In the process of industrial production, instruments play an important role in detecting, displaying, recording or controlling process parameters. The detection of the process production process is a basic means to understand and control industrial production. Only by accurately understanding the overall picture of the process at all times and controlling it can the production process proceed smoothly, and qualified products can be produced with high productivity and low consumption. Therefore, although instruments may seem unremarkable, they are indispensable! Today, the editor will take you to learn about the selection of various instruments, which is very comprehensive! Today, we will first introduce the selection of temperature instruments, pressure instruments, and flow instruments: I. Selection of on-site temperature instruments 1. Accuracy class General industrial thermometers: 1.5 or 1 class should be selected. Thermometers for precision measurement and laboratory use: 0.5 or 0.25 class should be selected. 2. Measurement range The maximum measured value shall not be greater than 90% of the upper limit of the instrument's measurement range, and the normal measured value shall be around 1/2 of the upper limit of the instrument's measurement range. The measured value of the pressure thermometer shall be between 1/2 and 3/4 of the upper limit of the instrument's measurement range. 3. Bimetallic thermometer When the requirements for measurement range, working pressure and accuracy are met, it should be preferred. The dial diameter is generally φ100mm. In places with poor lighting, high position and long observation distance, φ150mm should be selected. The connection mode between the instrument shell and the protective tube should generally be universal, and axial or radial can also be selected according to the principle of convenient observation. 4. Pressure thermometer It is suitable for low temperature below -80℃, unable to observe at close range, with vibration and low accuracy requirements for on-site or on-site panel display. 5. Glass thermometer It is only used in special occasions where the measurement accuracy is high, the vibration is small, there is no mechanical damage, and observation is convenient. However, due to mercury hazards, glass mercury thermometers should not be used. 6. Base-type instrument For on-site or on-site panel-mounted measurement and control (regulation) instruments, base-type temperature instruments should be selected. 7. Temperature switch It is suitable for occasions where contact signal output is required for temperature measurement. II. Selection of pressure instruments (selection of pressure gauges) 1. Select according to the service environment and the nature of the measured medium (1) In harsh environments with strong atmospheric corrosion, much dust and easy liquid spraying, closed all-plastic pressure gauges should be selected. (2) For dilute nitric acid, acetic acid, ammonia and other general corrosive media, acid-resistant pressure gauges, ammonia pressure gauges or stainless steel diaphragm pressure gauges should be selected. (3) For dilute hydrochloric acid, hydrochloric acid gas, heavy oil and other similar media with strong corrosiveness, solid particles and viscous liquids, diaphragm pressure gauges or diaphragm seals should be selected. The material of the diaphragm or diaphragm must be selected according to the characteristics of the measured medium. (4) For media such as crystallization, scarring and high viscosity, diaphragm pressure gauges should be selected. (5) In occasions with strong mechanical vibration, shock-proof pressure gauges or marine pressure gauges should be selected. (6) In flammable and explosive occasions, if electrical contact signals are needed, explosion-proof electrical contact pressure gauges should be selected. (7) Special pressure gauges should be selected for the following measured media: Gaseous ammonia, liquid ammonia: ammonia pressure gauge, vacuum gauge, pressure vacuum gauge; Oxygen: oxygen pressure gauge; Hydrogen: hydrogen pressure gauge; Chlorine: chlorine-resistant pressure gauge, pressure vacuum gauge; Acetylene: acetylene pressure gauge; Hydrogen sulfide: sulfur-resistant pressure gauge; Alkali solution: alkali-resistant pressure gauge, pressure vacuum gauge. 2. Selection of accuracy class (1) Pressure gauges, capsule pressure gauges and diaphragm pressure gauges for general measurement should be 1.5 or 2.5 class. (2) Pressure gauges for precision measurement and calibration should be 0.4, 0.25 or 0.16 class. 3. Selection of external dimensions (1) The nominal diameter of pressure gauges installed on pipes and equipment is φ100mm or φ150mm. (2) The nominal diameter of pressure gauges installed on instrument pneumatic pipelines and their auxiliary equipment is φ60mm. (3) The nominal diameter of pressure gauges installed in occasions with low illumination, high position and difficult reading of indicated values is φ200mm or φ250mm. 4. Selection of measurement range (1) When measuring stable pressure, the normal operating pressure value should be within 2/3 to 1/3 of the upper limit of the instrument's measurement range. (2) When measuring pulsating pressure (such as pressure at the outlet of pumps, compressors and fans), the normal operating pressure value should be within 1/2 to 1/3 of the upper limit of the instrument's measurement range. (3) When measuring high and medium pressure (greater than 4MPa), the normal operating pressure value should not exceed 1/2 of the upper limit of the instrument's measurement range. 5. Unit and scale (1) Pressure instruments shall use legal units of measurement, namely: Pascal (Pa), kilopascal (kPa) and megapascal (MPa). (2) For foreign-related design projects and imported instruments, international general standards or corresponding national standards can be adopted. III. Selection of flow instruments General principles 1. Scale selection The instrument scale should meet the requirements of the instrument scale modulus. When the scale reading is not an integer, it can also be selected as an integer for the convenience of reading conversion. (1) Square root scale range The maximum flow shall not exceed 95% of the full scale; the normal flow is 70% - 85% of the full scale; the minimum flow is not less than 30% of the full scale. (2) Linear scale range The maximum flow shall not exceed 90% of the full scale; the normal flow is 50% - 70% of the full scale; the minimum flow is not less than 10% of the full scale. 2. Instrument accuracy Flowmeters used for energy metering shall comply with the provisions of "General Rules for the Equipping and Management of Enterprise Energy Measuring Instruments (Trial)". (1) For metering of fuel inbound and outbound settlement, ±0.1%; (2) For metering of technical and economic analysis of workshops, teams and processes, ±0.5% - 2%; (3) For metering of industrial and domestic water, ±2.5%; (4) For metering of steam including superheated steam and saturated steam, ±2.5%; (5) For metering of natural gas, gas and household gas, ±2.0%; (6) For metering of oil for key energy-consuming equipment and process control, ±1.5%; (7) For metering of other energy-containing working substances (such as compressed air, oxygen, nitrogen, hydrogen, water, etc.) used for process control, ±2%. 3. Flow unit Volumetric flow is expressed in m³/h, l/h; mass flow is expressed in kg/h, t/h; gas volumetric flow under standard conditions is expressed in Nm³/h (0℃, 0.1013MPa). Selection of flow measurement instruments for general fluids, liquids and steam 1. Differential pressure flowmeters. Article sources: Civil Engineering Think Tank, Industrial Control South