A positive displacement flowmeter is an instrument that directly accumulates flow based on the volume of fluid discharged, also known as a fixed-displacement flowmeter. The following is a detailed introduction to positive displacement flowmeters.

I. Working Principle

A positive displacement flowmeter consists of a measuring chamber, moving parts, transmission, and display components. Its working principle is based on the pressure difference generated between the inlet and outlet of the fluid as it passes through the flowmeter. This pressure difference causes the rotating parts (rotors) of the flowmeter to rotate, discharging the fluid from the inlet to the outlet. In this process, the fluid repeatedly fills the "measurement space" of the flowmeter and is then continuously sent to the outlet. Under given flowmeter conditions, the volume of this measurement space is determined. By measuring the number of rotations of the rotor, the cumulative volume of fluid passing through the flowmeter can be obtained.

II. Classification

Positive displacement flowmeters can be classified into multiple types based on their measuring elements, such as oval gear flowmeters, scraper flowmeters, dual-rotor flowmeters, rotating piston flowmeters, reciprocating piston flowmeters, disc flowmeters, liquid-sealed rotating drum flowmeters, wet gas meters, and diaphragm gas meters.

III. Characteristics

1. Advantages:

• High measurement accuracy, making it a class of flow measurement instruments with high precision.

• Suitable for the measurement of high-viscosity liquids and suitable for clean single-phase fluids.

• Direct-reading instruments can directly obtain the cumulative total flow without external energy.

• Easy to install with no special requirements for upstream and downstream straight pipe sections.

2. Disadvantages:

• Not suitable for high or low temperature applications.

• Limited types and diameters of measured media.

• Relatively large pressure loss.

• Requires periodic calibration.

IV. Applications

Positive displacement flowmeters are widely used in the energy, oil, pharmaceutical, food, chemical, and other industries due to their high-accuracy measurement characteristics. Especially when measuring liquid media such as crude oil, it is necessary to measure storage, transportation, transfer, and distribution, and serve as a basis for financial accounting or as a legal measurement for tax-paying buyers and sellers to execute contracts.