A throttling device is installed on the gas flow pipeline, which contains an orifice plate with a circular hole at the center. The diameter of the hole is smaller than the inner diameter of the pipeline, and the gas flows steadily forward in front of the orifice plate. When the gas flows through the orifice plate, the diameter becomes smaller, the cross-sectional area shrinks, and the stable flow state is disrupted. Therefore, the flow velocity will change, the velocity will increase, and the static pressure of the gas will decrease accordingly, resulting in a pressure drop before and after the orifice plate, that is, differential pressure (the pressure is higher in the larger section in front of the orifice plate and lower in the smaller section through the orifice plate). There is a definite numerical relationship between the magnitude of differential pressure and gas flow rate, that is, when the flow rate is high, the differential pressure is high, and when the flow rate is low, the differential pressure is low. The flow rate is directly proportional to the square root of the differential pressure. An instrument that calculates flow based on the differential pressure generated by flow detection components installed in pipelines, known fluid conditions, and the geometric dimensions of the detection components and pipelines.

The differential pressure flowmeter consists of a primary device (detection component) and a secondary device (differential pressure conversion and flow display instrument). Differential pressure flow meters are usually classified in the form of detection components, such as orifice flow meters, Venturi flow meters, and averaging tube flow meters.

The secondary device includes various mechanical, electronic, and electromechanical integrated differential pressure gauges, differential pressure transmitters, and flow display instruments. It has developed into a large category of instruments with a high degree of standardization (serialization, generalization, and normalization) and a wide range of types and specifications. It can measure both flow parameters and other parameters (such as pressure, level, density, etc.).

The detection components of differential pressure flow meters can be divided into several categories according to their operating principles: throttling device, hydraulic resistance type, centrifugal type, dynamic pressure head type, dynamic pressure head gain type, and jet type.

Testing items can be divided into two categories based on their standardization level: standard and non-standard.

The so-called standard test piece is designed, manufactured, installed, and used according to standard documents, without the need for actual flow calibration to determine its flow value and estimate measurement errors.

Non standard testing components are those with poor maturity and have not yet been included in international standards.

Differential pressure flowmeter is a widely used type of flowmeter, with a high usage rate among various flow meters. In recent years, due to the emergence of various new types of flow meters, their usage percentage has gradually decreased, but they are still the most important type of flow meter at present.