Flow can be expressed as volumetric flow rate, mass flow rate or in terms of volume or total displaced mass.

The measurement is obtained using two devices: one, primary, which is placed in direct contact with the fluid and which generates a signal and one, secondary, which translates this signal into a movement or a signal to indicate, record, control or calculate the flow.

Other devices indicate or calculate the flow directly through the interaction of the fluid flowing in the pipeline and the measuring device which is placed directly or indirectly in contact with the fluid.