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A mass flow meter operates on the principle that the volume of a fluid is a function of its temperature and pressure, and thus constitutes a dependent variable; whereas the mass of the fluid is a quantity that does not change regardless of variations in time, temperature, or pressure. As mentioned earlier, for commonly used flowmeters such as orifice plate flowmeters, laminar mass flowmeters, turbine flowmeters, vortex flowmeters, electromagnetic flowmeters, rotameters, ultrasonic flowmeters, and gear flowmeters, the value measured is the volumetric flow rate of the fluid. In activities such as scientific research, production process control, quality management, economic accounting, and trade transactions, the quantity of fluid involved is generally mass. Relying solely on the aforementioned flow meters to measure the volumetric flow rate of a fluid often fails to meet people’s requirements; it is usually necessary to find a way to obtain the mass flow rate of the fluid as well. Previously, the mass of a fluid could only be determined indirectly, through methods such as correction, conversion, and compensation, after measuring parameters such as its temperature, pressure, density, and volume. With this measurement method, there are many intermediate steps, making it difficult to ensure and improve the accuracy of mass flow measurement. With the development of modern science and technology, various measurement methods and devices for directly measuring mass flow rate have emerged, thereby driving progress in flow measurement technology. Instruments used to measure fluid flow are collectively referred to as flowmeters or flow gauges. Flowmeters are one of the important instruments in industrial measurement. With the development of industrial production, there are increasing demands for higher accuracy and broader measurement ranges in flow measurement, leading to continuous advancements in this field. To meet various applications, different types of flowmeters have been developed over time. More than 100 types of flowmeters have been put into use so far. Classified according to the structural principle of the flow meter. There are positive displacement flowmeters, differential pressure flowmeters, float flowmeters, turbine flowmeters, electromagnetic flowmeters, vortex street flowmeters among fluid oscillation flowmeters, mass flowmeters, and insertion flowmeters. Based on the object of measurement, they are divided into two main categories: closed pipelines and open channels. According to the purpose of measurement, they can be further classified into total volume measurement and flow rate measurement, with the corresponding instruments being called volume meters and flowmeters respectively. A total flow meter measures the volume of fluid that passes through a pipe over a certain period of time; it is expressed as the total volume that flows through in a short period divided by that time. In fact, flow meters usually also come equipped with an accumulation device for use as a total flow meter, and total flow meters likewise have a flow signaling device. Therefore, in a strict sense, flow meters and totalizers no longer have any practical significance. Classified by measurement principle: 1. Mechanical principle: Instruments that fall under this principle include those of the differential pressure type and rotor type, which utilize Bernoulli’s theorem ； Using the impulse form of the momentum theorem, movable tube type ； Using the direct mass form of Newton’s second law ； Target type utilizing the principle of fluid momentum ； Turbine type utilizing the law of angular momentum ； Vortex-type and vortex street-type utilizing the principle of fluid oscillation ； Pitot-type methods that utilize the total static pressure difference, as well as volume-type methods and weirs, channels, etc. 2. Electrical principles: Instruments used for such principles include electromagnetic, differential capacitive, inductive, strain gauge-type instruments, etc. 3. Acoustic principles: Flow measurement using acoustic principles includes ultrasonic types, acoustic types (shock wave types), and others. 4. Optical principles: Instruments such as laser-type and photoelectric-type fall under this principle. 5. Based on physical principles: Nuclear magnetic resonance type, nuclear radiation type, and similar instruments fall under this category of principles