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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, corrosiveness, toxicity, pulsation, and other factors of the process are the main criteria for determining the selection of instruments; they 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 significance of the parameters at various measurement points serves as the basis for deciding on 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, indicator types can be used ； For important variables whose trend changes 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 installed ； For variables that require measurement or economic accounting in relation to material balance and energy consumption, it is advisable to establish accumulations ； For certain variables that may affect production or safety, alarms should be set. 3. Cost-effectiveness and consistency: The selection of instruments is also determined by the scale of investment; necessary cost analyses should be conducted to achieve an appropriate performance/price ratio, while meeting the requirements of the process and automatic control systems. 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 with proven reliable performance in actual field use ； It is also important to ensure that the selected instruments are readily available in supply, 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 (measurement) element: The selection of the insertion length should be based on the principle of inserting the detection (measurement) element to a position that is sensitive to temperature changes in the medium being measured and is representative of such changes. 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 the case of armored thermocouples), an additional protective sleeve should be used. Temperature instruments, temperature switches, temperature sensing elements, transmitters, etc., with local electrical contacts installed in flammable and explosive environments should be of the explosion-proof type. Selection of on-site 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: Class 0.5 or Class 0.25 should be selected. 2 Measurement range: The maximum measured value should not exceed 90% of the upper limit of the instrument’s measurement range; the normal measured value is approximately half of this upper limit. The reading of a pressure-type 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 casing is generally set at φ100mm; in locations with poor lighting, at greater heights, or at a greater observation distance, φ150mm should be used. For the connection between the instrument housing and the protection tube, a universal type is generally recommended; alternatively, an axial or radial type can be chosen based on the principle of ease of observation. 4 Pressure-type thermometers – Images. Suitable for low temperatures below –80°C, situations where close observation is not possible, environments with vibration, and for on-site 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 at the local panel, base-type temperature instruments are preferred. 7 Temperature switch: Suitable for applications where temperature measurement requires a contact signal output. Selection of centralized temperature instruments 1 Detection (measurement) element: (1) Choose a thermocouple, thermal resistor, or thermistor with an appropriate rating based on the temperature measurement range. (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 measurement object, the following time constant detection elements can be selected: Thermocouples: 600s, 100s, and 20s ； Thermal resistance: four levels of 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 areas ； Flameproof type: flammable and explosive environments ； 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 purge-type thermocouples should be used ； For the temperature measurement of equipment, outer surfaces of pipes, and rotating surfaces, surface or armored thermocouples and resistance temperature detectors should be 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, multi-element thermocouples should be used ； To save special protective tube materials such as tantalum, improve response speed, or when it is necessary to detect bending during the installation of components, armored thermocouples can be used. 2 Transmitters: In measurement or control systems that are paired with instruments for displaying standard signals, transmitters are used. Where design requirements are met, it is recommended to use transmitters that integrate measurement and transmission functions. 3 Display instruments: (1) For single-point display, conventional indicators are suitable; for multi-point display, digital indicators are preferred. When it is necessary to access historical data, conventional recorders are a good choice. (2) For signal alarm systems, it is advisable to select indicators or recorders with contact signal outputs. (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 points share one display instrument, a reliable changeover switch 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 type, 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℃ ； For -40 to +250°C, select the heat-resistant grade. c. In locations with intermittent electric heating or strong electric/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 – Selection of 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, plenty of dust, or exposure to splashing liquids, sealed all-plastic pressure gauges are recommended. (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 that are highly corrosive, contain solid particles, or are 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 vibrations, shock-resistant pressure gauges or marine pressure gauges should be used. (6) In flammable and explosive environments, when electrical contact signals are required, an explosion-proof electric contact pressure gauge should be selected. (7) For the following measuring media, special pressure gauges should be used: Ammonia gas, liquid ammonia: ammonia pressure gauges, vacuum gauges, 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, diaphragm box pressure gauges, and diaphragm pressure gauges used for general measurements, a 1.5 class or 2.5 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 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 positions, 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 used. (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 material 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 installation accessories: (1) When measuring water vapor and media with a temperature 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) chambers 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 no 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 and 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 metering natural gas, gas, and household propane; ±2.0% ； (6) Measurement of oil used for the control of key energy-consuming equipment and processes: ±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 measuring the flow rate 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. ②Non-standard throttling devices – Images. A venturi tube can be used when 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. Dual 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. A 1/4 circle nozzle can be selected if the following conditions are met: the medium being measured 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 cutouts can be selected for applications where the following conditions are met: the medium to be measured is a dirty medium that may produce deposits before and after the orifice plate (such as blast furnace gas, slurry, etc.) ； Horizontal or inclined pipes must be available. ③When selecting the pressure measurement method, it should be considered that a unified approach should be adopted throughout the entire project as much as possible. Angle connection or flange connection is generally used for pressure tapping. Depending on the operating conditions and measurement requirements, other pressure tapping methods such as radius-based pressure tapping can be used. (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: Low differential pressure: 6 kPa, 10 kPa ； Medium and low pressure: 16 kPa, 25 kPa ； High differential pressure: 40 kPa, 60 kPa. (3) Measures to improve measurement accuracy: For fluids with significant temperature and pressure fluctuations, temperature and pressure compensation measures should be considered ； When the length of the straight pipe section is insufficient or rotational flow occurs inside the pipe, fluid correction measures should be considered, such as selecting a straightener with an appropriate pipe diameter. (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. ②Built-in orifice plate flow meters are suitable for measuring small flow rates of clean liquids, steam, or gases that contain no suspended particles. They can be used when the range ratio is not greater than 3:1, and when high measurement accuracy is not required. These meters are an option 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 measurement of fluid flow rates in applications involving moderate to low flow rates and very low flow rates, where the pressure is less than 1 MPa and the temperature is below 100°C. The fluids in question 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 Ordinary metal tube rotameter: It 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). The ordinary metal tube rotameter can be used when on-site indication or remote signal transmission is required. ②Special-type metal tube rotameters – Metal tube rotameters with jackets. When the medium to be measured is prone to crystallization, vaporization, or has high viscosity, a metal tube rotameter with a jacket can be selected. A heating or cooling medium is passed through the jacket. Corrosion-resistant metal tube rotameters: For measuring the flow rate of corrosive media, corrosion-resistant metal tube rotameters can be used. (3) For rotameter images, vertical installation is required, 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. For measuring the flow rate of liquids with high viscosity and containing a small amount of solid particles, a target flow meter can be used when the required accuracy is not higher than grade 1.5 and the range ratio is not greater than 3:1. Target flow meters are generally installed on horizontal pipes. The length of the front straight section is 15–40D, and the length of the rear straight section is 5D. (2) Turbine flowmeter: Images. Turbine flowmeters can be used for measuring the flow rate of clean gases and clean liquids with a dynamic viscosity of no more than 5×10-6 m2/s. They are suitable when high accuracy is required and the range ratio is no more than 10∶1. The turbine flow meter should be installed on a horizontal pipe so that the liquid fills the entire pipe. Upstream and downstream shut-off valves and a bypass valve should be provided, along with a filter upstream and a drain valve downstream. Length of straight pipe section: not less than 20D upstream, and not less than 5D downstream. (3) Vortex flowmeters (Carman vortex street flowmeters or vortex flowmeters) 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 pipe flow velocity 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 ； Downstream is at least 5D. (4) Water meter: It accumulates the flow rate of water on-site; a water meter can be used when a range ratio of less than 30∶1 is required. The water meter should be installed on horizontal pipes, with the required length of straight pipe sections being at least 8D upstream and at least 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 good corrosion and wear resistance, with no pressure loss. It can measure various media such as strong acids, strong bases, salts, ammonia water, sludge, mineral slurries, pulp, etc. 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, vertical installation is preferred. 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 Non-standard throttling devices: Refer to the section on selecting flow measurement instruments for high-viscosity fluids mentioned earlier. 1 Positive displacement flowmeters: (1) Elliptical gear flowmeter. For clean liquids with high viscosity that require accurate flow measurement, an elliptical gear flowmeter 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 indicator dial lying in a vertical plane ； Upstream and downstream isolation valves as well as bypass valves should be installed. A filter should be installed upstream. For micro-flow rates, a miniature elliptical gear flow meter can be used. When measuring various easily vaporizable media, a degasser should be added. (2) Gear flow meter – Image: 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 flow meter: 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 flow meter can be chosen for this purpose. When installing a paddle 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 flow meters: For measuring the flow rate of liquids with relatively high viscosity and containing a small amount of solid particles, target flow meters can be used when the required accuracy level is no higher than Class 1.5 and the range ratio does not exceed 3:1. Target flow meters are generally installed on horizontal pipes. The length of the front straight section is 15–40D, and the length of the rear straight 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 bell-shaped average velocity tubes, inserted vortex flowmeters, inserted turbines, electromagnetic flowmeters, venturi tubes, and ultrasonic flowmeters can be chosen depending on the situation. 1 Flute-type average velocity tube flow meter: Used for measuring the flow rate of clean gases, steam, and clean liquids with a viscosity of less than 0.3 Pa·s. It is an appropriate choice when a low pressure loss is required. The fluted 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, venturi tubes – see above. Selection of New Flow Measurement Instruments 1 Ultrasonic Flow Meters Images Ultrasonic flow meters can be used for any fluid that allows sound to propagate through it. In addition to ordinary fluids, they can also be employed for media 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 directly, mass flow meters can be used. A mass flow meter 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 1 Impulse flow meters: For measuring the flow of freely falling powdery and granular solids, impulse flow meters are a suitable choice when it is necessary to transport the material in a closed system ； 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 predetermined 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. 2 Electronic belt scales are used for measuring the solid flow rate in belt conveyors, and they are 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 Track scales: For the continuous automatic weighing of railway freight cars, dynamic track scales are preferred. 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. This enables a proper assessment of the technical performance and economic benefits of the gauge, so that it can play its proper role in ensuring stable production, improving product quality, and enhancing 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 measurement requirements. (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 changes in density ； The amount of suspended particles in the liquid ； The degree of surface disturbance and the particle size of the solid material. (4) The display mode and functions of the instruments shall be determined according to the requirements of process operations and system composition. 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 at least 0.5 grade. (7) Electronic level instruments for explosive hazardous areas such as flammable gases, vapors, and combustible dusts. The appropriate explosion-proof construction type or other protective measures should be selected based on the determined category of the hazardous area and the degree of hazard posed by the medium being measured. (8) For electronic level instruments 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 Type Instruments (1) For continuous level measurement, differential pressure type instruments are recommended. 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) When there is a significant change in liquid density under normal operating conditions, differential pressure instruments should not be used. (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 measured medium 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 level of corrosive liquids, viscous liquids, crystalline liquids, molten liquids, or precipitating liquids, the blowing or flushing method 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 measuring the liquid level in a boiler drum using a differential pressure gauge, a temperature-compensated dual-chamber balance tank should be used. (8) The positive and negative drift of differential pressure instruments should be taken into consideration when selecting the instrument range. 2 Buoy-type measuring instruments: (1) 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, buoy-type instruments are suitable. Float-type instruments are suitable for vacuum applications and volatile liquids. 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 chosen; 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 chosen. (3) For measuring the liquid level in open storage tanks and uncovered liquid reservoirs, an internal float is recommended ； For liquid substances that do not crystallize or become viscous at operating temperatures, but may crystallize or become viscous at ambient temperatures, an internal float is also a suitable choice. 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 float 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 inside the container. (5) Electric float instruments are used in situations where the liquid level to be measured fluctuates frequently; 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 liquid level and interface positions in various types of storage tanks containing clean liquids, 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 densities of the two liquids must be constant, and the difference in specific densities should not be less than 0.2. (4) When internal float type level gauges are used for measuring the liquid level in large storage tanks, guide mechanisms should be provided to prevent the float from drifting ； To prevent the float from being affected by surface disturbances, a stabilizing sleeve should be installed. (5) For the continuous measurement of the liquid level or volume in large storage tanks, optical level gauges are suitable for single or multiple tanks where high measurement accuracy is required ； For single-tank applications where only average measurement accuracy is required, a steel-band float level gauge can be used. 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 open liquid 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 – Images: (1) For the continuous and level-based measurement of corrosive liquids, sedimentary fluids, and other chemicals used in various industrial processes, capacitive level gauges are the preferred choice. 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, a shaft-sleeve type electrode can be used ； For non-viscous conductive liquids, sleeve-type electrodes can be used ； For viscous, non-conductive liquids, bare electrodes can be used; the electrode surface should be made of a material that has little 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 Static pressure measuring instruments: (1) For continuous measurement of the liquid level in water supply tanks, wells, and reservoirs at depths of 5m to 100m, static pressure 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, it is not advisable to use hydrostatic pressure-type instruments. 7 Acoustic measuring instruments: (1) For the continuous and level measurement of corrosive liquids, highly viscous liquids, toxic liquids, and other types of liquids that are difficult to measure using conventional level gauges, acoustic measuring instruments are a suitable 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 instruments must be used for measuring the liquid level in containers where sound waves can be reflected and propagated, and must 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 should not be used. (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 should not be used. (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-based measuring instruments. (1) For non-contact continuous and point-level measurement of the liquid level in media with high temperature, high pressure, high viscosity, strong corrosiveness, explosiveness, or toxicity, nuclear radiation-based instruments can be selected when it is difficult to meet the measurement requirements using other level measuring instruments. (2) The intensity of the radiation source should be selected according to the measurement requirements. Meanwhile, after the radiation passes through the object being measured, the radiation dose at the workplace should be kept as low as possible. The safety dose standards must comply with the current “Regulations on Radiation Protection” (GB8703-88). Otherwise, protective measures such as isolation and shielding should be thoroughly considered. (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 (Cs-137) can be selected ； Cobalt 60 (Co60) can be used when thick-walled containers require strong penetration capability. (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 the continuous and level measurement of granular and powdery materials such as coal, plastic monomers, fertilizers, sand, etc., capacitive measurement instruments are suitable choices. (2) The extension cable of the detector should be a shielded cable, or measures to prevent electromagnetic interference should be considered. 2 Acoustic wave-based measuring instruments (1) For point-level measurement of granular materials with a particle size of 10 mm or less in silos and hoppers where there is little or no vibration, a tuning fork level gauge can be used. (2) For level measurement of powdered or granular materials with a particle size of 5 mm or less, an acoustic-blocking ultrasonic level gauge should be selected. (3) For continuous and level measurement of finely powdered materials, a reflective ultrasonic level gauge is recommended. Reflective ultrasonic level gauges are not suitable for measuring the level of materials 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 the continuous measurement of the material level in containers with complex structures or mechanical obstructions, as well as those that are difficult to install using conventional methods, laser-based measuring instruments should be selected. (2) Laser-based measuring instruments cannot be used for completely transparent materials with no reflection. 7 Gyroscopic level gauges: (1) For silos and hoppers that are subject to low pressure and do not experience pulsating pressures, gyroscopic level gauges can be used for measuring the level of granular and powder-like materials with a specific gravity of 0.2 or higher. (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 powder-like 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 form 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 present in high concentrations, weight-type measuring instruments equipped with a blowing device should be used.