The positive displacement (PD) flow meter is the only flow-measuring technology that directly measures the volume of fluid that passes though the flow meter. It achieves this by trapping pockets of fluid between rotating components housed within a high-precision chamber. This can be compared to repeatedly filling a beaker with fluid and pouring the contents downstream while counting the number of times the beaker is filled.
Rotor rotational velocity is directly proportional to flow rate since the flow of fluid is causing the rotation.
In electronic flow meters, the rotating components contain magnets that activate various sensor options located outside the fluid chamber. Mechanical flow meters rely on the rotation to drive either a magnetic coupling or a direct gear train connected to the mechanical counter.
In addition, PD flow meters do not need a power supply to operate.