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In chemical production, it is often necessary to measure the amount of a medium or to determine its flow rate, for which flow meters are required. Based on the different structures of flow meters, they can be roughly classified into: flow meters ； Volumetric flow meter ； Electromagnetic flowmeter ； Differential pressure flow meter ； Turbine flow meter ； Vortex flow meter ； Mass flow meter. We will briefly describe their structure, suitable environments, and requirements for selection. 1. The flow meter, also known as a float flow meter HK-FZ, is a relatively simple type of flow meter. It is mainly used for measuring flow rates in medium and small diameter pipes in applications where relatively low precision requirements apply. It is generally used for diameters smaller than ф50, with a wide range; typically 10:1, with a minimum of 5:1. Low pressure loss. Its sensing element consists of a vertical conical tube that widens from bottom to top, and a float that moves up and down along the conical tube as the fluid flow rate changes. As the fluid flows upward past the float, forces such as differential pressure, fluid dynamic pressure, and friction act on the float; these forces balance the downward weight of the float. Different positions of the float in the cone tube represent different flow rates. The common flow meters include glass tube flow meters and metal tube float flow meters. Glass tube flowmeters have a simple structure, are inexpensive, and easy to install and use; they are therefore a type of flowmeter that is widely used in production and scientific research experiments. However, due to the limitations of the glass tube material, it cannot be used in fragile or high-temperature and high-pressure environments. And it cannot be transmitted over long distances. Compared to glass tube flowmeters, metal tube flowmeters also feature resistance to high pressure and high temperatures, a robust structure, and the ability to avoid breaking. It can adapt to harsh working conditions. It can also transmit over long distances, with a standard signal output. The downside is that it’s relatively expensive. 2. Positive displacement flow meters: Positive displacement flow meters are the most accurate type of flow meters among all types of flow meters. The main types include helical gear type, worm gear type, screw type (double rotor type), scraper type, piston type and vane type, drum type, and membrane type (for gas measurement). The working principle of positive displacement flow meters relies on two specially shaped measuring elements that rotate while in contact (or out of contact) with each other; taking elliptical gears as an example, these are two intermeshing gears, one being the driving gear and the other the driven gear. When material enters, the driving wheel, under the influence of pressure, causes the driven wheel to operate; with each rotation of the rotor, four volumes of fluid from the crescent-shaped cavities formed by the rotor and the shell wall are discharged. By its relationship with time, the instantaneous flow rate and cumulative flow rate can be calculated. The main advantage of positive displacement flowmeters is their high measurement accuracy ； The installation of straight pipe sections has little impact on measurement accuracy ； Suitable for high-viscosity applications ； The range is relatively wide, generally between 5:1 and 10:1, with special cases reaching 30:1 or higher ； It can utilize medium dynamics, requires no external energy source, offers direct reading, and is simple and convenient. The disadvantage is that it is generally only suitable for small to medium diameters ； The price is relatively high ； Due to part deformation, it is generally not suitable for use in high and low temperatures, within the range of -30 to 160℃ ； In the measurement using positive-displacement flowmeters, it is necessary to eliminate particulate impurities; therefore, a filter is usually installed before the flowmeter, although this increases the pipeline resistance ； And gas-liquid mixing must be prevented ； Due to the small gap, the detection element is prone to getting stuck, resulting in poor safety. 3. Electromagnetic flowmeter HK-LDE: An electromagnetic flowmeter measures the flow rate of a fluid by taking advantage of its electrical conductivity. Its feature is that when measuring fluid flow, this instrument is not affected by temperature, pressure, density, viscosity, or the composition of the fluid. Since there are no protruding parts or moving components inside the tube, it is suitable for measuring wastewater containing suspended solid particles as well as coal slurry. It is particularly suitable for measuring corrosive media. It has a wide measurement range and can measure the flow rate of fluids in both forward and reverse directions. With current technology, it is required that the conductivity of the medium under test reach or exceed 0.01 u S/cm. 4. Differential pressure flow meters (with throttling elements): HK-LGBH or HK-LG. Differential pressure flow meters have a wide range of applications, among which the throttling-type differential pressure flow meters are the most widely used. Its working principle is as follows: the fluid filling the pipe, as it flows past the throttling element within the pipe, causes the flow stream to narrow locally at that element. At this point, the flow velocity increases while the static pressure decreases, resulting in a pressure difference before and after the throttling element. The greater the flow rate, the larger the pressure difference; thus, the flow rate can be determined based on this pressure difference. Classified by structural form, they can be divided into standard orifice plates, standard nozzle types, classic venturi tubes, venturi nozzles, 1/4 circle orifice plates, conical inlet orifice plates, annular orifice plates, eccentric orifice plates, etc. Its advantages and disadvantages are: it has a relatively simple structure, stable performance, a long service life, and is inexpensive ； A wide variety of options available, with many different types ； Repeatability and precision are average, with a narrow range, typically 3:1 to 4:1 ； The head loss is relatively high ； It has high installation requirements, such as the need for long straight pipe sections. 5. Turbine flowmeter HK-LWGQ: The turbine flowmeter is the main type among impeller flowmeters; it also includes anemometers and water meters. Its principle is as follows: when the fluid to be measured flows through the sensor, the force exerted by the fluid causes the impeller to rotate; the rotation speed is proportional to the average flow velocity in the pipeline. The rotation of the impeller changes the magnetic resistance value of the magnetoelectric converter, resulting in periodic changes in the magnetic flux within the detection coil. This generates a periodic induced electromotive force, namely a pulse signal, which, after being amplified by an amplifier, is sent to a secondary instrument for display. Its advantages are: high accuracy, ranging from ±0.25% to ±0.5%R, and precision of up to ±0.15%R (for liquids) ； Good repeatability ； No zero drift, strong resistance to interference ； Compact and lightweight, easy to install and maintain, with high flow capacity ； Good safety performance (even if the impeller fails and gets stuck, flow will not be interrupted) ； The disadvantage is that changes in the properties of the fluid (density, viscosity) have a significant impact on the dosimeter, requiring compensatory measures to be taken ； The instrument is greatly affected by the flow velocity distribution and swirling flow; therefore, a sufficient straight pipe section must be maintained upstream and downstream of the sensor ； High requirements are placed on the cleanliness of the medium ； Flowmeters with a diameter of DN50mm or less are significantly affected by the properties of the fluid, making it difficult for them to maintain excellent performance. 6. Vortex flow meter HK-LU: The working principle of a vortex flow meter involves the use of a vortex generator within the fluid; this causes regular vortices to be generated alternately on both sides of the generator. These vortices are arranged asymmetrically downstream of the vortex generator, resulting in a certain frequency. This frequency can be calculated using the formula f = St × v / (1 – 1.27d/D) × d, where St is the Strouhal number, a dimensionless value that is related to the vortex generator and the Reynolds number ； v is the flow velocity ； d is the frontal width of the occurring body ； The flow velocity can be determined using D (the nominal diameter). Generally speaking, the output signal (frequency) of a vortex flow meter is not affected by changes in the physical properties or composition of the fluid; this means that the instrument coefficient depends only on the shape and size of the vortex generator as well as the Reynolds number. Its advantages are: simple and robust structure, as well as easy installation and maintenance ； Suitable for a variety of fluids, including liquids, gases, vapors, and some mixed phases ； It has high accuracy, generally around ±1%R ； Wide flow range, up to 10:1 or 20:1 or higher ； Low head loss ； No zero drift ； The price is relatively cheap ； The disadvantage is that it is not suitable for low Reynolds number conditions (Re < 20,000), and its use is limited in cases of high viscosity, low flow velocity, and small diameters ； High requirements are placed on the environment; areas with vibrations should be avoided as much as possible, and there needs to be a long straight pipe section on the upstream side ； The instrument coefficient is low, and it becomes lower as the diameter increases. The signal resolution decreases, so the diameter should not be too large; it is generally used for DN25~DN300mm. 7. Mass flow meter (Siemens) – A mass flow meter is a new type of flow measurement device that can be used to directly measure the mass flow rate and density of a medium. It features high measurement accuracy, a wide range of operation, good stability, and low maintenance requirements, and is widely used in the petrochemical industry. They are divided into Coriolis mass flow meters and thermal mass flow meters. The Coriolis mass flow meter is commonly used. It is a flow meter based on the Coriolis force principle discovered by the Greeks. Inside the sensor’s housing, there is a pair of parallel measuring tubes that, under the action of electromagnetic drive coils mounted at the ends of the tubes, vibrate in a manner similar to that of a tuning fork. When a fluid flows through two parallel measuring tubes, an acceleration perpendicular to the direction of flow is generated, along with a corresponding Coriolis force. This force causes the measuring tubes to oscillate and twist, and this twisting phenomenon is known as the Coriolis effect. The degree of twist of the measuring tube is directly proportional to the mass flow rate passing through it. Mass flow meters are characterized by their high precision ； The measured mass flow rate is independent of the fluid’s temperature, pressure, viscosity, conductivity, and flow state, but the fluid must be fully filled ； It is necessary to ensure that the measuring tube is free from corrosion, wear, and scaling, as all of these can affect the accuracy of measurements ； The zero-point drift is large, and the head loss is high ； Avoid mixing liquid and gas ； Sufficiently strong supports must be installed in front of and behind the sensor flange to prevent pipeline vibrations from causing measurement errors.