/div> & nbsp;& nbsp;& nbsp;& nbsp; 1 Overview
 & nbsp;& nbsp;& nbsp;& nbsp; There are many kinds of flow measurement methods and instruments, and there are many classification methods. Up to now, there are more than 60 kinds of flow meters available for industry. The reason why there are so many varieties is that we have not found a kind of flow meter which is suitable for any fluid, any range, any flow state and any service condition. Each of these 60 kinds of flow meters has its specific applicability and limitations. According to the survey objects, there are two categories: closed pipeline and open channel; According to the purpose of measurement, it can be divided into gross measurement and flow measurement, and the instruments are called gross meter and flowmeter respectively.
 & nbsp;& nbsp;& nbsp;& nbsp; The total flow meter measures the flow through the pipe in a period of time, which is expressed by the quotient of the total flow in a short time divided by the time. In fact, the flowmeter is usually equipped with a cumulative flow device to be used as the total flow meter, and the total flow meter is also equipped with a flow signal device. Therefore, it has no practical significance to divide flowmeter and total meter in strict sense. According to the principle of measurement, there are mechanical principle, thermal principle, acoustic principle, electrical principle, optical principle, atomic physics principle, etc.
 & nbsp;& nbsp;& nbsp;& nbsp; In this paper, according to the most popular and extensive classification method, namely: volumetric flowmeter, differential pressure flowmeter, rotameter, turbine flowmeter, electromagnetic flowmeter, vortex flowmeter, mass flowmeter and plug-in flowmeter in fluid oscillation flowmeter, the principle, characteristics, application situation and development situation at home and abroad of various flowmeters are described respectively.
1.1 differential pressure flowmeter
Differential pressure flowmeter is an instrument that calculates the flow according to the differential pressure generated by the flow detector installed in the pipeline, the known fluid conditions and the geometric dimensions of the detector and pipeline. Differential pressure flowmeter is composed of primary device (detector) and secondary device (differential pressure conversion and flow display instrument). Differential pressure flow meters are usually classified in the form of test pieces, such as orifice flowmeter, venturi flowmeter, averaging tube flowmeter, etc.
 & nbsp;& nbsp;& nbsp;& nbsp; The secondary devices are all kinds of mechanical, electronic and electromechanical integrated differential pressure gauges, differential pressure transmitters and flow display instruments. It has been developed into a large class of instruments with high degree of serialization, generalization and standardization. It can measure not only flow parameters, but also other parameters (such as pressure, material level, density, etc.).
 & nbsp;& nbsp;& nbsp;& nbsp; According to its working principle, the detecting parts of differential pressure flowmeter can be divided into throttling device, hydraulic resistance type, centrifugal type, dynamic head type, dynamic head gain type and jet type. Test pieces can be divided into two categories according to the degree of Standardization: standard and non-standard. The so-called standard test piece is designed, manufactured, installed and used according to the standard documents, and its flow value and measurement error can be determined without real flow calibration. The non-standard test pieces are less mature and have not been listed in the international standards. Differential pressure flowmeter is one of the most widely used flowmeters, and its usage takes the first place in all kinds of flow instruments. In recent years, due to the advent of various new types of flowmeter, its usage percentage is gradually decreasing, but it is still the most important type of flowmeter at present.
Advantages:
 & nbsp;& nbsp;& nbsp;& nbsp; （ 1) The most widely used orifice flowmeter has firm structure, stable and reliable performance and long service life< Br > (2) it has a wide range of applications, and so far there is no comparable flowmeter< Br > (3) the detector, transmitter and display instrument are produced by different manufacturers, which is convenient for scale economy production.
Disadvantages:
(1) the measurement accuracy is generally low< Br > (2) the range is narrow, generally only 3:1 ~ 4:1< Br > (3) the requirements of field installation conditions are high< Br > (4) large pressure loss (orifice, nozzle, etc.).
Application Overview:
Differential pressure flowmeter is widely used in various objects of closed pipe flow measurement, such as fluid: single phase, mixed phase, clean, dirty, viscous flow, etc; Working conditions: normal pressure, high pressure, vacuum, normal temperature, high temperature, low temperature, etc; Pipe diameter: from several mm to several m; Flow conditions: subsonic, sonic, pulsating flow, etc. Its consumption in various industrial sectors accounts for about 1 / 4 ~ 1 / 3 of the total consumption of flowmeter.
1.2 Rotameter
Rotameter, also known as rotameter, is a kind of variable area flowmeter. In a vertical cone tube expanded from bottom to top, the gravity of the float with circular cross section is borne by liquid power, so that the float can rise and fall freely in the cone tube. Rotameter is the most widely used flowmeter after differential pressure flowmeter, especially in small and micro flow. In the mid-1980s, Japan, Western Europe and the United States accounted for 15% ~ 20% of the total sales of flow meters. China's output in 1990 is estimated to be 120000-140000 units, of which more than 95% are glass cone rotameters.
Features:
(1) the glass cone tube rotameter is simple in structure and easy to use, but it has the disadvantages of low pressure resistance and high risk of glass tube breakability< Br > (2) suitable for small diameter and low flow rate< Br > (3) the pressure loss is low.
　　1. 3 volumetric flowmeter
Volumetric flowmeter, also known as constant displacement flowmeter, referred to as PD flowmeter, is one of the most accurate flow meters. It uses mechanical measuring elements to continuously divide the fluid into a single known volume part, and measures the total volume of the fluid according to the number of times that the measuring chamber repeatedly fills and discharges the fluid of the volume part. According to the classification of measuring elements, volumetric flowmeter can be divided into oval gear flowmeter, scraper flowmeter, double rotor flowmeter, rotary piston flowmeter, reciprocating piston flowmeter, disc flowmeter, liquid sealed rotary cylinder flowmeter, wet gas meter and membrane gas meter.
Advantages:
(1) high measurement accuracy< Br > (2) the installation condition of pipeline has no effect on the measurement accuracy< Br > (3) it can be used to measure high viscosity liquid< Br > (4) wide range< Br > (5) the direct reading instrument can directly obtain the total amount without external energy, which is clear and easy to operate.
Disadvantages:
(1) the results are complex and bulky< Br > (2) the type, diameter and working state of the measured medium are limited< Br > (3) not suitable for high and low temperature< Br > (4) most instruments are only suitable for clean single phase fluid< Br > (5) produce noise and vibration.
Application Overview:
Volumetric flowmeter, differential pressure flowmeter and rotameter are the three most widely used flowmeters, which are often used in the total amount measurement of expensive media (oil, natural gas, etc.). In recent years, the sales amount of PD flowmeter (excluding domestic gas meter and domestic water meter) in developed countries accounts for 13% ~ 23% of flow meters; China accounts for about 20% of the total output. In 1990, the output (excluding domestic gas meters) is estimated to be 340000 units. Among them, elliptical gear type and waist wheel type account for about 70% and 20% respectively.
1.4 turbine flowmeter
Turbine flowmeter is the main type of velocity flowmeter. It uses multi blade rotor (turbine) to sense the average velocity of fluid and deduce the flow or total amount. Generally, it is composed of sensor and display instrument, and can also be made into integral type. Turbine flow meter, volumetric flow meter and Coriolis mass flow meter are three kinds of products with the best repeatability and accuracy. As one of the ten types of flow meters, their products have developed into the scale of multi variety and multi series mass production.
 & nbsp;& nbsp;& nbsp;& nbsp; advantage:
 & nbsp;& nbsp;& nbsp;& nbsp; ( 1) High precision, among all flowmeters, it is the most accurate one< BR> & nbsp;& nbsp;& nbsp;& nbsp; ( 2) Good repeatability< BR> & nbsp;& nbsp;& nbsp;& nbsp; ( 3) Element zero drift, good anti-interference ability< BR> & nbsp;& nbsp;& nbsp;& nbsp; ( 4) The range is wide< BR> & nbsp;& nbsp;& nbsp;& nbsp; ( 5) Compact structure.
Disadvantages:
 & nbsp;& nbsp;& nbsp;& nbsp; ( 1) The calibration characteristics cannot be maintained for a long time< BR> & nbsp;& nbsp;& nbsp;& nbsp; ( 2) The physical properties of fluid have great influence on the flow characteristics.
Application Overview:
Turbine flowmeter is widely used in the following measurement objects: petroleum, organic liquid, inorganic liquid, liquefied gas, natural gas and low temperature fluid. In Europe and the United States, turbine flowmeter is second only to orifice flowmeter in terms of consumption. In the Netherlands alone, more than 2600 natural gas pipelines of various sizes are used, The gas turbine flowmeter with pressure from 0.8 to 6.5MPa has become an excellent natural gas measuring instrument.
1.5 electromagnetic flowmeter
Electromagnetic flowmeter is a kind of instrument for measuring conductive liquid based on Faraday's law of electromagnetic induction. Electromagnetic flowmeter has a series of excellent characteristics, which can solve the problems that other flowmeters are not easy to apply, such as the measurement of dirty flow and corrosion flow. In the 1970s and 1980s, the electromagnetic flow meter made a great breakthrough in technology, which made it a widely used type of flowmeter. The percentage of its usage in the flow meter was increasing.
Advantages:
(1) the measurement channel is a smooth straight pipe, which is suitable for measuring liquid-solid two-phase fluid containing solid particles, such as pulp, mud, sewage, etc< Br > (2) no pressure loss caused by flow detection, good energy saving effect< Br > (3) the measured volume flow rate is not affected by the changes of fluid density, viscosity, temperature, pressure and conductivity< Br > (4) large flow range and wide diameter range< Br > (5) corrosive fluid can be used.
< br > disadvantages:
(1) liquid with low conductivity, such as petroleum products, cannot be measured< Br > (2) gases, vapors and liquids with large bubbles cannot be measured< Br > (3) can not be used at higher temperature.
< br > Application Overview:
Electromagnetic flowmeter is widely used in water supply and drainage engineering; Small and medium calibers are often used in high requirements or difficult to measure occasions, such as cooling water control of blast furnace tuyere in iron and steel industry, pulp and black liquor measurement in paper industry, strong corrosive liquid in chemical industry, pulp in non-ferrous metallurgy industry; Small caliber and micro caliber are commonly used in places with hygienic requirements such as pharmaceutical industry, food industry and biochemistry.
　　16 vortex flowmeter
Vortex flowmeter is an instrument in which a non streamline vortex generator is placed in the fluid, and the fluid is separated alternately on both sides of the generator to release two series of regularly staggered vortex. Vortex flowmeter can be divided into stress type, strain type, capacitance type, thermal type, vibrating body type, photoelectric type and ultrasonic type according to frequency detection mode. Vortex flowmeter is the youngest type of flowmeter, but it has developed rapidly and has become a general type of flowmeter.
Advantages:
(1) the structure is simple and firm< Br > (2) there are many kinds of suitable fluids< Br > (3) high precision< Br > (4) wide range< Br > (5) small pressure loss.
Disadvantages:
(1) it is not suitable for low Reynolds number measurement< Br > (2) long straight pipe section is needed< Br > (3) the instrument coefficient is lower (compared with turbine flowmeter)< Br > (4) the instrument is lack of application experience in pulsating flow and multiphase flow.
1.7 ultrasonic flowmeter
Ultrasonic flowmeter is an instrument that measures the flow by detecting the effect of fluid flow on ultrasonic beam (or ultrasonic pulse). According to the principle of signal detection, ultrasonic flowmeter can be divided into propagation velocity difference method (direct time difference method, time difference method, phase difference method and frequency difference method), beam migration method, Doppler method, cross-correlation method, spatial filtering method and noise method. Like the electromagnetic flowmeter, the ultrasonic flowmeter belongs to the unimpeded flowmeter because there is no obstruction in the flow channel of the instrument. It is a kind of flowmeter which is suitable for solving the difficult problem of flow measurement, especially in the aspect of large diameter flow measurement. In recent years, it is one of the types of flowmeter which is developing rapidly.
Advantages:
(1) it can be used for non-contact measurement< Br > (2) no flow obstruction, no pressure loss< Br > (3) it can measure non-conductive liquid, which is a supplement to the electromagnetic flowmeter.
Disadvantages:
(1) the propagation time method can only be used to clean liquids and gases; The Doppler method can only be used to measure the liquid containing a certain amount of suspended particles and bubbles< Br > (2) the measurement accuracy of Doppler method is not high.
Application Overview:
(1) the propagation time method is applied to clean, single-phase liquids and gases. Typical applications include factory discharge liquid, strange liquid, liquefied natural gas, etc< Br > (2) gas application has good experience in the field of high pressure natural gas< Br > (3) Doppler method is suitable for two-phase fluids with low heterogeneous content, such as untreated wastewater, plant effluent and dirty process fluid; Usually not suitable for very clean liquids.
1.8 Coriolis mass flowmeter
Coriolis mass flowmeter (CMF) is a kind of direct mass flow meter based on the principle of Coriolis force which is proportional to mass flow when fluid flows in vibrating tube. The application of CMF in China started late. In recent years, several manufacturers (such as Taihang instrument factory) have developed their own supply market; Several other manufacturers have set up joint ventures or quoted foreign technology to produce series instruments.
 & nbsp;& nbsp;& nbsp;& nbsp; 1.9 open channel flowmeter
Different from the above, it is a flow meter for measuring free surface natural flow in non full tubular open channel. Open channel flow meter is used to measure the flow in open channel. In addition to the circular shape, the open channel flowmeter also has U-shaped, trapezoidal, rectangular and other shapes. Open channel flowmeter is applied to all urban water supply diversion channels; Water diversion and drainage channels, sewage treatment inflow and discharge channels of thermal power plants; Industrial and mining enterprises water discharge and water conservancy projects and agricultural irrigation channels. It is estimated that 1995 sets of flow meters account for about 1.6% of the total flow meters, but there is no estimated data for domestic application.
 & nbsp;& nbsp;& nbsp;& nbsp; Research and development of new working principle flow meter
2.1 electrostatic flow meter
Tokyo Institute of technology, Japan, developed an electrostatic flowmeter for low conductivity liquid flow measurement in oil pipeline. The metal measuring tube of the electrostatic flowmeter is insulated and connected with the pipe system. The charge in the measuring tube can be known by measuring the static charge on the capacitor. They have carried out real flow tests on metal and plastic measuring tube instruments with inner diameter of 4 ~ 8mm, such as copper, stainless steel, etc. The results show that the relationship between flow rate and charge is nearly linear.
2.2 combined effect meter
The working principle of the instrument is based on the deformation of the instrument cavity caused by the momentum and pressure of the fluid, and the flow rate can be calculated by measuring the deformation of the composite effect. The instrument is developed by GMI School of engineering and management, USA, and has applied for two patents.
2.3 tachometer flow sensor
It is developed by industrial instrument company of Russian science and Engineering Center and is based on the theory of levitation effect. The instrument has been successfully applied in several fields (for example, more than 2000 sets have been installed in nuclear power plant to measure hot water flow for 8 years), and it is still being improved to expand its application field.
 & nbsp;& nbsp;& nbsp;& nbsp; Application and development trend of three kinds of flow meters
3.1 Coriolis mass flowmeter (CMF)
Foreign CMF has developed more than 30 series, the technical focus of each series development is: flow detection and measurement tube structure design innovation; Improve the zero stability and accuracy of the instrument; Increase the deflection of the measuring tube to improve the sensitivity; It can improve the stress distribution of the measuring tube, reduce the fatigue damage and strengthen the anti vibration interference ability.
　　3. 2 electromagnetic flowmeter (EMF)
Since EMF came into industrial application in the early 1950s, its application field has been expanding day by day. Since the late 1980s, it has accounted for 16% ~ 20% of the sales amount of flow meters in various countries.
 & nbsp;& nbsp;& nbsp;& nbsp; China's rapid development in recent years, estimated sales in 1994 6500 ~ 7500 units. Enf with the largest diameter of 2 ~ 6m has been produced in China, and it has the capacity of real flow calibration with the diameter of 3M.
3.3 vortex flowmeter (USF)
USF entered into industrial application in the late 1960s, and since the late 1980s, it has accounted for < br > 4% ~ 6% of the sales amount of flow meters in various countries. The estimated worldwide sales volume in 1992 was 3.54%In the same period, the number of domestic products is estimated to be 8000-9000.
 & nbsp;& nbsp;& nbsp;& nbsp; 4 Conclusion
It can be seen from the above that although the flowmeter has become more and more mature today, its types are still extremely diverse, and so far there is no flowmeter suitable for any occasion.
 & nbsp;& nbsp;& nbsp;& nbsp; Every kind of flowmeter has its application scope and limitation. This requires us to:
 & nbsp;& nbsp;& nbsp;& nbsp; ( 1) When selecting the instrument, we must be familiar with both the instrument and the measured object, and consider other factors, so that the measurement can be accurate< BR> & nbsp;& nbsp;& nbsp;& nbsp; ( 2) Efforts should be made to develop new instruments to make them more perfect on the existing basis.