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Meters used to measure the flow rate of fluids 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 a wider range 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. From different perspectives, flow meters can be classified in various ways. There are two common classification methods: one is to categorize them based on the measurement principle used by the flow meter, and the other is to classify them according to the structural principle of the flow meter. I. Classification 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 ； Pirometer types that utilize the total static pressure difference, as well as volumetric types and weir, channel types, etc. (2) Electrical principles: Instruments used for such principles include electromagnetic, differential capacitive, inductive, strain-gauge types, etc. (3) Acoustic principles: Flow measurement using acoustic principles includes ultrasonic types, acoustic types (shock wave types), and others. (4) Thermal principles: Flow measurement methods that utilize thermal principles include calorimetric, direct calorimetry, and indirect calorimetry methods. (5) Optical principle: Instruments such as laser-type and photoelectric-type fall under this principle. (6) Based on physical principles: NMR-type and nuclear radiation-type instruments fall under this category of principles. (7) Other principles: there are the labeling principle (tracing principle, NMR principle), correlation principle, etc. II. Classification by the structural principle of flow meters: Based on the actual characteristics of current flow meter products, and according to their structural principles, they can be roughly classified into the following types: 1. Positive displacement flow meters. Positive displacement flow meters function like containers with a standard volume, continuously measuring the flowing medium. The greater the traffic, the more times it is measured, and the higher the frequency of output. The principle of positive displacement flow meters is relatively simple, making them suitable for measuring fluids with high viscosity and low Reynolds numbers. Depending on the shape of the rotating element, the products currently available include: elliptical gear flowmeters suitable for measuring liquid flow rates, gear pump flowmeters (Roths flowmeters), rotary piston flowmeters, and scraper-type flowmeters ； Servo-type volumetric flowmeters, diaphragm flowmeters, and rotameter types that are suitable for measuring gas flow rates. 2. Impeller flowmeters: The working principle of impeller flowmeters is to place an impeller in the fluid being measured; the impeller rotates as a result of the force exerted by the flowing fluid, and the speed of rotation of the impeller is used to indicate the flow rate. Typical impeller-type flow meters are water meters and turbine flow meters, whose structure can be of mechanical drive output type or electrical pulse output type. Mechanical transmission water meters generally have lower accuracy, with an error of around ±2%, but they feature a simple structure and low cost. They are already produced in bulk in China, and have been standardized, generalized, and serialized. Turbine flowmeters that output electrical pulse signals have high accuracy, with an error range of generally ±0.2% to 0.5%. 3. Differential pressure flow meter (pressure drop type flow meter): A differential pressure flow meter consists of a primary unit and a secondary unit. The primary unit, known as the flow measurement element, is installed in the pipeline of the fluid being measured; it generates a pressure difference that is proportional to the flow rate, which is then used by the secondary unit to display the flow rate. The secondary device is called a display instrument. It receives the differential pressure signal generated by the sensing element and converts it into the corresponding flow rate for display. The primary element of a differential pressure flow meter is usually a throttling device or a dynamic pressure measuring device (such as a pitot tube, venturi tube, etc.). Secondary devices consist of various mechanical, electronic, and combined differential pressure gauges equipped with flow display instruments. The differential pressure sensing elements of these differential pressure gauges are mostly elastic elements. Since the differential pressure and flow rate are related by a square root relationship, flow display instruments are equipped with square root functions to linearize the flow scale. Most instruments are also equipped with a flow accumulation device to display the cumulative flow for economic accounting purposes. This method of measuring flow rate using differential pressure has a long history and is quite mature; it is generally used in important applications around the world, accounting for about 70% of all flow measurement methods. These meters are used for measuring the flow rates of main steam, feedwater, condensate, etc., in power plants. 4. Variable-area flowmeters (equal-pressure-drop flowmeters): A float placed in a conical flow channel that is larger at the top and smaller at the bottom moves as a result of the force exerted by the fluid flowing from bottom to top. The float comes to rest when this force is in balance with the float’s \"apparent weight\" (the weight of the float itself minus the buoyant force exerted by the fluid on it). The height at which the float remains stationary can be used as a measure of the flow rate. Since the flow cross-sectional area of the flowmeter varies with the height of the float, and the pressure difference between the upper and lower sections is equal when the float remains stationary, this type of flowmeter is known as a variable-area flowmeter or an equal-pressure-drop flowmeter. The typical instrument for this type of flowmeter is the rotor (float) flowmeter. 5. Momentum flow meters are flow meters that utilize the momentum of the flowing fluid to determine the flow rate. Since the momentum P of a flowing fluid is proportional to the density of the fluid and the square of its velocity, i.e., P = ρv², and when the cross-sectional area through which the fluid flows is constant, the velocity v is proportional to the volumetric flow rate Q; therefore, P is proportional to Q². Let the proportionality constant be A; then Q = A. Therefore, by measuring P, it is possible to determine the flow rate Q. Flow meters of this type generally use sensing elements to convert momentum into pressure, displacement, or force, and then measure the flow rate. Typical instruments for this type of flowmeter are target-type and rotating vane-type flowmeters. 6. Impulse flow meters: Flow meters that use the impulse theorem to measure flow rate are known as impulse flow meters. They are commonly used to measure the flow of particulate solid materials, as well as mud, crystalline liquids, and abrasive materials. The flow measurement range ranges from a few kilograms per hour to nearly 10,000 tons. A typical instrument is the horizontal-component impulse flow meter. Its principle of operation is that when the fluid to be measured falls freely from a certain height h onto a detection plate with an inclination angle, an impulse is generated; the horizontal component of this impulse is proportional to the mass flow rate. Therefore, by measuring this horizontal component, it is possible to determine the value of the mass flow rate. According to the detection method of signal (9), this type of flow meter is divided into displacement detection type and direct force measurement type. 7. Electromagnetic flowmeter: An electromagnetic flowmeter is designed based on the principle that a conductive material moving in a magnetic field generates an induced electromotive force, and this induced electromotive force is proportional to the flow rate; by measuring this electromotive force, the flow rate in the pipeline can be determined. It has high measurement accuracy and sensitivity. It is widely used in industry to measure the flow rate of media such as water and slurry. The maximum measurable pipe diameter reaches 2m, with extremely low pressure loss. However, media with low electrical conductivity, such as gases and vapors, cannot be used. Electromagnetic flowmeters are expensive, and their signals are susceptible to interference from external magnetic fields, which limits their widespread use in industrial fluid flow measurement. To this end, the products are continuously being improved and updated, moving toward microcomputerization. 8. Ultrasonic flow meter: The ultrasonic flow meter is designed based on the principle that the speed at which ultrasound travels through a flowing medium equals the sum of the average flow velocity of the medium being measured and the speed of the sound wave itself. It also reflects the flow rate by measuring the velocity of the flow. Although ultrasonic flowmeters were introduced in the 1970s, they have become very popular due to their ability to be designed as contactless devices, their capability to be used in conjunction with ultrasonic level gauges for measuring flow rate in open channels, and their lack of disturbance or resistance to the fluid flow. They represent a flowmeter with great potential for further development. Ultrasonic Doppler flowmeters based on the Doppler effect have received widespread attention in recent years, and are considered ideal instruments for non-contact measurement of two-phase flows. 9. Fluid oscillation flow meter: The fluid oscillation flow meter is designed based on the principle that fluids undergo oscillations when flowing through certain channels, and the frequency of these oscillations is proportional to the flow velocity. When the cross-sectional area through which the fluid flows remains constant, the flow velocity is directly proportional to the volumetric flow rate. Therefore, the flow rate can be determined by measuring the oscillation frequency. This type of flow meter was developed in the 1970s. It has great potential for further development due to its advantages of having no moving parts and providing a pulse digital output. Typical products at present include vortex flow meters and rotameter-type vortex flow meters. 10. Mass flow meters: Since the volume of the fluid is affected by parameters such as temperature and pressure, when expressing the flow rate in terms of volumetric flow rate, it is necessary to specify the parameters of the medium. With continuously changing medium parameters, it is often difficult to meet this requirement, resulting in distorted readings on the instrument. Therefore, mass flow meters have been widely used and valued. Mass flow meters are divided into direct-type and indirect-type. Direct-type mass flow meters utilize principles that are directly related to mass flow for measurement; common types currently in use include calorimetric, angular momentum, vibrating gyro, Magnus effect, and Coriolis force mass flow meters. An indirect mass flow meter calculates the mass flow rate by multiplying the density by the volumetric flow rate. In modern industrial production, operating parameters such as the temperature and pressure of the flowing fluid are continuously increasing. Under high-temperature and high-pressure conditions, the use of direct mass flow meters faces difficulties due to issues related to material properties and structure. Indirect mass flow meters, on the other hand, are often not suitable for practical use because density meters are limited by their tolerance to humidity and pressure. Therefore, temperature and pressure-compensated mass flow meters are widely used in industrial production. It can be regarded as a type of indirect mass flow meter; instead of using a density meter, it takes advantage of the relationship between temperature, pressure, and density. Temperature and pressure signals are processed through functional operations to generate a density signal, which is then multiplied by the volumetric flow rate to obtain the mass flow rate. Although temperature and pressure-compensated mass flow meters are now in practical use, it can be difficult or even impossible to achieve accurate compensation when the parameters of the medium being measured change significantly or rapidly. Therefore, further research into mass flow meters and density meters suitable for actual industrial applications remains an ongoing task. There are many flowmeters based on the aforementioned common structural principles, as compared to those of various other structures; for example, various weir-type and channel-type flowmeters suitable for measuring flow in open channels ； Insertion flow meter suitable for large-diameter flow measurement ； Laminar flow meter for measuring laminar flow rates ； Correlation flowmeter suitable for two-phase flow measurement ； As well as laser methods, nuclear magnetic resonance flowmeters, various tracing methods, dilution methods for flow measurement, etc. With the advancement of technology and the demands of practical applications, new types of flow meters will continue to emerge, resulting in a more diverse range of flow meter options.