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Types and Principles of Flow Meters I. Classification by Measurement Principle (1) Mechanical principle: Instruments based on this principle include differential pressure types and rotor types that make use of 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 volume types and weirs, troughs, etc. (2) Electrical principles: Instruments used for such principles include electromagnetic, differential capacitive, inductive, strain gauge-type instruments, etc. (3) Acoustic principles: Flow measurement based on acoustic principles includes ultrasonic type, acoustic type (shock wave type), etc. (4) Thermal principles: Methods for measuring flow rate using thermal principles include heat-type, direct calorimetry, indirect calorimetry, etc. (5) Optical principle: Laser-type, photoelectric-type, etc., are instruments that operate based on this principle. (6) Based on physical principles: Instruments such as nuclear magnetic resonance types and nuclear radiation types fall under this category of principles. (7) Other principles: There are the labeling principle (tracing principle, nuclear magnetic resonance 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: gear flow meters suitable for measuring liquid flow rates, vane flow meters (Rotary vane flow meters), rotary piston flow meters, and scraper-type flow meters ； Servo-type volumetric flow meters suitable for measuring gas flow rates, diaphragm flow meters, rotameter types, etc. 2. Impeller flow meters The working principle of impeller flow meters 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 measurement device (such as a pitot tube or venturi tube). Secondary devices consist of various mechanical, electronic, and combined differential pressure gauges, along with flow display instruments. The differential pressure sensing elements of these differential pressure gauges are mostly elastic elements. Since there is a square root relationship between differential pressure and flow rate, flow meters are equipped with square root devices to linearize the flow rate 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. The products currently manufactured include: orifice flow meters, wedge flow meters, venturi flow meters, and average pitot tubes. 4. Variable-area flow meters (isobaric drop flow meters): 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 is at rest can be used as a measure of the flow rate. Since the flow cross-sectional area of the flow meter 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 flow meter is known as a variable-area flow meter or an equal-pressure-drop flow meter. A typical instrument for this type of flow meter is the rotameter (float meter). 5. Momentum flow meter: A flow meter that uses the momentum of the flowing fluid to determine the flow rate is called a momentum flow meter. 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 meter A flow meter that uses the impulse theorem to measure flow rate is called an impulse flow meter; it is commonly used to measure the flow of granular solid materials, as well as sludge, 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. Its measurement accuracy and sensitivity are both relatively high. 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 prone to interference from external magnetic fields, which limits their widespread use in industrial fluid flow measurement. To this end, the products are constantly being improved and updated, moving towards miniaturization. 8. Ultrasonic flow meters An ultrasonic flow meter is designed based on the principle that the speed at which ultrasound propagates through a flowing medium is equal to the geometric 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. Classification of ultrasonic flowmeters: 1. Doppler ultrasonic flowmeter: Transducer 1 emits ultrasonic signals with a frequency of f1; after passing through the suspended particles or bubbles in the liquid within the pipeline, the frequency shifts and is reflected back to transducer 2 at a frequency of f2. This is the principle behind the Doppler effect, and the difference between f2 and f1 is known as the Doppler frequency shift fd. Let the fluid flow velocity be v, the ultrasonic wave speed be c, and the Doppler shift fd be proportional to the fluid flow velocity v. Once the pipeline conditions, transducer installation position, transmission frequency, and sound speed are determined, c, f1, and θ become constants. The fluid flow velocity is proportional to the Doppler shift; by measuring this shift, the fluid flow velocity can be determined, and thus the fluid flow rate can be calculated. 2. Time-difference ultrasonic flow meter: A time-difference ultrasonic flow meter measures fluid flow rate by utilizing the principle that the time difference between the propagation of sound waves in the fluid in the forward direction and in the reverse direction is proportional to the fluid flow velocity. 9. Fluid oscillation flow meter The fluid oscillation flow meter is designed based on the principle that fluids undergo oscillations when flowing through specific 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, as it combines the 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 meter 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 dielectric 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 given considerable attention. Mass flow meters are divided into direct and indirect types. 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. Indirect mass flow meters determine the mass flow rate by directly multiplying a densitometer reading by the volumetric flow rate. There are also various weir-type flow meters and flume-type flow meters suitable for open-channel flow measurement ； Insertion flow meter suitable for large-diameter flow measurement ； Laminar flow meters for measuring laminar flow rates ； Correlation-type flowmeter suitable for two-phase flow measurement ； As well as laser methods, nuclear magnetic resonance flow meters, and various tracing methods and dilution-based flow measurement techniques.