Introduction to the seven most commonly used types of flow meters

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　　There are a wide variety of flow measurement methods and instruments, as well as many ways to classify them. To date, as many as 60 types of flow meters are available for industrial use. The reason for there being so many varieties is that to date, no flow meter has been found that is suitable for any fluid, any range, any flow condition, or any set of operating conditions. 　　Below, in accordance with the most popular and widely used classification system at present, namely differential pressure flowmeters, float flowmeters, turbine flowmeters, electromagnetic flowmeters, vortex flowmeters, mass flowmeters, and insert-type flowmeters, the principles, characteristics, general applications, as well as the development trends both domestically and internationally of various types of flowmeters are discussed separately. 　　1.1 Differential pressure flow meter A differential pressure flow meter is a device that calculates flow rate based on the differential pressure generated by a flow sensing element installed in the pipeline, along with the known properties of the fluid and the geometric dimensions of the sensing element and the pipeline. 　　A differential pressure flow meter consists of a primary device (the sensing element) and a secondary device (the differential pressure conversion and flow display instrument). Differential pressure flowmeters are usually classified by their sensing elements, such as orifice flowmeters, venturi flowmeters, and average velocity tube flowmeters. 　　Secondary devices include various mechanical, electronic, and mechatronic differential pressure gauges, differential pressure transmitters, and flow display instruments. It has evolved into a large category of instruments with a high degree of serialization, generalization, and standardization, featuring a wide variety of types and specifications. These instruments can be used to measure flow rates as well as other parameters such as pressure, level, density, etc. 　　Differential pressure flowmeters are the most widely used type of flowmeter, accounting for the largest share among all types of flow measurement devices. In recent years, due to the emergence of various new types of flow meters, their usage percentage has gradually declined; however, they remain the most important category of flow meters. 　　Advantages: 　　(1) The orifice plate flow meter, which is the most widely used type, features a robust structure, stable and reliable performance, and a long service life; 　　(2) It has a wide range of applications, and no other type of flow meter can compare with it to date; 　　(3) The sensing element, the transmitter, and the display instrument are manufactured by different manufacturers, which facilitates cost-effective production on a large scale. 　　Disadvantages: (1) The measurement accuracy is generally low; (2) The range is narrow, usually only 3:1 to 4:1; (3) High requirements are placed on the installation conditions in the field; (4) High pressure loss (referring to orifice plates, nozzles, etc.). 　　1.2 Float Flow Meters Float flow meters, also known as rotor flow meters, are a type of variable-area flow meter. In a vertical conical tube that widens from bottom to top, the gravity of a float with a circular cross-section is counteracted by the fluid dynamics, allowing the float to rise and fall freely within the conical tube. 　　Float flowmeters are the type of flowmeter with the widest range of applications, second only to differential pressure flowmeters; they play a crucial role, especially in measuring small and micro flow rates. 　　Features: 　　(1) The glass cone tube float flow meter has a simple structure and is easy to use, but its disadvantage is its low pressure resistance, as well as the significant risk of the glass tube breaking; 　　(2) It is suitable for small pipe diameters and low flow rates; 　　(3) It has low pressure loss. 　　1.3 Turbine flowmeters Turbine flowmeters are one of the main types of velocity-type flowmeters; they use a multi-bladed rotor (turbine) to detect the average flow velocity of the fluid, thereby determining the flow rate or total volume. 　　Generally, it consists of a sensor and a display unit, or it can be designed as an integrated unit. 　　Turbine flowmeters, positive displacement flowmeters, and Coriolis mass flowmeters are considered to be the three types of flowmeters with the best repeatability and accuracy. As one of the ten major types of flowmeters, these products have been developed into a range of varieties and series that are produced on a large scale. 　　Advantages: 　　(1) High precision – it is the most accurate flow meter among all types of flow meters; 　　(2) Good repeatability; 　　(3) No zero drift, and excellent resistance to interference; 　　(4) Wide measurement range; 　　(5) Compact design. 　　Disadvantages: (1) It cannot maintain its calibration characteristics over a long period of time; (2) The properties of the fluid have a significant impact on the flow rate characteristics. 　　1.4 Electromagnetic flowmeter An electromagnetic flowmeter is a device for measuring conductive liquids, based on Faraday’s law of electromagnetic induction. 　　Electromagnetic flowmeters possess a range of excellent features that enable them to address issues that are difficult for other types of flowmeters to handle, such as the measurement of dirty or corrosive fluids. In the 1970s and 1980s, significant technological advancements were made in electromagnetic flowmeters, which enabled them to become a widely used type of flowmeter; their share in the total number of flow measurement devices continued to increase. 　　Advantages: 　　(1) The measurement channel is a smooth straight tube that does not get clogged, making it suitable for measuring liquid-solid two-phase fluids containing solid particles, such as pulp, sludge, and sewage. 　　(2) It does not cause pressure losses resulting from flow measurement, thus offering good energy-saving effects. 　　(3) The measured volumetric flow rate is essentially unaffected by changes in fluid density, viscosity, temperature, pressure, or conductivity. 　　(4) It has a wide range of flow rates and a broad range of pipe diameters. 　　(5) It can be used with corrosive fluids. 　　Disadvantages: (1) It cannot measure liquids with very low conductivity, such as petroleum products; (2) It cannot measure gases, vapors, or liquids containing large bubbles; (3) It cannot be used at high temperatures. 　　1.5 Vortex Flow Meter A vortex flow meter is a device in which a non-streamlined vortex generator is placed within the fluid; the fluid separates on either side of this generator, resulting in two sequences of vortexes that are arranged in a regular, alternating pattern. 　　Vortex flowmeters can be classified according to their frequency detection methods into stress-type, strain-type, capacitive type, thermosensitive type, vibrating-body type, photoelectric type, and ultrasonic type, among others. 　　Vortex flowmeters belong to the youngest category of flowmeters, but they have developed rapidly and are now become a commonly used type of flowmeter. 　　Advantages: (1) Simple and robust structure; (2) Suitable for a wide range of fluids; (3) High precision; (4) Wide measurement range; (5) Low pressure loss. 　　Disadvantages: (1) Not suitable for measurements at low Reynolds numbers; (2) Requires a long straight pipe section; (3) Has a lower coefficient of performance compared to turbine flowmeters; (4) Lacks experience in application with pulsating flows and multiphase flows. 　　1.6 Ultrasonic flowmeters Ultrasonic flowmeters are instruments that measure flow rate by detecting the effect of fluid flow on an ultrasonic beam (or ultrasonic pulse). 　　Based on the principles of signal detection, ultrasonic flowmeters can be classified into methods based on the difference in propagation speed (direct time difference method, time difference method, phase difference method, and frequency difference method), beam deflection method, Doppler method, cross-correlation method, spatial filtering method, and noise method, among others. 　　Like electromagnetic flowmeters, ultrasonic flowmeters also belong to the category of obstruction-free flowmeters, as there are no obstacles in their flow channels. They are suitable for addressing difficulties in flow measurement, and they have particular advantages in measuring large-diameter flows. In recent years, they have been one of the types of flowmeters that have seen rapid development. 　　Advantages: 　　(1) It enables contactless measurement; 　　(2) It is a measurement without flow obstruction, resulting in no pressure loss; 　　(3) It can measure non-conductive liquids, serving as a complement to electromagnetic flowmeters that require unobstructed measurement. 　　Disadvantages: (1) The propagation time method can only be used for clean liquids and gases, while the Doppler method can only be used to measure liquids containing a certain amount of suspended particles and bubbles; (2) The measurement accuracy of the Doppler method is not high. 　　1.7 Open-channel flow meters Different from the previous types, these are instruments used to measure the flow rate of free-surface flow in open channels that are not completely filled. 　　A water channel with flow that is not at full capacity is called an open channel, and a device used to measure the flow rate of water in such channels is known as an open-channel flow meter. 　　In addition to circular shapes, open-channel flow meters also come in various other shapes such as U-shaped, trapezoidal, and rectangular. 　　Applications of open-channel flow meters include urban water supply channels; intake and discharge channels in thermal power plants, as well as channels for sewage treatment and discharge; water discharge from industrial and mining enterprises; and channels used in hydraulic engineering and agricultural irrigation. Some estimates put the figure at 1,995 units, accounting for about 1.6% of all flow meters, but there are no estimated figures available for their use in China.