Selection of flow meters

1. Overview　　　An electromagnetic flowmeter (hereinafter referred to as EMF) is a device that measures the volumetric flow rate of conductive liquids, and it is based on Faraday’s law of electromagnetic induction. EMF saw industrial application in the early 1950s, and in recent years, its global production accounts for approximately 5% to 6.5% of the total number of industrial flow meters. Since the 1970s, the key-controlled low-frequency rectangular wave excitation method has emerged, gradually replacing the power-frequency AC excitation method used in earlier times. This has led to a significant improvement in the performance of these instruments, resulting in their more widespread use. 　　2. Principles and Mechanisms The basic principle of EMF is Faraday’s law of electromagnetic induction, which states that when a conductor moves through a magnetic field, cutting across the magnetic field lines, an induced electromotive force is generated at its ends. As shown in Figure 1, a conductive fluid flows within a non-magnetic measuring tube perpendicular to the magnetic field, generating an induced electromotive force that is proportional to the flow rate in a direction perpendicular to the flow direction. The direction of this electromotive force follows Fleming’s right-hand rule, and its value is given by the following formula: where E represents the induced electromotive force, i.e., the flow rate signal, in volts; k is a coefficient ； B-----magnetic flux density, T ； D----Inner diameter of the measurement tube, m ； --- Average flow velocity, m/s. Let the volumetric flow rate of the liquid be , then in the equation, K is the instrument constant, with K = 4 KB/πD. The EMF consists of two main components: a flow sensor and a converter. The typical structure of the sensor is shown in Figure 2; excitation coils are installed at the top and bottom of the measurement tube, and when an excitation current flows through them, a magnetic field is generated that passes through the measurement tube. A pair of electrodes is placed on the inner wall of the measurement tube in contact with the liquid, thereby generating an induced electromotive force which is sent to the converter. The excitation current is provided by the converter. 　　3. Advantages The measurement channel of EMF is a smooth, straight tube without any flow-blocking elements; therefore, it is not prone to blockage and is suitable for measuring liquid-solid two-phase fluids containing solid particles or fibers, such as pulp, coal water slurry, mineral slurry, mud, and sewage. EMF does not cause any pressure loss resulting from flow measurement; the resistance of the instrument is merely that of a pipe of the same length, resulting in significant energy savings. It is particularly suitable for large-diameter water supply pipes where low resistance losses are required. The volumetric flow rate measured by EMF is actually not significantly affected by changes in fluid density, viscosity, temperature, pressure, and conductivity (as long as they remain above a certain threshold). Compared to most other flow meters, the requirement for a straight pipe section ahead is lower. EMF has a large measurement range, typically 20:1 to 50:1, with a wide selectable flow range. The full-scale liquid flow rate can be selected within the range of 0.5 to 10 m/s. Some models of meters allow the flow rate to be increased or decreased on-site as needed (for example, they are equipped with 4-digit potentiometers for setting the meter constants), eliminating the need to remove them for offline calibration. The diameter range of EMF flow meters is wider than that of other types of flow meters, ranging from a few millimeters to 3 meters. It can measure flow in both forward and reverse directions, as well as pulsating flow, provided that the pulsation frequency is much lower than the excitation frequency. The instrument output is essentially linear. It is easy to select the material type for fluid-contact parts, and it can be used with corrosive fluids. 　　4. Disadvantages EMF cannot measure liquids with very low conductivity, such as petroleum products and organic solvents. Gases, vapors, and liquids containing many large bubbles cannot be measured. Due to the limitations of the lining material and electrical insulation materials, general-purpose EMFs cannot be used with liquids at higher temperatures ； Some models of instruments are used for liquids below room temperature, and the insulation is damaged due to condensation (or frost) on the outside of the measuring tube. 　　5. Classification 　　General-purpose products and specialized instruments available on the market can be classified from different perspectives. Classified by excitation current, there are DC excitation, AC (power frequency or other frequencies) excitation, low-frequency rectangular wave excitation, and dual-frequency rectangular wave excitation. The waveforms of several excitation methods are shown in Figure 3. Classified by the configuration of the output signal wiring and the excitation (or power supply) wiring, there are four-wire systems and two-wire systems. Classified by the assembly method of the converter and sensor, there are separate-type and integrated-type. Classified by the method of connecting the flow sensor to the pipeline, there are flange connection, flange-clamped connection, sanitary connection, and threaded connection. Classified by whether the flow sensor electrodes are in contact with the liquid being measured, there are contact type and non-contact type. Classified by the structure of the flow sensor, there are short-tube type and insertion type. Classified by application, there are general-purpose, explosion-proof, sanitary, water-resistant, and submersible types. 　　6. Selection Considerations 　　 　　6.1 Application Overview 　　EMF has a wide range of application areas. Large-diameter instruments are widely used in water supply and drainage projects. Small and medium diameter pipes are commonly used for measuring difficult-to-measure fluids with both solid and liquid phases, or in applications with high requirements. Examples include measuring pulp and black liquor in the papermaking industry, slurry in the non-ferrous metallurgy industry, coal slurry in coal processing plants, highly corrosive liquids in the chemical industry, as well as for controlling and detecting leaks in the cooling water systems of blast furnaces in the steel industry. They are also used for measuring and controlling flow rates in the hydraulic transport of coal over long distances through pipelines. Small-diameter and ultra-small-diameter pipes are commonly used in industries such as the pharmaceutical industry, food industry, and bioengineering, where hygiene standards are important. 　　6.2 Accuracy Level and Functions The performance of general-purpose EMFs available on the market varies greatly; some have high accuracy and numerous functions, while others have lower accuracy and simpler functions. Instruments with high precision have a basic error of (±0.5%～±1%)R, while those with lower precision have an error of (±1.5%～±2.5%)FS; the price difference between the two types is 1 to 2 times. Therefore, in applications where high measurement accuracy is not required (for example, in non-trade accounting where control is the only goal and only high reliability and good repeatability are needed), it is not economical to use high-precision instruments. Some models of meters claim to have higher accuracy, with a basic error of only (±0.2%–±0.3%)R, but they require strict installation conditions and reference parameters; for example, the ambient temperature should be between 20–22°C, and the lengths of the straight sections before and after the meter must be greater than 10D and 3D respectively (usually 5D and 2D). In addition, it is recommended that the flow sensor be integrated with the straight sections before and after it in a flow standard apparatus for actual-flow calibration, in order to minimize the impact of improper installation. Therefore, when comparing different models, one should not focus solely on high specifications; instead, it is necessary to read the manufacturer’s brochures or instructions in detail for a comprehensive analysis. The functions of EMFs available on the market vary greatly; some simple models are designed solely to measure one-way flow, outputting only analog signals to drive instruments located downstream ； The multi-functional meter features the ability to measure two-way flow, range switching, upper and lower flow limit alarms, empty pipe and power outage alarms, small signal rejection, flow display and total volume calculation, automatic verification and fault self-diagnosis, communication with a host computer, and motion configuration. Some models of instruments offer a serial digital communication function with various communication interfaces and dedicated chips (ASICs) to connect to HART protocol systems, PROFINET, Modbus, CONFIG, FF fieldbuses, and others. 　　6.3 Flow rate, full-scale flow, range, and diameter The diameter of the instrument selected need not be the same as that of the pipe; it should be determined based on the flow rate. The process industry transports liquids with different viscosities such as water, and the pipe flow velocity is generally the economic flow velocity of 1.5 to 3 m/s. EMF is used in such pipes; the sensor diameter needs to be the same as that of the pipe. At the EMF full-scale flow rate, the liquid flow velocity can be selected within the range of 1–10 m/s, which is a fairly wide range. In principle, there is no limit to the upper flow velocity; however, it is generally recommended not to exceed 5 m/s, unless the lining material can withstand the scouring effect of the fluid flow. In practical applications, the flow velocity rarely exceeds 7 m/s, and values above 10 m/s are even rarer. The lower limit of the flow velocity at full flow rate is generally 1 m/s, while for some model instruments it is 0.5 m/s. In some newly constructed systems where the flow rate is low or the flow velocity is low during the initial operation phase, from the perspective of measurement accuracy, it is necessary to use instruments with an aperture smaller than that of the pipe, connecting them via reducers. For fluids containing substances prone to adhesion, deposition, and scaling, a flow rate of not less than 2 m/s is recommended; ideally, it should be increased to 3–4 m/s or higher, to facilitate self-cleaning and prevent adhesion and deposition. For highly abrasive fluids such as slurry, the recommended flow rate should be below 2–3 m/s in order to reduce wear on the lining and electrodes. When measuring low-conductivity liquids close to the threshold, it is advisable to choose a lower flow rate (below 0.5–1 m/s), as an increase in flow rate raises flow noise and leads to fluctuations in the output. The range of EMF is quite large; it is usually not less than 20, and for instruments with automatic range switching, it can exceed 50–100. The diameter of standardized products available domestically ranges from 10 mm to 3000 mm; although small and medium diameters are more commonly used in practical applications, large-diameter instruments account for a larger proportion compared to most other flow meters based on different principles (such as positive displacement, turbine, vortex shedding, or Coriolis mass flow meters). Among the nearly 10,000 instruments in a certain company, those with small diameters of less than 50 mm account for 37%, those with medium diameters ranging from 65 to 250 mm account for 45%, those with large diameters ranging from 300 to 900 mm account for 15%, and those with ultra-large diameters of over 1000 mm account for 3%. 　　6.4 Liquid Conductivity 　　A prerequisite for using EMF is that the liquid being measured must be conductive, and its conductivity must not be below a certain threshold (i.e., the minimum value). A conductivity below the threshold causes measurement errors until it becomes unusable; above the threshold, measurements can still be taken even if there are changes, and the error in the indicated value remains relatively constant. The threshold for general-purpose EMFs ranges from 10-4 to (5×10-6) S/cm, depending on the model. In use, it also depends on the length of the flow signal line between the sensor and the converter as well as its distributed capacitance; the manufacturer’s instructions typically specify the length of the signal line corresponding to a certain conductivity. Meters with non-contact capacitive coupling for large-area electrodes can measure liquids with a conductivity as low as 5×10-8 S/cm. The conductivity of industrial water and its aqueous solutions is greater than 10-4 S/cm, while that of acids, bases, and salts ranges from 10-4 to 10-1 S/cm; in these cases, there are no issues with their use. Low-purity distilled water has a conductivity of 10-5 S/cm, and again, there are no problems with it. Petroleum products and organic solvents cannot be used if their conductivity is too low. Table 1 lists the conductivity of several liquids. According to available information, some pure liquids or aqueous solutions have low conductivity and are considered unsuitable for use. However, in practical applications, there are cases where they can still be used due to the presence of impurities that contribute to an increase in conductivity. For aqueous solutions, the conductivity values provided in the data were measured in the laboratory using pure water as a base; however, the actual solutions used in practice may be prepared with industrial water, resulting in a conductivity level that is higher than those listed, which in turn is beneficial for flow measurement. 　　Table 1: Conductivity of various liquids at 20°C
Liquid name | Conductivity
--- | ---
Petroleum | (3–5)×10-13
Propane | (2–6)×10-8
Pure water, highly distilled water | 4×10-8
Benzene | 7.6×10-8
Liquid ammonia | 1.3×10-7
Methanol | (4.4–7.2)×10-7
Drinking water | ≈10-4
Seawater | ≈4×10-2
Sulfuric acid (5%–99.4%) | (2.1×10-1)–(8.5×10-3)
Ammonia solution (4%–30%) | (1×10-3)–(2×10-4)
Sodium hydroxide (4%–50%) | (1.6×10-1)–(8×10-2)
Saltwater (2.5%) | 2×10-1
Based on practical experience, the conductivity of liquids used in actual applications should be at least one order of magnitude higher than the threshold specified by the instrument manufacturers. This is because the lower limit specified in the manufacturer’s instrument specifications represents the lowest value that can be measured under optimal operating conditions. It is subject to certain usage constraints, such as conductivity uniformity, connected signal lines, and external noise; otherwise, issues like output fluctuations may occur. We have encountered on many occasions the situation where when measuring low-purity distilled water or deionized water, its conductivity is close to the threshold of 5×10-6 S/cm, resulting in fluctuations in the output during use. 　　6.5 Contaminants in the liquid 　　Tiny bubbles mixed into the flow can still allow normal operation, but the measured value is the volumetric flow rate of the mixture including those bubbles ； If the gas content increases to the point where a plug-like flow is formed, the electrodes may be covered by the gas, causing the circuit to be interrupted momentarily; this can result in fluctuations in the output or even prevent normal operation. The volumetric flow rate of the two phases can also be measured for solid-liquid biphasic fluids containing non-ferrimagnetic particles or fibers. Fluids with a high solid content, such as drilling mud, drilling cement slurry, and pulp, are actually non-Newtonian fluids. Since the solids flow together with the carrier fluid, there is sliding between them and a difference in velocities; therefore, instruments calibrated for single-phase fluids will introduce additional errors when used with solid-liquid two-phase flows. Although there are no systematic experimental reports on the effect of solids in solid-liquid two-phase fluids using EMF, foreign studies indicate that when the solid content is 14%, the error remains within 3% ； An experimental report from the Institute of Water Resources Science of China’s Yellow River Water Resources Commission states that when measuring the flow rate of water with a high sediment content, the volume ratio of sediment to water ranges from 17% to 40% (with an average particle size of sand of 0.35 mm), and the measurement error of the instruments is less than 3%. When larger particles in the slurry scrape against the electrode surface, spike-like slurry noise is generated in the EMF produced by low-frequency rectangular excitation, causing instability in the flow rate signal. In such cases, it is necessary to use instruments with a higher frequency or those capable of effectively suppressing slurry noise; alternatively, instruments powered by AC mains or those with dual-frequency excitation can also be used. Fluids containing ferromagnetic materials introduce measurement errors in normal EMF readings, as the magnetic permeability within the measuring tube changes depending on the amount of ferromagnet present. However, the EMF compensated by a flux detection coil placed in the magnetic circuit can reduce the influence of interference from ferromagnets. In its experimental report on AC-excited meters, Shanghai Guanghua Instrument Factory states that when using a slurry containing iron ore with a liquid-to-solid weight ratio of approximately 4:1 and particle sizes of ≤0.15 mm, and conducting comparison tests on flow rates for clean water and this slurry using meters with an 80 mm diameter, the reading of ordinary meters changes by 7% to 10%; whereas meters equipped with flux detection coils have a reading error within ±2% of the full scale. When applying this to slurry containing ore particles, attention should be paid to the degree of wear on the sensor lining, as an increase in the inner diameter of the measurement tube will cause additional errors. For such applications, ceramic linings or polyurethane rubber linings with good wear resistance should be used. It is also recommended to install the sensors on vertical pipes, so as to ensure even wear across the pipe and eliminate the problem of severe localized wear in the lower part when the pipe is installed horizontally. A nozzle-shaped sleeve can also be installed at the sensor inlet to relatively extend its service life. 　　6.6 Adhesion and Precipitation When measuring fluids in which substances tend to adhere to and precipitate on the tube walls, if the substance adhering is a conductive one with a higher electrical conductivity than the liquid, the signal potential will be short-circuited and the device will not function. In the case of non-conductive layers, attention should first be paid to electrode contamination; for example, it is advisable to use electrodes with sharp or hemispherical protrusions that are less prone to adhesion, replaceable electrodes, or scraper-type cleaning electrodes. The scraper electrode allows for the regular manual removal of deposits outside the sensor. Foreign products once used ultrasonic transducers on the electrodes to remove surface scale, but this is now rare. It is also possible to temporarily disconnect the measurement circuit and pass a low-voltage, high-current through the electrodes for a short period of time to burn away the adhered grease layer. Areas prone to fouling can have their flow rate increased to achieve self-cleaning, and easier-to-clean pipe connections can be used, allowing the sensors to be cleaned without disassembly. For contactless electrodes with an EMF non-conductive film layer, the instrument can still function; however, if it is a highly conductive layer, the instrument will not work either. 　　6.7 Selection of Materials for Components in Contact with Fluids The sensor components that come into contact with fluids include liners (or measurement tubes made of insulating materials), electrodes, grounding rings, and gaskets. The corrosion resistance, wear resistance, and maximum operating temperature of these materials affect the instrument’s suitability for working with various fluids. Due to its few components, simple shape, and flexible material selection, the electromagnetic flowmeter has strong adaptability to fluids. 　　(1) Lining material (or the measuring tube in direct contact with the medium) Common lining materials include fluoroplastics, polyurethane rubber, neoprene, and ceramics. In recent years, linings made of high-purity alumina (999.7% Al2O3) ceramics have been used, but only for sensors with small to medium diameters. Neoprene and fiberglass are used for non-corrosive or mildly corrosive liquids such as industrial water, wastewater, and weak acids and bases, and they are the most cost-effective options. Fluoroplastics have excellent chemical resistance, but poor wear resistance, and cannot be used for measuring slurry. Polytetrafluoroethylene was the first fluoroplastic to be used; since it only adheres to the measuring tube by pressure without any bonding force, it cannot be used in negative-pressure pipelines. Later, various modified versions were developed that allow for injection molding, resulting in a stronger bond with the measuring tube and making them suitable for use in negative-pressure environments. Polyurethane rubber has excellent wear resistance, but its resistance to acid and alkali corrosion is poor. Its wear resistance is 10 times that of natural rubber, and it is suitable for coal slurry, mineral slurry, etc ； The medium temperature should be below 40–60/70°C. Alumina ceramics exhibit excellent wear resistance as well as resistance to wear and corrosion caused by strong acids and bases; their wear resistance is about 10 times that of polyurethane rubber, making them suitable for use in corrosive slurries ； However, it is brittle; care must be taken during installation to avoid breaking it. It can be used at higher temperatures (120–140/180°C), but sudden temperature changes must be avoided. For example, when using steam sterilization, the temperature change should not exceed 100°C, and it should take 10 minutes to raise the temperature to 150°C. The general pressure and temperature range of applicability for several materials in general-purpose EMFs can be referred to in Figure 4. 　　(2) Electrode and grounding ring materials The corrosion resistance of the electrodes against the measuring medium is the primary factor in selecting materials; secondary considerations include whether surface effects such as passivation will occur and the noise generated as a result. 　　1) Selection of corrosion-resistant materials: EMF electrodes require high corrosion resistance. Commonly used metal materials include molybdenum-containing acid-resistant steel Icr18Ni12Mo2Ti, Hastelloy (corrosion-resistant nickel-based alloys) types B and C, titanium, tantalum, and platinum-iridium alloys, which can handle almost all types of chemical solutions. In addition, there are low-noise electrodes suitable for slurries and the like, which are conductive rubber electrodes, conductive fluoroplastic electrodes, and porous ceramic electrodes, or metal electrodes coated with these materials. In principle, the selection of electrode materials should be determined by the user based on the actual applications of this material in other devices and past experience. Sometimes, necessary experiments need to be conducted, such as collecting liquid samples on-site for corrosion tests on the materials to be used in the laboratory. The best experiment is to conduct the test on-site with actual samples, as this is a corrosivity test that most closely resembles real-world application conditions, allowing for relatively reliable conclusions regarding suitability. 　　2) Avoiding electrode surface effects: The corrosion resistance of electrodes is an important factor in selecting materials. However, even though an electrode material may have excellent corrosion resistance against the medium being tested, it is not necessarily the suitable material; it is also necessary to avoid electrode surface effects. Electrode surface effects are divided into three aspects: surface chemical reactions, electrochemical and polarization phenomena, and the catalytic action of the electrode. Chemical reaction effects include the formation of a passivation film or oxide layer when the electrode surface comes into contact with the medium being tested. They may provide positive protection for corrosion resistance, but they could also increase surface contact resistance. For example, tantalum oxidizes when in contact with water, forming an insulating layer. Regarding the matching of dielectric-electrode materials to avoid or mitigate electrode surface effects, there is not as much available information as there is for corrosion; only limited experience exists, which still needs to be accumulated through practice. Grounding rings are connected to both ends of flow sensors in plastic pipes or metal pipes with insulating linings; their corrosion resistance requirements are lower than those of electrodes, and they can withstand a certain degree of corrosion before requiring replacement on a regular basis. Acid-resistant steel or Hastelloy is usually selected. Due to their large size, precious metals such as tantalum and platinum are less commonly used for economic reasons. If the metal process piping is in direct contact with the fluid, a grounding ring is not required. 　　7. Precautions for installation and use 　　7.1 General precautions during use: The liquid should have the conductivity required for measurement, and its conductivity distribution should be roughly uniform. Therefore, the flow sensor should be installed away from areas where conductivity variations are likely to occur; for example, if chemicals are added near its upstream, it is best to place the addition point downstream of the sensor. When in use, the sensor measurement tube must be filled with liquid (exceptions apply to non-full-tube types). When there is mixing, its distribution should be roughly uniform. The liquid should be at the same potential as the ground and must be grounded. When insulating materials such as plastic are used for process pipelines, factors such as frictional static electricity generated by the flowing liquid can result in a potential difference between the liquid and the ground. 　　7.2 Installation of flow sensors 　　(1) The enclosure of electromagnetic flow sensors usually has a protection rating of IP65 (the dust and moisture protection level specified in GB 4208), and the installation location must meet the following requirements. 　　1) When measuring mixed-phase fluids, select a location that will not cause phase separation ； When measuring two-component liquids, avoid filling them downstream where the mixing has not yet been uniform ； When measuring chemical reaction pipes, it should be installed downstream of the section where the reaction is fully complete ； 　　2) Try to avoid negative pressure forming inside the measurement tube ； 　　3) Choose a location with minimal vibration, especially for integrated instruments ； 　　4) Avoid having large motors, large transformers, etc., in the vicinity to prevent electromagnetic interference ； 　　5) Locations where it is easy to implement separate grounding for sensors ； 　　6) Avoid areas with high concentrations of corrosive gases in the surrounding environment as much as possible ； 　　7) The ambient temperature ranges from –25/–10 to 50/600°C; for the all-in-one design, the temperature is also constrained by electronic components, resulting in a narrower range ； 　　8) The relative environmental humidity is within the range of 10% to 90% ； 　　9) Avoid direct sunlight as much as possible ； 　　10) Avoid exposure to rainwater; it will not be submerged in water. If the protection rating is IP67 (dust and water resistance) or IP68 (dust and submersion resistance), then the requirements in items 8) and 10) above are not necessary. 　　(2) Requirement for the length of the straight pipe section: To achieve proper measurement accuracy, there should also be a certain length of straight pipe section upstream of the electromagnetic flow meter, although this length requirement is lower compared to that of most other flow meters. For 90º elbows, T-pipes, concentric reducers, and fully open gate valves, it is generally sufficient to have a straight pipe section with a length of 5 times the diameter (5D) away from the electrode centerline (not from the connection surface at the sensor inlet); for valves at other opening degrees, 10D is required ； The straight pipe section downstream is (2–3)D or no requirement ； But it is necessary to prevent the butterfly valve disc from extending into the sensor’s measurement tube. The lengths of the upstream and downstream straight sections specified in various standards or testing procedures also vary; these values are summarized in Table 2, and the requirements are higher than those usual. This is to ensure compliance with the requirements for instruments with a current precision level of 0.5. Name of the flow disturbance element, Standard or testing specification number: ISO 6817, ISO 9104, JIS B7554, ZBN 12007, JJG 198. Upstream: Elbows, shaped tubes, fully open gate valves, converging tubes – 10D or as specified by the manufacturer; 10D, 5D, 5D, 10D. Converging tubes can be treated as straight tubes. Other types of valves: 10D. Downstream: Various types – no specific requirement; 5D, no specific requirement, 2D, 2D. If the valve can be used in operation, it should be installed at an angle of 45º between its flow direction and the axis of the electrodes, which will significantly reduce additional errors. 　　(3) Installation position and flow direction The sensor can be installed horizontally, vertically, or at an angle; there are no restrictions. However, it is best to install the device vertically when measuring solid-liquid two-phase fluids, with flow from bottom to top. This prevents severe localized wear on the lower part of the lining during horizontal installation, as well as issues such as solid phase precipitation at low flow rates. When installed horizontally, the electrode axis should be parallel to the horizon and not perpendicular to it; otherwise, the electrodes at the bottom are prone to being covered by sediment, while the top electrodes can have their surfaces obscured by bubbles present in the liquid, causing fluctuations in the output signal. In the piping system shown in Figure 5, c and d are the appropriate positions ； a, b, and e are unsuitable locations; the area at b may not be fully filled with liquid, gas tends to accumulate at a and e, and the short pipe section behind the sensor at e may also not be fully filled. It is best for the discharge outlet to have the shape shown as f. For a solid-liquid two-phase flow, point c is also an unsuitable location. 　　(4) Bypass pipe, for easy cleaning of connections and pre-installed access holes: A bypass pipe should be installed to facilitate checking and adjusting the zero point when flow continues in the process pipeline while the sensor has no flow. However, large-diameter piping systems are often difficult to implement due to investment and space constraints. It is difficult to correct the measurement values based on the degree of electrode contamination, or to establish a criterion for determining the degree of contamination that does not affect the measurement values. In addition to what has been mentioned earlier, instruments that use contactless electrodes or electrodes equipped with scraping devices can solve certain problems; however, when it is necessary to remove deposits from the inner walls, these can be removed on-site without removing the sensor, as shown in Figure 6. For piping systems with a diameter greater than 1.5–1.6 m, pre-installed access holes are provided on the pipes near the EMF, so as to clean the inner wall of the sensor measurement tube when the piping system is shut down. 　　(5) Installation of negative pressure piping Fluoroplastic-lined sensors must be used with caution in negative pressure piping systems ； Positive-pressure piping systems should be protected from the formation of negative pressure. For example, in piping systems where the fluid temperature is higher than room temperature, when the upstream and downstream shut-off valves of the sensor are closed to stop operation, the cooling and contraction of the fluid can result in negative pressure; therefore, a negative-pressure prevention valve should be installed near the sensor, as shown in Figure 7. Manufacturers specify that for PTFE and PFA plastic linings used in negative-pressure piping systems, the absolute pressure at temperatures of 200°C, 1000°C, and 1300°C must be greater than 27, 40, and 50 KPa respectively. (6) Grounding: The sensor must be grounded separately (with a grounding resistance of less than 100 Ω). In principle, for separate types, grounding should be on the sensor side, while the converter’s grounding should be at the same grounding point. If the sensor is installed on a pipeline protected against cathodic corrosion, in addition to grounding the sensor together with the grounding ring, a thicker copper wire (16 mm2) should be used to connect across the two connection flanges of the pipeline around the sensor, thereby isolating the cathodic protection current from the sensor. Sometimes, the back-ground stray current is too high; for example, if the leakage current from the electrolyte along the electrolyzer affects the accurate measurement of the EMF, electrical isolation between the flow sensor and the process to which it is connected can be employed. This method can also be employed on pipelines with cathodic protection, where the cathodic protection current affects EMF measurements. 7.3 Converter Installation and Connection Cables – Integrated EMF; no separate converter required ； Separate converters are installed near the sensors or in the control room; there is more flexibility in terms of location, and the environmental conditions there are better than those at the sensors. Their protection rating is IP65 or IP64 (dust and splash protected). 　　The requirements for the installation location are the same as those specified in items 3), 4), 6), 8), 9), and 10) of (1) in Section 7.2; the ambient temperature is restricted by electronic components, and the operating temperature range is narrower than that specified in item 7). The distance between the converter and the sensor is determined by the conductivity of the medium being measured and the type of signal cable, namely factors such as the cable’s distributed capacitance, conductor cross-section, and number of shielding layers. Use the signal cable that comes with the instrument from the manufacturer (or the specified model). When the conductivity of the liquid is low and the transmission distance is long, it is also specified to use a three-layer shielded cable. The “user manual” for general instruments specifies the corresponding transmission distance ranges for liquids with different conductivities. Single-layer shielded cables used for industrial water or acidic/basic liquids can typically transmit over a distance of 100 meters. To avoid interference signals, the signal cable must be run separately inside a grounded protective steel pipe; the signal cable and power cable cannot be installed in the same pipe.

Advantages and disadvantages of various flow meters: The turbine flow meter is the main type among velocity-type flow meters; it uses 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 variety of models that are produced in large quantities. Advantages: 1) High precision – it is the most accurate flow meter among all types; (2) Good repeatability; (3) Excellent resistance to zero-point drift; (4) Wide measurement range; (5) Compact design. Disadvantages: 1) It cannot maintain its calibration properties over the long term; 2) The properties of the fluid have a significant impact on the flow characteristics. Application overview: Turbine flowmeters are widely used for measuring various fluids such as oil, organic liquids, inorganic liquids, liquefied gas, natural gas, and cryogenic fluids. In Europe and the United States, they are the second-most commonly used flowmeters for measuring natural gas, after orifice plate flowmeters. In the Netherlands alone, more than 2,600 gas turbine flowmeters of different sizes, operating under pressures ranging from 0.8 to 6.5 MPa, are used in natural gas pipelines; these flowmeters have proven to be excellent instruments for measuring natural gas. 1.2 Vortex Flow Meter A vortex flow meter is a device in which a non-streamlined vortex generator is placed within the fluid; as the fluid flows past this generator, it separates alternately on either side, 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 common 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. 1.3 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 section of smooth straight pipe that does not get blocked, 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, and liquids containing large bubbles; (3) It cannot be used at high temperatures. Application Overview: Electromagnetic flowmeters have a wide range of applications. Large-diameter models are commonly used in water supply and drainage projects; medium and small-diameter models are often employed in situations where high precision is required or where measurement is difficult, such as in the control of cooling water for blast furnaces in the steel industry, in measuring pulp and black liquor in the paper industry, in highly corrosive fluids in the chemical industry, and in mineral slurries in the non-ferrous metallurgy industry. Small and ultra-small diameter models are typically used in industries such as pharmaceuticals, food processing, and biochemistry, where hygiene standards are important. 1.4 Differential pressure flow meters: Differential pressure flow meters are instruments that calculate 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. 　　Based on their working principles, the sensing elements of differential pressure flow meters can be classified into several major categories: throttling devices, hydraulic resistance types, centrifugal types, dynamic head types, dynamic head gain types, and jet types. 　　Test specimens can also be divided into two major categories based on their degree of standardization: standard and non-standard. 　　The so-called standard test pieces are those whose flow rate values and measurement error can be determined without the need for actual flow calibration, as long as they are designed, manufactured, installed, and used in accordance with standard documents. 　　Non-standard test pieces are those with a lower level of maturity and have not yet been included in international standards. 　　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) It is the most widely used orifice plate flow meter; it has 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, transmitter, and display instrument are manufactured by different manufacturers, which facilitates mass production for cost savings. 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 on-site; (4) High pressure loss (referring to orifice plates, nozzles, etc.). Application overview: Differential pressure flow meters have a very wide range of applications. They are used for measuring flow rate in closed pipelines for various types of fluids, such as single-phase, mixed-phase, clean, dirty, and viscous flows. In terms of operating conditions, they can be used at normal pressure, high pressure, vacuum, normal temperature, high temperature, or low temperature. The pipe diameter can range from a few millimeters to several meters. As for flow conditions, they can handle subsonic, sonic, and pulsating flows. Its usage across various industrial sectors accounts for about 1/4 to 1/3 of the total usage of flowmeters. 1.5 Float flow meter: A float flow meter, also known as a rotameter, is 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 flow meters are the type of flow meter with the widest range of applications, second only to differential pressure flow meters; they play a crucial role, especially in measuring small and micro flow rates. In the mid-1980s, sales in Japan, Western Europe, and the United States accounted for 15% to 20% of the total sales for flow meters. China’s production was estimated to be between 120,000 and 140,000 units in 1990, of which over 95% were glass cone tube float flowmeters. 　Features: 1) The glass cone tube float flow meter has a simple structure and is easy to use; however, its disadvantage is its low pressure resistance, along with a 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. 1.6 Positive Displacement Flow Meters: Positive displacement flow meters, also known as volume-based flow meters or simply PD flow meters, are the most accurate type of flow measurement instruments. It uses mechanical measuring elements to continuously divide the fluid into individual, known volume portions, and measures the total volume of fluid by counting the number of times each volume portion is filled and emptied in the measuring chamber. 　　Based on their measuring elements, positive displacement flowmeters can be classified into elliptical gear flowmeters, scraper flowmeters, twin-rotor flowmeters, rotary piston flowmeters, reciprocating piston flowmeters, disc flowmeters, liquid-sealed drum flowmeters, wet gas meters, and diaphragm gas meters, among others. Advantages: (1) High measurement accuracy; (2) The conditions of the piping system in which it is installed have no impact on the measurement accuracy; (3) It can be used for measuring high-viscosity liquids; (4) Wide measurement range; (5) As a direct-reading instrument, it can provide cumulative and total values without the need for external power, offering clear readings and simple operation. Disadvantages: (1) The results are complex and the device is large in size; (2) There are significant limitations regarding the type of medium being measured, its diameter, and the operating conditions of the medium; (3) It is not suitable for use in high or low temperature environments; (4) Most of these instruments are only applicable to clean, single-phase fluids; (5) They generate noise and vibration. 　Application overview: Positive displacement flowmeters, together with differential pressure flowmeters and float flowmeters, are among the three most commonly used types of flowmeters. They are frequently employed for measuring the total volume of expensive fluids such as oils and natural gas. 　　In industrially developed countries, the sales value of PD flowmeters (excluding household gas meters and domestic water meters) accounted for 13% to 23% of the total sales volume of flowmeters in recent years; in China this figure is around 20%. In 1990, the production volume (excluding household gas meters) was estimated to be 340,000 units, of which elliptical gear type and gear wheel type flowmeters accounted for approximately 70% and 20% respectively. 1.7 Ultrasonic flow meters: Ultrasonic flow meters 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. Application overview: (1) The propagation time method is applied to clean, single-phase liquids and gases. Typical applications include factory effluents, strange liquids, liquefied natural gas, etc.; 2) In terms of gas applications, there is good experience in using it in the field of high-pressure natural gas; 3) The Doppler method is suitable for two-phase fluids with a low content of heterogeneous components, such as untreated wastewater, factory effluents, and dirty process fluids; it is generally not suitable for very clean liquids. 1.8 Coriolis Mass Flow Meter The Coriolis mass flow meter (hereinafter referred to as CMF) is a direct-type mass flow measuring instrument that operates on the principle that, as fluid flows through a vibrating tube, a Coriolis force is generated which is proportional to the mass flow rate. 　　The application of CMF in our country started relatively late. In recent years, several manufacturers (such as Taihang Instrument Factory) have developed their own products and supplied them to the market; other manufacturers have established joint ventures or utilized foreign technology to produce a range of instruments. 1.9 Open-channel flow meters differ from the previous types; they are instruments used to measure the flow rate of free-surface flow in open channels that are not completely filled. 　　A water channel with non-full pipe flow 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 flowmeter. 　　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 drainage channels in thermal power plants, as well as channels for wastewater treatment intake and discharge; water discharge from industrial and mining enterprises; and channels used in hydraulic engineering and agricultural irrigation. Some estimates put the number 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.

Generally, imported ones are more accurate and have a longer lifespan, but they are very expensive. Those produced domestically have a short lifespan and inaccurate measurements.

I’ve used vortex flow meters; the selection process is quite complicated, and I don’t really understand it.