When fluid flows through the flow meter, a pressure difference is generated between the inlet and outlet. The rotating part of the flow meter (referred to as the rotor) rotates under this pressure difference, discharging the fluid from the inlet to the outlet. During this process, the fluid repeatedly fills the flow meter's "measuring space" and is continuously sent to the outlet. Given a specific flow meter, by measuring the rotor's rotation speed, the volume of the measuring space can be determined, thus obtaining the cumulative volume of fluid flowing through the flow meter.

Working principle of an elliptical gear flow meter:

Two elliptical gears with a special shape mesh and roll in contact. p1 and p2 represent the inlet pressure and outlet pressure, respectively, and obviously p1 > p2. In Figure 1(a), the lower gear rotates counterclockwise under the pressure difference on both sides, acting as the driving gear; the upper gear, under equal pressure on both sides, does not generate torque and acts as the driven gear, rotating clockwise under the drive of the lower gear. At the position shown in Figure 1(b), both gears generate torque under the pressure difference and continue to rotate. When installed at the position shown in Figure 1(c), the upper gear becomes the driving gear, and the lower gear becomes the driven gear, continuing to rotate to the same position as shown in Figure 1(a), completing one cycle. One cycle discharges the fluid volume of the four crescent-shaped cavities closed between the gear and the housing wall; this volume is called the "circulation volume" of the flowmeter.

What is the working principle of an elliptical gear flowmeter?

Let the "circulation volume" of the flowmeter be υ, and the number of rotations of the gear in a certain time be N. Then the fluid volume flowing through the flowmeter in that time is V.

Therefore, V = Nυ (1) The rotation of the elliptical gear is transmitted to the counter through the magnetic seal coupling and the transmission reduction mechanism, directly indicating the total amount flowing through the flowmeter. If a transmitting device and an electric display instrument are added, the remote transmission will only show the instantaneous flow or cumulative flow.

Working principle of Roots flow meter with rotary wheel: A rotary wheel flow meter, also known as a Roots flow meter, is characterized by a metering chamber inside the flow meter housing containing one or two pairs of tangentially rotating rotary wheels. Outside the flow meter housing, a pair of drive gears are coaxially mounted with the two rotary wheels and mesh with each other, causing the two rotary wheels to move in tandem. A constant flow of fluid passes through the side wheels and components, separating from the flow meter and flowing from the inlet to the outlet. Knowing the volume of the metering chamber and recording the number of rotations N of the rotary wheels, the volume V of the fluid flowing through the flow meter can be calculated. Clearly, for the flow rate of the rotary wheel flow meter: V = 4Nv (2-2)

Working principle of dual rotor (screw) flow meter: A pair of specially toothed helical rotors in a dual rotor (spiral) flow meter mesh directly without relative sliding or the need for synchronizing gears. The rotors are driven to rotate by a small pressure difference between the inlet and outlet. Therefore, the rotor's rotational speed is proportional to the cumulative flow rate of the fluid, and the rotor's rotational speed is proportional to the instantaneous flow rate of the fluid. The rotor's rotational speed is transmitted to the instrument counter via a magnetic coupler, which displays the flow rate passing through the flow meter (through the pipe).

The dual-rotor (screw) flow meter is a uniquely designed positive displacement flow meter primarily used for liquid flow measurement.

Where is positive displacement used?

Positive displacement flow meters are particularly suitable for the flow of viscous fluids such as oil, condensate, resins, and liquid foods. For high-viscosity media, other flow meters struggle to measure flow rates, but positive displacement flow meters can measure them accurately, with an precision of ±0.2%. Therefore, positive displacement flow meters are often used for total volume measurement of expensive media (oil, natural gas, etc.).