lead to increased measurement uncertainty with regard to the volume flow measurement.

The volume flow is determined physically via a differential pressure transmitter and a differential pressure transducer. To measure differential pressures, two system pressures are measured and the difference between their values is determined. The measuring element is usually a piezoresistive differential pressure sensor with a silicon-based measuring diaphragm. As the differential pressure changes, the measuring diaphragm expands in one direction or the other, thus changing its electrical properties. The volume flow can therefore be determined as a function of the differential pressure using a formula if the cross-sectional area of the channel and the k factor of the differential pressure sensor are known.