Flow measurement methods and instruments of a wide range of classification methods are many, rotameter, flowmeter throttle, slit flow, volume flow meter, electromagnetic flow meters, ultrasonic flow meters, and weirs. In a variety of flow meters, each product has its specific applicability, but also has its limitations. By measuring the object by a closed pipe and open channel into two categories; by measurement purposes can be divided into the total measurement and flow measurement, the instrument called the total scale and the flow meter, respectively.
Meter to measure the total flow through the pipeline for some time the traffic flow is a short period of time divided by the total amount of time to represent the business. In fact, the cumulative flow meter device is usually also available for tables of total use, while the total flow of the table are also available for the sender device. Therefore, in the strict sense to sub-meter and the total scale moot.
Measuring principle divided by mechanics, thermal theory, acoustics, electrical theory, optics and atomic physics and so on.
An ultrasonic flowmeter
Ultrasonic wave propagation in the flow when the fluid flow rate on the information contained by the received ultrasound can detect the velocity of the fluid, thus converted into traffic. According to the test the way, can be divided into velocity difference method, Doppler method, beam-shift method, the noise law and related laws, such as different types of ultrasonic flowmeter. Ultrasonic flow meter the past ten years with the rapid development of integrated circuit technology started as a fluid metering applications.
Ultrasonic flow meter is a non-contact measuring instruments, suitable for measuring exposure is not easy, not easy to observe the flow of the fluid and large diameter. It is linked with the water level can be open water flow measurement. Do not use the ultrasonic flowmeter is installed in the fluid measuring device, it will not change the state of fluid flow, no additional resistance. Installation and maintenance of the instrument can be run without affecting production pipeline, which is an ideal energy-saving flow meter.
Various types of ultrasonic flow meter can be installed in the tube, non-contact flow measurement, instrumentation, and test cost is essentially independent of the size of pipe diameter; and other types of flow meters with the diameter increases, a substantial increase in cost, so the larger diameter ultrasonic flow than the same function in the function of other types of flow the more favorable price, is considered a better measurement of large diameter flow meters. Doppler ultrasonic flowmeter to measure the flow of two-phase media, it can be used for dirty sewers and sewage water flow measurements. In power plants, using portable ultrasonic flowmeter into the water turbine, steam turbine and other large diameter circular water flow, the skin than in the past much more convenient detached flow meter. Ultrasound is also used for gas flow measurement of juice. Diameter scope 2CM ~ 5M, from a few meters wide, wide open channel or culvert to the river 500M can be used.
In addition, the ultrasonic measurement accuracy of flow measurement instruments almost independent of the measured fluid temperature, pressure, viscosity and density parameters, but also made of non-contact and portable measuring instruments, it can solve other types of instruments are difficult to measure corrosive, non-conductivity, radioactive and explosive medium flow measurements. View of the characteristics of non-contact measurement, which together with reasonable electronic circuit, a measuring instrument can be adapted to various diameters and a variety of flow range measurements. Ability to adapt ultrasonic flowmeter is unmatched by other instruments. Ultrasonic Flowmeter with some of the above advantages, it more and more attention, and the product series and common development, has been made of the different channels standard, high temperature, explosion-proof type and wet type instrument, to adapt to different media channels on different occasions and different conditions of flow measurement.
2 electromagnetic flowmeter
Electromagnetic flowmeter fifties and sixties of the 20th century with the rapid development of electronic technology and the development of a new type of flow measuring instruments. Electromagnetic flowmeter is made according to Faraday's law of electromagnetic induction, used to measure the volume of conductive liquid flow meter. Because of its unique advantages, industry and agriculture has been widely used in a variety of conductive liquid flow measurement, such as various acids, alkalis, salt and other corrosive media and various slurry flow measurement, has developed a unique application areas.
In structure, the electromagnetic flowmeter by the electromagnetic flow sensor and converter of two parts. Sensors installed in the pipeline, it is the role of the liquid flowing into the pipeline to transform the linear volume flow into the induced electric potential signal, and this signal is sent through a transmission line converter. Converters installed in a place not too far away from the sensor, it sent the traffic sensor signal amplification, and with the traffic signals into electrical signals proportional to the standard output for display, accumulation and regulation control.
3 flow in the irrigation application
The national economy, the strong construction of farmland water conservancy is an important measure for agricultural development in China. Can not do without irrigation water conservancy construction, especially irrigation systems correctly measured flow control method is essential. There are mainly two methods, the speed - an area of law and the hydraulic method, each measurement method has its unique flow sensing (detection) part.
Rate - area method to measure flow is multiplied by the average velocity of the flow channel cross-sectional area, while the flow rate through the meter in some specific points on the measurement. This method there measured flow rate is generally 3 different ways: First, the velocity distribution law; Second, 0.6H Act (H is the water surface to the channel bottom of the distance); third 0.2H ~ 0.8H law. Distribution method is to measure water velocity and the channel bottom surface of the flow between the number of points, and then by the surface measurement point to determine the speed of a vertical curve, the area defined by the speed curve divided by the depth, you can find the average speed; 0.6H method 0.6H below the surface measurement of the depth of a single point velocity, and assumes that the value is the average of all speed; 0.2H ~ 0.8H measuring the water depth beneath the surface 0.2H, and 0.8H velocity of two points, the two readings can be estimated as the average of the average velocity value.
These 3 methods are in any particular calculation of average speed measurement areas. Sounding along the entire width of the channel number and the number of points in a single vertical plane measurements are different. Many European countries to request a single surface sounding 5-point test, while the United States that measure 2 points enough. Sounding frequency depends on the width of the channel. Sounding number in Europe is 5 to 15, 8 to 18 in the United States. Sounding measurement error is based on the number and the measurement time at each decision point, using velocity - area method the average error of less than 6%.
With the speed - flow area method to measure two technologies should pay particular attention to two points: First, the use of ultrasonic flow meter, and the other is the use of electromagnetic flow meter.
Ultrasound technology is by measuring the ultrasonic propagation time across the channel to calculate the velocity of the transmitter and receiver sub-placed sources on both sides, so that the waves of the path and the angle between the direction of flow in the 45 ° ~ 60 ° range. This geometry makes the upstream and downstream direction to the sound of the time difference between pulses. Channels, the shape of the decision to use single or multi-path system path system, that uses one or more transmitting and receiving pairs.
Electromagnetic flow measurement technology is the use of conductive currents created in the artificial magnetic field, and then measured the synthesis of both sides across the water potential difference. Mainly by the electromagnetic flow meter drains, excitation coil and a pair of electrodes.
4 Flowmeter Selection
Flowmeter selection is in accordance with production requirements, the supply of instrumentation products from the actual situation, considering the measurement of the security, accuracy and economy, and according to the nature and measured the flow of the fluid flow sampling devices to determine the manner and measuring instruments, and measuring instruments in the form and specifications.
Safe and reliable flow measurement is the first reliable measurement, sampling devices that will not happen in the operation of mechanical strength or electrical circuit failure caused the accident; followed by the production of measuring instruments, whether or failure in the normal case not to affect the production system's security.
Ensure the safe operation of the instrument, based on, and strive to improve the instrument's accuracy and energy efficiency. To do this, not only to choose to meet the requirements of the display instrument accuracy, but also to choose according to the characteristics of the measured medium reasonable measurement. For the power plant main steam flow measurement, because of its critical power plant safety and economy, generally installed by cutting the amount of mature standard with differential pressure flow meter. Chemical water treatment of sewage and fuel, respectively, are dirty and low Reynolds number viscous flow stream, do not apply to standard cutting pieces. A general selection round of dirty plates and other non-standard cutting pieces missing with differential pressure gauge or Doppler flow meter, and viscous flow respectively by positive displacement, target type or wedge flowmeter. Turbine inlet water, condenser water loop and return back to the hot steam heat units are all large diameter (400MM or more) of the flow measurement parameters, due to processing difficulties and create a large pressure loss, generally do not use standard throttling device. According to the measured medium characteristics and measurement accuracy requirements, were used to insert the flow meter, velocity components with differential pressure meter, ultrasonic flow meter, or use markers and simulation methods such as inability to measure traffic loss
In order to ensure the life and accuracy of flow meter, the meter must pay attention to when selecting vibration requirements. In humid areas, to choose heat meter. Select the correct specifications of the instrument instrument is to ensure the accuracy of life and an important part, should pay special attention to the choice of static pressure and temperature. Static pressure instrument (ie, pressure level) should be slightly larger than the measured medium working pressure, and generally 1.25 times to ensure that no leaks or accidents. Scale range of choices of the scale limit the choice of instrument. Select small, easy to overload, damage to the instrument; election big, hinder the measurement accuracy. Selected as the actual operation of the general value of the maximum flow of 1.2 ~ 1.3 times.
Pipe installed in the production of long-running contact on the instrument, flow measurement devices should also consider the energy loss caused. Under normal circumstances, in the same production pipeline pressure loss should not use a large number of measuring devices, such as cutting components.
In short, no single measurement or meter the flow of various fluids and can be adapted. Different measurement methods and structural requirements of the different measurement operation, using methods and conditions of use, each type has its unique advantages and disadvantages. Therefore, should the various measurement and instrument to do a comprehensive comparison of features based on selection for production requirements, both quiet and economical, durable and reliable the best type. With the development of China's agricultural modernization and the implementation of precision agriculture, irrigation flow meter will play an increasingly larger role.
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