A primary flow element is a device inserted into the flowing fluid that produces a physical property that can be accurately related to flow.
For example, an orifice plate produces a pressure drop that is a function of the square of the volume rate of flow through the orifice.
A vortex meter primary flow element produces a series of oscillations of pressure. Generally the physical property generated by the primary flow element is more convenient to measure than the flow itself.
The properties of the primary flow element, and the fidelity of the practical installation to the assumptions made in calibration, are critical factors in the accuracy of the flow measurement.