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This post was last edited by qy576100527 on 2021-5-5 08:32. Explanation of instrument selection: In industrial production processes, instruments play an important role in detecting, displaying, recording, or controlling process parameters. Monitoring the manufacturing process is a fundamental means of understanding and controlling industrial production. Only by being able to accurately understand the overall situation of the process at all times and exert control over it can the production process proceed smoothly, enabling the manufacture of qualified products with high productivity and low consumption. So, despite their unassuming appearance, instruments are the most essential! This article provides a comprehensive overview of the selection process for automatic instruments, temperature instruments, pressure instruments, flow meters, and level sensors I. General principles for selecting automated instruments The general principles for selecting measuring instruments (components) and control valves are as follows: 1. Conditions of the process The temperature, pressure, flow rate, viscosity, corrosivity, toxicity, pulsation, and other factors of the process are the main criteria for determining the selection of instruments. These factors affect the rationality of instrument selection, their service life, as well as issues related to fire prevention, explosion protection, and safety in the workshop. 2. Operational importance: The operational importance of the parameters at each monitoring point serves as the basis for determining functions such as instrument indication, recording, accumulation, alarm generation, control, and remote operation. Generally, it has little impact on the manufacturing process; for variables that require frequent monitoring, an indicator type can be used ； For important variables whose trends need to be monitored frequently, a recording type should be selected ； And for those variables that have a significant impact on the manufacturing process and need to be monitored at all times, controls should be established ； For variables that require measurement or economic accounting in relation to material balance and energy consumption, it is advisable to establish accumulations ； Alarms should be installed for certain variables that may affect production or safety. 3. The selection of economical and unified instruments is also determined by the scale of investment; necessary economic calculations should be carried out to ensure that the requirements of the process and automatic control are met, thereby achieving an appropriate performance/price ratio. To facilitate the maintenance and management of instruments, attention should also be paid to their uniformity when selecting them. Try to choose products from the same series, with the same specifications and model, and manufactured by the same manufacturer. 4. Use and availability of instruments: The instruments selected should be mature products whose reliability has been proven through field use ； At the same time, it should be noted that the selected instruments must be readily available in the market, so as not to affect the construction progress of the project. II. Selection of temperature instruments – General principles 1. Unit and scale: The scale unit for temperature instruments is uniformly the Celsius temperature (℃). 2 Insertion length of the detection (measuring) element: The selection of the insertion length should be based on the principle that the detection (measuring) element is inserted into a position where it is sensitive and representative regarding temperature changes in the medium being measured. But under normal circumstances, for the sake of ease of interchange, the length of one or two gears is usually chosen uniformly for the entire device. When installing on flues, furnaces, and equipment with insulated cladding, the appropriate type should be selected based on actual needs. The material of the protective cover for the detected (measured) components should not be inferior to that of the equipment or pipelines. If the protective sleeve of a finished product is too thin or not corrosion-resistant (such as in armored thermocouples), an additional protective sleeve should be used. Temperature instruments, temperature switches, temperature sensing elements, and transmitters installed in flammable and explosive areas must be of the explosion-proof type. Selection of local temperature instruments 1 Accuracy class: For general industrial thermometers, a class of 1.5 or 1 is selected. Thermometers for precision measurement and laboratory use: Grades 0.5 or 0.25 should be selected. 2 Measurement range: The maximum measured value shall not exceed 90% of the upper limit of the instrument’s measurement range, while the normal measured values are around 1/2 of the upper limit of that range. The measured value of a pressure thermometer should be between 1/2 and 3/4 of the upper limit of the instrument’s measurement range. 3 Bimetallic thermometers should be given priority when they meet the requirements regarding measurement range, operating pressure, and accuracy. The diameter of the gauge case is generally chosen to be φ100mm; in locations with poor lighting conditions, at elevated positions, or where the observation distance is relatively long, φ150mm should be selected. For the connection method between the instrument enclosure and the protective tube, a universal type is generally recommended; alternatively, an axial or radial type can be chosen based on the convenience of observation. 4. Pressure-type thermometers are suitable for use in low temperatures below –80°C, in situations where close observation is not possible, in environments with vibrations, and for local or panel display where high precision is not required. 5 Glass thermometers are used only in special situations where high accuracy is required, there is minimal vibration, no mechanical damage, and easy observation is possible. However, due to mercury hazards, glass mercury thermometers should not be used. 6. Base-type instruments: For measurement and control (regulation) instruments installed on-site or in local panels, base-type temperature instruments are preferred. 7 Temperature switch: Suitable for applications where temperature measurement requires a contact signal output. Selection of centralized temperature measuring instruments 1 Sensing elements (1) Based on the temperature measurement range, select a thermocouple, thermal resistor, or thermistor with the appropriate calibration number. (2) Thermocouples are suitable for general applications. Thermal resistors are suitable for vibration-free environments. Thermistors are suitable for applications that require fast response times in measurements. (3) Depending on the response speed requirements of the object being measured, the following time constant detection elements can be selected: Thermocouples: 600s, 100s, and 20s ； Thermal resistors: four levels—90–180s, 30–90s, 10–30s, and <10s ； Thermistor: ＜1s. (4) Depending on the operating environment conditions, select the junction box according to the following principles: Standard type: locations with favorable conditions ； Splash-proof, waterproof: humid or outdoor locations ； Flameproof type: areas where flammable and explosive substances are present ； Socket type: Suitable only for special occasions. (5) Threaded connections can generally be used; flanged connections should be employed in the following situations: when installing on equipment, lined pipes, and non-ferrous metal pipes ； Crystallization, scarring, clogging, and highly corrosive media ； Flammable, explosive, and highly toxic media. (6) Thermocouples and thermoresistors for use in special conditions: In environments with temperatures above 870°C, reducing gases with a hydrogen content of more than 5%, inert gases, and vacuum conditions, tungsten-rhodium thermocouples or gas-jet thermocouples should be used ； For the temperature of equipment, pipeline outer walls, and rotating surface surfaces, surface or armored thermocouples and thermal resistors are used ； For media containing hard solid particles, use wear-resistant thermocouples ； When multiple temperature measurements are required within the same protection sleeve of the detection element, multiple thermocouples should be used ； To save special protective tube materials such as tantalum, improve response speed, or when it is necessary to detect bending of the sensing element, armored thermocouples can be used. 2 Transmitters: For measurement or control systems used in conjunction with indicating instruments that accept standard signals; select appropriate transmitters. Provided that design requirements are met, it is recommended to choose a transmitter that integrates measurement and transmission functions. 3 Display instruments: (1) For single-point display, conventional indicators are suitable; for multi-point display, digital indicators are preferable. If it is necessary to access historical data, conventional recorders are the better choice. (2) For the signal alarm system, it is advisable to use an indicator or recorder with contact-based signal output. (3) For multi-point recording, a medium-sized recorder (such as a 30-point recorder) is recommended. 4 Selection of auxiliary equipment (1) When multiple devices share one display instrument, a switch with reliable quality should be selected. (2) Thermocouples are used to measure temperatures below 1600°C. When changes in the cold junction temperature cause the measurement system to fail to meet accuracy requirements, and the accompanying display instrument does not have an automatic cold junction temperature compensation function, an automatic cold junction temperature compensator should be used. (3) Compensation wire a. Depending on the number of thermocouple elements, their calibration class, and the operating conditions, a compensation wire or cable that meets the required specifications should be selected. b. Select different grades of compensation wires or cables according to the operating temperature: ordinary grade for –20～+100℃ ； Choose the heat-resistant grade for -40～+250℃. c. In areas with intermittent electric heating or strong electric and magnetic fields, shielded compensation wires or shielded compensation cables should be used. d. The cross-sectional area of the compensation wire shall be determined based on the round-trip resistance value over its installation length, as well as the allowable external resistance for input by the associated display instruments, transmitters, or computer interfaces. III. Selection of pressure instruments – Choosing pressure gauges 1. Selection based on the operating environment and the properties of the medium being measured: (1) In environments with high levels of atmospheric corrosion, a lot of dust, or exposure to splashing liquids, it is advisable to use hermetically sealed all-plastic pressure gauges. (2) For dilute nitric acid, acetic acid, ammonia, and other generally corrosive media, acid-resistant pressure gauges, ammonia pressure gauges, or stainless steel diaphragm pressure gauges should be used. (3) For media such as dilute hydrochloric acid, hydrogen chloride gas, heavy oils, and similar substances with strong corrosiveness, solid particles, or viscous liquids, diaphragm pressure gauges should be used. The material of its diaphragm must be selected based on the properties of the medium being measured. (4) For media such as those with crystallization, scarring, and high viscosity, diaphragm pressure gauges should be used. (5) In environments with strong mechanical vibration, shock-resistant pressure gauges or marine pressure gauges should be used. (6) In flammable and explosive environments, where electrical contact signals are required, explosion-proof electrical contact pressure gauges should be used. (7) Specialized pressure gauges should be used for the following measurement media: gaseous ammonia and liquid ammonia: ammonia pressure gauges, vacuum gauges, and pressure-vacuum gauges ； Oxygen: Oxygen pressure gauge ； Hydrogen: Hydrogen pressure gauge ； Chlorine: Chlorine-resistant pressure gauges, pressure-vacuum gauges ； Acetylene: Acetylene pressure gauge ； Hydrogen sulfide: sulfur-resistant pressure gauge ； Alkaline solution: Alkali-resistant pressure gauges, pressure and vacuum gauges. 2 Selection of accuracy class (1) For pressure gauges, capsule pressure gauges, and diaphragm pressure gauges used for general measurements, a 1.5 or 2.5 accuracy class should be selected. (2) For precision measurement and calibration pressure gauges, grades 0.4, 0.25, or 0.16 should be selected. 3 Selection of external dimensions (1) The pressure gauges to be installed on pipes and equipment have a nominal diameter of φ100mm or φ150mm. (2) The pressure gauges installed on the instrument pneumatic pipelines and their auxiliary equipment have a nominal diameter of φ60mm. (3) Pressure gauges installed in areas with low illumination, at high locations, or where it is difficult to observe the readings have a nominal diameter of φ200mm or φ250mm. 4 Selection of measurement range (1) When measuring a stable pressure, the normal operating pressure value should be between 2/3 and 1/3 of the upper limit of the instrument’s measurement range. (2) When measuring pulsating pressure (such as the pressure at the outlet of pumps, compressors, and fans), the normal operating pressure value should be between 1/2 and 1/3 of the upper limit of the instrument’s measurement range. (3) When measuring high and medium pressures (greater than 4 MPa), the normal operating pressure value should not exceed 1/2 of the upper limit of the instrument’s measurement range. 5 units and scales (graduations) (1) Pressure instruments must use legally mandated measurement units. That is: Pa, kPa, and MPa. (2) For foreign-related design projects and the introduction of instruments, internationally recognized standards or corresponding **standards can be adopted. Selection of transmitters and sensors (1): When transmitting with a standard signal (4–20 mA), a transmitter should be chosen. (2) In flammable and explosive environments, pneumatic transmitters or explosion-proof electric transmitters should be used. (3) For crystalline, scaly, clogged, viscous, and corrosive media, flanged transmitters should be selected. The materials in direct contact with the medium must be selected based on the properties of that medium. (4) In applications with favorable operating conditions and low requirements for measurement accuracy and reliability, resistive or inductive remote pressure gauges, or Hall pressure transmitters can be used. (5) When measuring very low pressures (less than 500 Pa), a differential pressure transmitter can be used. Selection of mounting accessories (1) When measuring water vapor and media with temperatures above 60°C, spiral or U-shaped elbows should be used. (2) When measuring easily liquefiable gases, if the pressure tapping point is above the instrument, a separator should be used. (3) When measuring dust-containing gases, a dust collector should be used. (4) When measuring pulsating pressure, a damper or buffer should be used. (5) When the operating ambient temperature is close to or below the freezing point or solidification point of the measured medium, insulation or heating measures should be taken. (6) Instrument protection (temperature) boxes should be used in the following situations. Pressure switches and transmitters installed outdoors. Pressure switches and transmitters installed in factories with severe atmospheric corrosion, high dust levels, and other harmful substances. IV. Selection of flow meters – General principles 1. Scale selection: The scale of the meter should meet the requirements of its scale modulus; when the scale readings are not integers, it is also possible to choose an integer value for easier reading conversion. (1) Square root scale range: The maximum flow rate shall not exceed 95% of the full scale ； The normal flow rate is 70% to 85% of the full scale ； The minimum flow rate shall be no less than 30% of the full scale. (2) Linear scale range: The maximum flow rate shall not exceed 90% of the full scale ； The normal flow rate is 50% to 70% of the full scale ； The minimum flow rate shall be not less than 10% of the full scale. 2 Instrument accuracy: Flowmeters used for energy measurement shall comply with the provisions of the \"General Rules for the Provision and Management of Energy Measurement Instruments in Enterprises (Trial)\". (1) Used for measuring fuel inflow and outflow at the plant, ±0.1% ； (2) For the measurement in technical-economic analysis of workshop teams and production processes, ±0.5％～2％ ； (3) Used for metering in industrial and domestic water applications, ±2.5% ； (4) For steam metering including superheated steam and saturated steam, ±2.5% ； (5) Used for the metering of natural gas, gas, and household gas, ±2.0% ； (6) Measurement of oil used for the control of key energy-consuming equipment and process streams, ±1.5% ； (7) Measurement of other energetic media used for process control (such as compressed air, oxygen, nitrogen, hydrogen, water, etc.), ±2%. 3 Flow units: Volumetric flow rate is expressed in m3/h and l/h ； Mass flow rate is expressed in kg/h and t/h ； The volumetric flow rate of gas under standard conditions is expressed in Nm3/h (0°C, 0.1013 MPa). Selection of flow measurement instruments for general fluids, liquids, and steam 1 Differential pressure flow meters (1) Throttling devices ① Standard throttling devices For the flow measurement of general fluids, standard throttling devices (standard orifice plates, standard nozzles) should be used. The selection of standard throttling devices must comply with the provisions of GB2624-81 or the international standard ISO 5167-1980. If there are new **standard regulations, those new regulations should be followed. ②For non-standard throttling devices, a venturi tube can be used if the following conditions are met: accurate measurement is required with low pressure loss ； The medium under test is a clean gas or liquid ； The inner diameter of the pipeline ranges from 100 to 800 mm ； The fluid pressure is within 1.0 MPa. Double orifice plates can be used when the following conditions are met: the medium being measured is a clean gas or liquid ； The Reynolds number is within the range of greater than (or equal to) 3000 and less than (or equal to) 300,000. Those who meet the following conditions may choose a 1/4 circle nozzle ； The medium under test is a clean gas or liquid ； The Reynolds number is in the range of greater than 200 and less than 100,000. Orifice plates with circular missing holes can be chosen for applications where the medium being measured may contain precipitates or contaminants that can form ahead of and behind the orifice plate (such as blast furnace gas, sludge, etc.) ； Horizontal or inclined pipes must be available. ③The selection of pressure tapping methods should take into account the use of a unified approach throughout the entire project as much as possible. Corner tapping or flange tapping is generally used. Depending on the operating conditions and measurement requirements, other pressure tapping methods such as radial distance pressure tapping can be employed. (2) Selection of the differential pressure range for differential pressure transmitters: The differential pressure range should be determined through calculations. Generally, depending on the operating pressure of the fluid, the following values can be chosen: For low differential pressures: 6 kPa, 10 kPa ； Medium differential pressure: 16 kPa, 25 kPa ； High differential pressure: 40 kPa, 60 kPa. (3) Measures to improve measurement accuracy: For fluids with large temperature and pressure fluctuations, temperature and pressure compensation measures should be considered ； When the length of the straight section of the pipe is insufficient or rotational flow occurs within the pipe, fluid correction measures should be considered, and a straightener of an appropriate diameter should be selected. (4) Special differential pressure flowmeters ① Steam flowmeters: For measuring the flow rate of saturated steam, steam flowmeters can be used when the required accuracy is not higher than grade 2.5, and for local or remote accumulation purposes. ②An orifice plate flow meter built into the pipeline is suitable for measuring small flow rates of clean liquids, steam, or gases that contain no suspended particles. It can be used when the range ratio is not greater than 3:1, when high measurement accuracy is not required, and when the pipe diameter is DN < 50 mm. When measuring steam, the steam temperature should not exceed 120°C. 2 Area-type flow meters: When the required accuracy is not higher than grade 1.5 and the range ratio is not greater than 10∶1, rotor flow meters can be used. (1) Glass rotameters can be used for on-site indication of fluid flow rates that are low or extremely low, with a pressure of less than 1 MPa and a temperature below 100°C; such fluids should be clean, transparent, non-toxic, free from risks of combustion or explosion, and should not cause corrosion or adhesion to glass. (2) Metal tube rotameter: ① The ordinary type metal tube rotameter is suitable for measuring low-flow rates of fluids that are easy to vaporize or condense, toxic, flammable, explosive, free of magnetic materials, fibers, and abrasive substances, and that do not cause corrosion to stainless steel (1Crl8Ni9Ti). It can be used when on-site indication or remote signal transmission is required. ②Special-type metal tube rotameters: Jacketed metal tube rotameters can be used when the medium being measured tends to crystallize, vaporize, or has high viscosity. A heating or cooling medium is passed through the jacket. Corrosion-resistant metal tube rotameters can be used for measuring the flow rate of corrosive media. (3) Rotameters must be installed vertically, with an inclination of no more than 5°. The fluid should flow from bottom to top; the installation location should experience minimal vibration and should be easy to observe and maintain. Upstream and downstream shut-off valves as well as bypass valves should be provided. For dirty media, a filter must be installed at the inlet of the flow meter. 3 Velocity-type flow meters (1) Target flow meter: It is used for measuring the flow rate of liquids with high viscosity and containing a small amount of solid particles. The target flow meter can be employed when the required accuracy is not higher than grade 1.5 and the range ratio is not greater than 3:1. Target flowmeters are generally installed in horizontal pipes. The length of the preceding straight pipe section is 15–40D, while that of the subsequent straight pipe section is 5D. (2) Turbine flow meters: These are used for measuring the flow rate of clean gases and clean liquids with a dynamic viscosity not exceeding 5×10-6 m2/s. They can be employed when accurate measurement is required and the range ratio is not greater than 10:1. The turbine flowmeter should be installed in a horizontal pipe, with the liquid filling the entire pipe, and upstream and downstream shut-off valves as well as a bypass valve should be provided; in addition, a filter should be installed upstream and a discharge valve downstream. Length of straight pipe section: not less than 20D upstream, and not less than 5D downstream. (3) Vortex flowmeters (Carnot vortex flowmeters or vortex flow meters) can be used for measuring medium to high flow rates of clean gases, steam, and liquids. Vortex flowmeters are not suitable for measuring low-speed fluids and liquids with a viscosity greater than 20×10-3 Pa·s. The pipeline flow rate should be checked when making the selection. This flow meter features low pressure loss and easy installation. Requirements for straight pipe sections: 15–40D upstream (depending on the piping arrangement) ； When a rectifier is added upstream, the distance upstream should be no less than 10D ； The downstream should be at least 5D. (4) The water meter accumulates the water flow on-site; it can be used when a range ratio of less than 30∶1 is required. The water meter is installed on horizontal pipes, with the required length of straight pipe sections being: no less than 8D upstream and no less than 5D downstream. Selection of flow measurement instruments for corrosive, conductive, or solid-particle-containing fluids 1 Electromagnetic flowmeters are used for measuring the flow rate of liquids with a conductivity greater than 10 μS/cm, or of homogeneous liquid-solid two-phase media. It has excellent corrosion and wear resistance, with no pressure loss. It can measure various media such as strong acids, strong bases, salts, ammonia water, sludge, pulp, and paper pulp. The installation direction can be vertical, horizontal, or inclined; when installed vertically, the liquid must flow from bottom to top. For liquid-solid two-phase media, it is best to install it vertically. When installed in a horizontal pipe, the pipe section should be filled with liquid, and the electrodes of the transmitter should be at the same level ； Length of the straight pipe section: not less than 5–10D upstream, and not less than 3–5D downstream, or no specific requirement (requirements vary depending on the manufacturer). Transmitters should not be installed in areas where the magnetic field strength is greater than 398 A/m. 2 For non-standard throttling devices, refer to Section 1 on the selection of flow measurement instruments for high-viscosity fluids. 1 Positive-displacement flowmeters: (1) Elliptical gear flowmeters – These are suitable for clean, highly viscous liquids where accurate flow measurement is required; they can be used when the range ratio is less than 10∶1. The helical gear flowmeter should be installed in a horizontal pipeline, with the indicating dial positioned in a vertical plane ； Upstream and downstream isolation valves as well as bypass valves should be provided. A filter should be installed upstream. For micro-flow rates, a miniature elliptical gear flow meter can be used. When measuring various volatile media, a degasser should be added. (2) Gear flow meter: For the measurement of clean gases or liquids, especially lubricating oils, where high accuracy is required, a gear flow meter can be used. The flow meter should be installed horizontally, a bypass line should be provided, and a filter should be installed at the inlet. (3) Scraper flowmeter: It is used for continuously measuring the liquid flow rate in closed pipelines, and is particularly suitable for accurate measurement of various oils. A scraper flowmeter can be chosen for this purpose. When installing a scraper flow meter, the pipeline should be filled with fluid, and the meter should be installed horizontally so that the digits on the counter are in a vertical position. When measuring various oils and requiring precise measurement, a degasser should be added. 2. Target flowmeters are used for measuring the flow rate of liquids with high viscosity and containing a small amount of solid particles. They can be employed when the required accuracy is not higher than grade 1.5 and the range ratio is not greater than 3:1. Target flowmeters are generally installed in horizontal pipes. The length of the preceding straight pipe section is 15–40D, while that of the subsequent straight pipe section is 5D. Selection of flow measurement instruments for large-diameter pipes: When the pipe diameter is large, pressure loss has a significant impact on energy consumption. Conventional flowmeters are expensive; when the pressure loss is high, options such as fluted averaging tubes, inserted vortex flowmeters, inserted turbine flowmeters, electromagnetic flowmeters, venturi tubes, and ultrasonic flowmeters can be chosen depending on the situation. The pitot tube flowmeter is used for measuring the flow rate of clean gases, vapors, and clean liquids with a viscosity of less than 0.3 Pa·s. When low pressure loss is required, the pitot tube flowmeter can be selected. The flared constant velocity tube is installed on a horizontal pipe, with the straight section length being at least 6–24D upstream and at least 3–4D downstream. 2 Insertion-type turbine flowmeters, insertion-type vortex flowmeters, electromagnetic flowmeters, and venturi tubes are described above. Selection of new types of flow measurement instruments 1. Ultrasonic flow meters: Ultrasonic flow meters can be used for any fluid that allows sound to pass through it. In addition to ordinary fluids, they can also be employed for fluids that operate under harsh conditions such as high corrosivity, non-conductivity, flammability, or radioactivity, when contact-based measurement methods are not feasible. 2 Mass flow meters: When it is necessary to accurately measure the mass flow rate of liquids, high-density gases, and slurries, mass flow meters can be used. Mass flow meters can provide accurate and reliable mass flow data, unaffected by changes in fluid temperature, pressure, density, or viscosity. Mass flow meters can be installed in any direction without the need for straight pipe sections. Selection of flow measurement instruments for powdery and granular solids: Impulse flow meters are suitable for measuring the flow of freely falling powdery and granular solids; when it is necessary to convey the material in a closed system, impulse flow meters are the preferred choice ； The impulse flow meter is suitable for various bulk materials of any particle size, and can provide accurate measurements even in environments with high dust levels; however, the weight of the bulk material must not exceed 5% of the weight of the preset impact plate. For the installation of impulse flow meters, it is required that the material fall freely, with no external forces acting on the object being measured. There are specific requirements for the installation angle of the punching die, as well as the angle and height between the feed inlet and the punching die; these factors are related to the range selection, and calculations should be carried out prior to making a choice. The 2-electron belt scale is used for measuring the solid flow rate in belt conveyors, and it is installed on belt conveyors that meet standard performance requirements. It is stated that the installation requirements for the weighing frame are strict; the position of the frame on the belt and its distance from the discharge opening both affect the measurement accuracy, so it is necessary to choose the appropriate installation location. 3. Dynamic weighing scales: For the continuous and automatic weighing of railway freight cars, dynamic weighing scales are the preferred choice. V. Selection of level gauges – General principles: (1) It is necessary to have a thorough understanding of the process conditions, the properties of the medium to be measured, and the requirements of the measurement and control system, so as to properly assess the technical performance and economic benefits of these instruments, enabling them to play their role in ensuring stable production, improving product quality, and increasing economic efficiency. (2) For liquid level and interface measurement, differential pressure instruments, float-type instruments, and buoy-type instruments should be used. When the requirements are not met, capacitive, resistive (electrical contact), acoustic, and other types of instruments can be used. The method for measuring the material surface should be selected based on the particle size of the material, its angle of repose, its electrical conductivity, the structure of the silo, and the requirements for measurement. (3) The structural design and material of the instrument should be selected based on the properties of the medium to be measured. The main factors to consider are pressure, temperature, corrosivity, and electrical conductivity ； Are there phenomena such as aggregation, viscosity, precipitation, crystallization, caking, vaporization, or foaming? ； Density and density changes ； The amount of suspended particles in the liquid ； The degree of surface disturbance and the particle size of the solid material. (4) The display method and functions of the instruments shall be determined according to the requirements of the process operation and system configuration. When signal transmission is required, instruments with analog signal output functionality or digital signal output functionality can be selected. (5) The instrument range should be determined based on the range that needs to be displayed or the actual range of variation of the process object. Except for level instruments used for volume measurement, the normal level should generally be around 50% of the instrument’s range. (6) The accuracy of instruments should be selected according to process requirements; however, for level instruments used for volume measurement, their accuracy class should be 0.5 or higher. (7) Electronic level instruments for explosive hazard areas such as flammable gases, vapors, and combustible dusts. An appropriate explosion-proof design type should be selected, or other protective measures taken, based on the identified category of hazardous location and the degree of hazard associated with the medium being measured. (8) For electronic level gauges used in environments with corrosive gases and harmful dusts, an appropriate enclosure protection type should be selected based on the operating conditions. Selection of Level and Interface Measurement Instruments 1 Differential pressure measurement instruments (1) For continuous level measurement, differential pressure instruments are preferable. For interface measurement, a differential pressure gauge can be used, provided that the total liquid level is always above the upper pressure-taking port. (2) When high measurement accuracy is required, and the measurement system needs complex precise calculations that are difficult to achieve with conventional analog instruments, a differential pressure intelligent transmitter can be used, with an accuracy of 0.2 grade or higher. (3) Differential pressure instruments are not suitable when there are significant changes in liquid density under normal operating conditions. (4) For corrosive liquids, crystalline liquids, viscous liquids, easily vaporizable liquids, and liquids containing suspended solids, flat-flange differential pressure instruments are recommended. For highly crystalline liquids, highly viscous liquids, gelling liquids, and precipitating liquids, plug-type flange differential pressure instruments are recommended. For the liquid levels of the media under measurement mentioned above, in cases where there is a large amount of condensate or sediment in the gas phase, or when it is necessary to isolate the high-temperature liquid from the transmitter, or when the media under measurement needs to be replaced and the measuring head requires thorough purification, a double-flange differential pressure gauge can be used. (5) When it is difficult to use flanged differential pressure instruments to measure the liquid levels of corrosive liquids, viscous liquids, crystalline liquids, molten liquids, or precipitating liquids, the method of blowing air or injecting fluid can be employed in conjunction with ordinary pressure gauges, pressure transmitters, or differential pressure transmitters for measurement. (6) For cases where, at ambient temperature, the vapor phase may condense, the liquid phase may vaporize, or there is liquid separation in the vapor phase, and it is difficult to use flanged differential pressure instruments while measurement must be carried out using ordinary differential pressure instruments, components such as isolators, separators, vaporizers, and balance vessels should be installed as appropriate depending on the specific circumstances, or the measurement pipelines should be insulated or heated. (7) When using a differential pressure gauge to measure the liquid level in a boiler drum, a temperature-compensated dual-chamber balance vessel should be employed. (8) The positive and negative drift of differential pressure instruments should be taken into consideration when selecting the instrument range. 2. Float-type measuring instruments: (1) Float-type instruments are suitable for the continuous measurement of liquid levels in liquids with a measurement range of up to 2000 mm and a specific gravity of 0.5–1.5, as well as for the continuous measurement of liquid interfaces in liquids with a measurement range of up to 1200 mm and a specific gravity difference of 0.1–0.5. For vacuum applications and volatile liquids, float-type instruments are recommended. Pneumatic float-type instruments are suitable for on-site level indication or control. Float-type instruments must be used for clean liquids. (2) Float-type instruments should be selected; when high precision is required and the signal needs to be transmitted over long distances, a force-balanced type is advisable ； When high precision is not required, and on-site indication or adjustment is sufficient, a displacement-balanced type can be selected. (3) For measuring the liquid level in open storage tanks and open liquid reservoirs, an internal float is recommended ； Internal floats are also suitable for liquid substances that do not crystallize or thicken at the operating temperature, but may crystallize or thicken at ambient temperature. For process equipment where parking is not allowed, internal float tanks should not be used; instead, external float tanks should be chosen. For highly viscous, crystalline, or high-temperature liquid substances, external floating tanks should not be used. (4) For internal float gauges, a stabilizing sleeve should be installed to protect them from the effects of fluid disturbance within the container. (5) Electric float gauges are used in applications where the liquid level under measurement fluctuates frequently, and a damper should be added to their output signal. 3 Float-type measuring instruments: (1) For the continuous measurement of the liquid level and volume calculation of clean liquids in large storage tanks, as well as for the determination of the level and interface of clean liquids in various types of storage tanks, float-type instruments should be used. (2) Float-type instruments are not suitable for dirty liquids, as well as liquids that freeze at ambient temperature. For the continuous measurement of viscous liquids as well as multi-point measurements, float-type instruments are also not suitable. (3) When float-type measuring instruments are used for interface measurement, the specific gravity ratio of the two liquids should remain constant, and the difference in specific gravities should not be less than 0.2. (4) When using internal float-type level gauges for measuring the liquid level in large storage tanks, guiding devices should be provided to prevent the float from drifting ； To prevent the float from being affected by disturbances on the liquid surface, a stabilizing sleeve should be installed. (5) For the continuous measurement of the liquid level or volume in large storage tanks, photoelectric level gauges are suitable for use in single or multiple tanks where high measurement accuracy is required ； For single storage tanks where moderate measurement accuracy is sufficient, a steel tape float-type level gauge can be selected. For single or multi-tank systems that require high-precision continuous measurement of liquid level, interface, volume, and mass, tank measurement systems should be used. (6) For multi-point level measurement in open storage tanks and reservoirs, as well as for multi-point level measurement of hazardous liquids that are corrosive or toxic, magnetic float level gauges are recommended. (7) For the level measurement of viscous liquids, a lever-type float level controller is recommended. 4 Capacitive measuring instruments: (1) For the continuous and level-based measurement of the liquid level in corrosive liquids, sedimentary fluids, and other chemicals used in industrial processes, capacitive level gauges are suitable choices. When used for interface measurement, the electrical properties of the two liquids must meet the technical requirements of the product. (2) The specific model of the capacitive level gauge, the type of electrode structure, and the electrode material should be determined based on factors such as the electrical properties of the medium being measured and the material of the container. (3) For non-viscous, non-conductive liquids, shaft-sleeve type electrodes can be used ； For non-viscous conductive liquids, cannula-type electrodes can be used ； For viscous, non-conductive liquids, bare electrodes can be used; the electrode surface should be made of a material with low affinity for the liquid being measured, or automatic cleaning measures should be employed. (4) Capacitive level gauges cannot be used for the continuous measurement of the level of viscous, conductive liquids. (5) Capacitive measuring instruments are susceptible to electromagnetic interference; shielded cables should be used, or other measures to counter electromagnetic interference should be adopted. (6) Capacitive level gauges used for on-off measurement should preferably be of the horizontally mounted type ； For capacitive level gauges used in continuous measurement, a vertically mounted type is recommended. 5 Resistive (electrical contact) measuring instruments: (1) For measuring the level of corrosive conductive liquids, as well as the interface between conductive and non-conductive liquids, resistive (electrical contact) instruments can be used. (2) For conductive liquids that tend to cause scaling on the electrodes, as well as in cases where electrolysis occurs between the electrodes due to the process medium, resistive (electrical contact) instruments are generally not suitable. For liquids that are non-conductive and tend to adhere to electrodes, resistive (electrical contact) instruments should not be used. 6 Hydrostatic measuring instruments: (1) For continuous measurement of the liquid level in water supply tanks, wells, and reservoirs at depths of 5m to 100m, hydrostatic instruments should be used. For continuous measurement of the liquid level in pressure-free containers, hydrostatic instruments can be used. (2) Under normal operating conditions, when there are significant changes in liquid density, static pressure instruments are not suitable. 7 Acoustic wave measuring instruments: (1) For the continuous and level measurement of corrosive liquids, highly viscous liquids, toxic liquids, and other substances whose levels are difficult to determine using conventional level gauges, acoustic wave measuring instruments are an appropriate choice. (2) The specific model and structural type of the acoustic wave instrument should be determined based on factors such as the properties of the medium to be measured. (3) Acoustic wave instruments must be used for measuring the liquid level in containers that can reflect and transmit sound waves; they shall not be used in vacuum containers. It is not suitable for use with liquids containing bubbles or solid particles. (4) For containers with internal obstacles that affect the propagation of sound waves, acoustic instruments are not suitable. (5) For acoustic wave-based instruments used for continuous level measurement, if there are significant changes in the temperature or composition of the liquid being measured, it is necessary to consider compensating for the changes in the speed of sound propagation in order to improve the accuracy of the measurements. (6) The cable between the detector and the converter should be a shielded cable, or measures to prevent electromagnetic interference should be considered. 8 Microwave measuring instruments: (1) For the continuous measurement of the level of corrosive liquids, high-viscosity liquids, and toxic liquids in large fixed-roof tanks and floating-roof tanks, where it is difficult for conventional level gauges to achieve high precision, microwave measuring instruments should be used. The measurement method of microwave-based measuring instruments involves continuous scanning of microwaves within a specific frequency range. When the distance between the liquid level and the antenna changes, a frequency difference arises between the sensing signal and the reflected signal. This frequency difference is proportional to the distance between the liquid level and the antenna; therefore, by determining this frequency difference, it is possible to determine the liquid level. (2) The structural design and material of the antenna should be determined based on factors such as the properties of the medium being measured and the pressure inside the tank. (3) For storage tanks with internal obstacles that affect microwave propagation, microwave-based instruments are not suitable. (4) When the densities of water vapor and hydrocarbon vapor inside the tank change significantly under normal operating conditions, compensation for the changes in microwave propagation speed should be considered ； For boiling or turbulent liquid surfaces, it is necessary to consider using stationary pipes with tapered flares and other compensatory measures to improve measurement accuracy. 9 Nuclear radiation type level gauges: (1) For non-contact, continuous measurement as well as level detection of liquids with high temperature, high pressure, high viscosity, strong corrosiveness, explosiveness, or toxicity, nuclear radiation type gauges can be used when other level gauges are insufficient to meet the measurement requirements. (2) The intensity of the radiation source should be selected according to the measurement requirements; at the same time, the radiation dose at the work site after the rays pass through the object being measured should be kept as low as possible. The safety dose standards shall comply with the current \"Radiation Protection Regulations\" (GB8703-88). Otherwise, protective measures such as isolation and shielding should be given due consideration. (3) The type of radiation source should be selected based on the measurement requirements and the characteristics of the object being measured, such as the density of the medium, the geometric shape of the container, its material, and wall thickness. When a low source intensity is required, radium (Re) can be used ； When a higher source intensity is required, cesium-137 (Cs137) can be used ； Cobalt 60 (Co60) can be used when thick-walled containers require strong penetration capabilities. (4) To avoid measurement errors caused by the decay of the radiation source, improve operational stability, and reduce the number of calibrations, the measuring instrument should be capable of compensating for decay. 10 Laser-based measuring instruments: (1) For the continuous measurement of the liquid level in containers with complex structures or mechanical obstacles, as well as in containers that are difficult to install using conventional methods, laser-based measuring instruments should be used. (2) For completely transparent liquids with no reflection, laser-based measuring instruments cannot be used. Selection of level measurement instruments 1 Capacitive measurement instruments (1) For granular and powdery materials such as coal, plastic monomers, fertilizers, sand, etc., capacitive measurement instruments are suitable for both continuous level measurement and point-level measurement. (2) The extension cable of the detector should be a shielded cable, or measures to prevent electromagnetic interference should be considered. 2 Acoustic measuring instruments: (1) For level measurement of granular materials with a particle size of 10 mm or less in silos or hoppers that experience little to no vibration, a fork-type level gauge can be used. (2) For the measurement of the material level of powdery materials with a particle size of 5 mm or less, an acoustic-blocking ultrasonic level gauge should be used. (3) For continuous and level-based measurement of fine powdery materials, reflective ultrasonic level gauges are recommended. Reflective ultrasonic level gauges are not suitable for measuring the level of material in silos or hoppers where dust is present, nor are they appropriate for measuring levels in areas with an uneven surface. 3 Resistive (electrical contact) measuring instruments: (1) For the level measurement of granular and powdery materials with good electrical conductivity or poor electrical conductivity but containing moisture, such as coal and coke, resistive measuring instruments can be used. (2) The electrode-to-ground resistance value specified for the product must be met to ensure the reliability and sensitivity of the measurements. 4 Microwave measuring instruments (1) For the level measurement and continuous monitoring of bulk or granular materials that are high in temperature, highly adhesive, corrosive, or toxic, microwave measuring instruments are suitable choices. (2) It is not suitable for level measurement on surfaces with unevenness. 5 Nuclear radiation type measuring instruments: (1) For the level measurement and continuous monitoring of bulk, granular, or powdery materials that are subject to high temperatures, high pressures, have strong adhesiveness, are highly corrosive, or are toxic, nuclear radiation type measuring instruments can be used. (2) Other requirements shall comply with the aforementioned provisions. 6 Laser-based measuring instruments: (1) For containers with complex structures or mechanical obstacles, as well as those for which it is difficult to install measuring devices using conventional methods, laser-based measuring instruments should be used for continuous measurement of the liquid level. (2) Laser-based measuring instruments cannot be used for completely transparent materials with no reflection. 7 Rotating-element level gauges (1) For silos and hoppers that are subject to low pressure and have no pulsating pressure, as well as for measuring the level of granular and powder-like materials with a specific gravity of 0.2 or higher, rotating-element level gauges can be used. (2) The size of the rotor should be selected based on the specific density of the material. (3) To prevent the material from hitting the rotor and causing incorrect readings of the instruments, a protective plate should be installed above the rotor. 8 Diaphragm-type measuring instruments: (1) For measuring the level of granular or powdery materials in silos or hoppers, diaphragm-type measuring instruments can be used. (2) Since the operation of the diaphragm is susceptible to the influence of powder adhesion and the flow pressure of the powder, it cannot be used in applications requiring high precision. 9. Weight-type measuring instruments: (1) For the regular and continuous measurement of the material level in large silos and bulk storage areas where the material level is high and varies over a wide range, as well as in open or sealed, pressure-free containers containing bulk materials, granular materials, or powdery materials with low adhesiveness, weight-type measuring instruments should be used. (2) The shape of the weight should be selected based on factors such as the particle size and moisture content of the material. (3) For level measurement in silos and containers where dust is highly prevalent, weight-type measuring instruments equipped with a blowing device should be used.