Classification of flow meters for electronic measuring instruments

　　1. Velocity flowmeter (such as vortex flowmeter, swirl flowmeter, turbine flowmeter, ultrasonic flowmeter, etc.); differential pressure flowmeter (orifice flowmeter, V-cone flowmeter, etc.); positive displacement flowmeter (elliptical gear flowmeter, Roots flowmeter, membrane gas meter, etc.); mass flowmeter (Coriolis mass flowmeter, calorimetric mass flowmeter, etc.); open channel flowmeter (Parshall flume, triangular weir, etc.)

　　2. The differential pressure flowmeter is a flowmeter that calculates the differential pressure by installing a flow detection element in an industrial pipeline and calculating the differential pressure flow by using known fluid conditions and the geometric dimensions of the detection element and the pipeline.

　　The differential pressure flowmeter consists of a primary detection component and a secondary instrument (differential pressure converter or transmitter and flow display instrument). Differential pressure flowmeters are classified according to the form of detection components, including orifice flowmeters, venturi flowmeters, and average velocity tube flowmeters. Secondary instruments include various mechanical, electronic, and electromechanical integrated differential pressure flowmeters, differential pressure transmitters, and flow display instruments.

　　The differential pressure flow meter is the most widely used flow meter in the family of flow meters. At present, it has been serialized, universalized and standardized at home and abroad. The differential pressure flow meter can measure flow parameters alone and other parameters (pressure, level, density), etc. The detection parts of the differential pressure flow meter can be divided into several categories according to their working principles: throttling device, hydraulic resistance, dynamic pressure head type, dynamic pressure head gain and jet type, and centrifugal type.

　　There are two types of detection components: standardized or non-standard. Standard detection components are designed, manufactured, installed and used according to standard documents. Their flow values ​​and estimated measurement errors can be determined without actual flow calibration. Non-standard detection components are generally not included in the international standard detection components. Differential pressure flowmeter is also the most widely used flow meter, ranking first among all kinds of flowmeters. The main advantages are: (1) The most widely used orifice flowmeter has a solid structure, stable and reliable performance, and a long service life; (2) It has a wide range of applications, and no other flowmeter can compare with it so far; (3) The detection components, transmitters, and display instruments are produced by different manufacturers, which is convenient for economies of scale.

　　The main disadvantages are: (1) the measurement accuracy is generally low; (2) the range is narrow, generally only 3:1 to 4:1; (3) high requirements for on-site installation conditions; (4) large pressure loss (referring to orifice plates, nozzles, etc.).

　　3. Positive displacement flowmeter, also known as fixed displacement flowmeter, PD flowmeter for short, is the most accurate type of flow meter. It uses mechanical measuring elements to continuously divide the fluid into a single known volume portion, and measures the total volume of the fluid according to the number of times the measuring chamber is repeatedly filled and discharged with the volume portion of the fluid.

　　Positive displacement flow meters are classified according to their measuring elements: there are oval gear flow meters, rotary piston flow meters, reciprocating piston flow meters, disc flow meters, wet gas meters and diaphragm gas meters, liquid-sealed drum flow meters, etc.

　　Main advantages: (1) High measurement accuracy; (2) The installation pipeline conditions have no effect on the measurement accuracy; (3) It can be used to measure high-viscosity liquids; (4) Wide range; (5) Direct-reading instruments can directly obtain the accumulation and total amount without external energy, which is clear and easy to operate. Main disadvantages: (1) Complex results and large size; (2) The type, caliber and working state of the measured medium are greatly limited; (3) It is not suitable for high and low temperature occasions; (4) Most instruments are only suitable for clean single-phase fluids; (5) Noise and vibration are generated.

　　4. Float flowmeter, also known as rotor flowmeter, is a type of variable area flowmeter. In a vertical cone that expands from bottom to top, the gravity of the float with a circular cross-section is borne by the liquid power, so that the float can rise and fall freely in the cone. Float flowmeter is a type of flowmeter that is widely used after differential pressure flowmeter and is suitable for monitoring small flow. Main advantages: (1) Simple structure and easy to use; (2) Suitable for small pipe diameter and low flow rate; (3) Low pressure loss. Disadvantages: Low pressure resistance and glass tube is fragile.

　　Turbine flowmeters, positive displacement flowmeters and Coriolis mass flowmeters are collectively referred to as the three types of flowmeters with the best repeatability and accuracy. Currently, they are developing towards multiple varieties and multiple series.

　　Main advantages: (1) High accuracy, the most accurate flow meter among all flow meters; (2) Good repeatability; (3) No zero drift, good anti-interference; (4) Wide measuring range; (5) Compact structure.

　　Main disadvantages: (1) Calibration characteristics cannot be maintained for a long time; (2) Physical properties of the fluid have a significant impact on flow characteristics.

　　6. Vortex flowmeter

　　The structure of the vortex flowmeter is to place a non-streamlined vortex generator in the fluid. When the fluid is alternately separated on both sides of the vortex generator, two strings of regularly staggered vortices are released. Vortex flowmeters are generally divided into stress type, strain type, capacitance type, thermal type, photoelectric type, ultrasonic type, vibration type, etc. according to the frequency detection method. ) Vortex flowmeter is a new type of flow meter at home and abroad.

　　Main advantages: (1) Simple and strong structure; (2) Applicable to flow rates of various fluid types; (3) High measurement accuracy; (4) Wide measurement range and small pressure loss.

　　Main disadvantages: (1) Not suitable for measuring low Reynolds number fluids; (2) Requires a longer straight pipe section; (3) Compared with turbine flowmeters, the instrument coefficient is lower.

　　7. Electromagnetic flow meter

　　Electromagnetic flowmeter consists of sensor, converter and display. It is a general flowmeter for measuring conductive fluids based on Faraday's law of electromagnetic induction. Electromagnetic flowmeter has unique advantages that other flowmeters cannot match, and is particularly suitable for measuring dirty and corrosive fluids. Electromagnetic flowmeter has made major breakthroughs in electromagnetic flow technology in the 1970s and 1980s, making it a flow monitoring instrument widely used in modern industrial fields.

　　Main advantages: (1) Since the measuring channel is a smooth straight tube, it will not be blocked and is particularly suitable for liquid-solid two-phase fluids with solid particles, such as pulp, sewage, mud, etc.; (2) No pressure loss and good energy-saving effect; (3) Not affected by changes in fluid humidity, density, viscosity, pressure and conductivity; (4) Large flow range and wide caliber range; (5) Suitable for the measurement of corrosive fluids.

　　Main disadvantages: (1) Not suitable for measuring fluids released from petroleum products; (2) Not suitable for gases, steam and liquids containing large bubbles; (3) Not suitable for high temperature occasions.

　　8. Ultrasonic flow meter

　　Ultrasonic flowmeter is designed based on the principle that the speed of ultrasonic wave propagation in the flowing medium is equal to the geometric sum of the average flow velocity of the measured medium and the speed of the sound wave itself. It also reflects the flow rate by measuring the flow velocity. Although ultrasonic flowmeter only appeared in the 1970s, it is very popular because it can be made into a non-contact type and can be linked with ultrasonic water level meter to measure the open flow, and it does not cause disturbance and resistance to the fluid. It is a very promising flowmeter.

　　Classification of ultrasonic flowmeters: 1 Doppler ultrasonic flowmeter: transducer 1 transmits an ultrasonic signal with a frequency of f1. After passing through the suspended particles or bubbles in the liquid in the pipeline, the frequency shifts and is reflected to transducer 2 at a frequency of f2. This is the Doppler shift. The difference between f2 and f1 is the Doppler frequency difference fd. Assume that the fluid flow velocity is v, the ultrasonic sound velocity is c, and the Doppler frequency shift fd is proportional to the fluid flow velocity v. When the pipeline conditions, transducer installation position, transmission frequency, and sound velocity are determined, c, f1, and θ are constants. The fluid flow velocity is proportional to the Doppler frequency shift. By measuring the frequency shift, the fluid flow velocity can be obtained, and then the fluid flow rate can be obtained. 2 Time difference ultrasonic flowmeter: The time difference ultrasonic flowmeter measures the fluid flow rate by using the principle that the time difference between the propagation of sound waves downstream and upstream in the fluid is proportional to the fluid flow velocity.