Working Principle of Positive Displacement Flow Meters

Volumetric flow measurement uses a fixed small volume to repeatedly measure the volume of fluid passing through the flowmeter. Therefore, there must be a space that constitutes a standard volume inside the volumetric flowmeter, which is usually called the "metering space" or "metering chamber" of the volumetric flowmeter. This space is composed of the inner wall of the instrument case and the rotating parts of the flowmeter.

The working principle of the volumetric flowmeter is: when the fluid passes through the flowmeter, a certain pressure difference will be generated between the inlet and outlet of the flowmeter. The rotating part of the flowmeter (referred to as the rotor) rotates under the action of this pressure difference and discharges the fluid from the inlet to the outlet. In this process, the fluid fills the "metering space" of the flowmeter again and again, and then is continuously sent to the outlet. Under given flowmeter conditions, the volume of the metering space is determined. As long as the number of rotations of the rotor is measured, the cumulative value of the volume of the fluid passing through the flowmeter can be obtained.

Assume that the volume of the flow meter's measuring space is v (m3), and the number of rotor rotations in a certain period of time is N. Then the volume of the fluid flowing through the meter during this period of time is

V = Nv

. Assume that the gear ratio constant of the meter is α. The value of α is determined by the gear ratio of the gear set that transmits the rotor rotation and the scale value of the meter pointer rotating one circle. If the meter indication value is I, its relationship with the number of rotor rotations N is

I = α N.

From equations (1) and (2), we can get the relationship between the volume of fluid passing through the meter within a certain period of time and the meter indication value

V = (v/α)I