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Common Flowmeter Applications - Focus on Microfluidic Control Technology

Instruments for measuring fluid flow are collectively referred to as flow meters or flow meters. Flow meters are one of the most important instruments in industrial measurement. They are widely used in metallurgy, electric power, coal, chemical industry, petroleum, transportation, construction, textile, food, and medicine. Various fields of national economy such as agriculture, environmental protection, and people’s daily life are important tools for the development of industrial and agricultural production, energy conservation, improvement of product quality, and improvement of economic efficiency and management level. They play an important role in the national economy. In order to adapt to various uses, various types of flow meters have come out one after another. More than 60 flow meters have been put into use. In accordance with the current zui popular, zui wide classification, namely: differential pressure flowmeter, vortex flowmeter, turbine flowmeter, float flowmeter, digital target flowmeter, electromagnetic flowmeter, ultrasonic flowmeter.
1 differential pressure flow meter
1.1 Differential Pressure Flowmeter
The differential pressure flowmeter is a meter that calculates the flow rate based on the differential pressure generated by the flow detection element installed in the pipeline, the known fluid condition, and the geometry of the detection element and the pipeline. Differential pressure flowmeter is one of the industrially used flowmeters, its measurement accuracy is determined by the measurement principle, structure, manufacturing process level, the nature of the fluid to be measured and the conditions of use, etc. Device (detection) and secondary device (differential pressure conversion and flow display meter). Differential pressure flowmeters are often classified in the form of test pieces, such as orifice flowmeters.
, V cone flowmeter and so on.
1.1.1 Orifice Flow Meter
Orifice plate flow meter works: In the fluid flow pipe is equipped with a throttling device, which is equipped with an orifice plate, the center is opened with a circular hole, its aperture is smaller than the pipe ID, the fluid in the front of the orifice plate stable forward flow When the fluid flows through the orifice, the pore diameter becomes smaller, the cross-sectional area shrinks, and the stable flow state is disrupted. As a result, the flow velocity will change, the velocity will increase, and the static pressure of the gas will decrease, so the pressure drop will occur before and after the orifice plate. That is, the differential pressure (where the pressure is large at the area where the orifice plate has a large cross-section, and the pressure is small at the place where the cross-section of the orifice plate is small). The magnitude of the differential pressure and the fluid flow have a definite numerical relationship. That is, when the flow rate is large, the differential pressure is large, the flow rate is small, and the differential pressure is small. The flow is proportional to the square root of the differential pressure
1.1.2 V Cone Flowmeter
The V-cone flowmeter originates from MCROMETER in the United States. It is a new type of differential pressure flow meter with superior advantages. Since its inception twenty years ago, it has been widely used and well received in the flow measurement field due to its many advantages that cannot be compared with conventional differential pressure meters. The V-cone flowmeter is a brand-new differential pressure flow metering device. It gradually shrinks the throttling mode with a unique side wall. It is a revolutionary result of the differential pressure flowmeter that has changed almost all the shortcomings of the traditional throttling device. Its principle is the same as that of other differential pressure flowmeters. It is the principle of energy conservation and flow continuity in classical closed pipes, and it has the functions of self-rectification, self-cleaning and self-protection; the requirements for straight pipe segments are very short, no fouling, and no plugging. Can maintain long-term stability. The V-cone flowmeter works in the same way as an orifice flowmeter. When the medium passes through the V cone, due to the presence of the V cone of the flow blocker, the flow area of the fluid changes and the flow rate changes. According to the Bernoulli equation, the change in flow rate causes a change in pressure, and the pressure changes between the flow rate and the flow rate. There is a certain relationship. The purpose of measuring the flow rate is to measure the pressure difference. Although the same principle as the orifice plate, the essence of Zui is that the orifice plate is a center-contraction type throttling device, and the V-cone is a side-wall contraction type throttling device. Compared with the V-cone and orifice plate, the V-cone has a much lower signal-to-noise ratio. Since the signal-to-noise ratio is small, the V-cone can accurately measure even when the measured differential pressure is small under a small flow rate measurement. The V-cone flowmeter also has some disadvantages, such as: need for calibration, higher price, etc.
2 vortex flowmeter
2.1 Vortex Flowmeter Features
Vortex flowmeter is mainly used for the flow measurement of medium fluids in industrial pipelines, such as gases, liquids, vapors and other media. Its characteristics are low pressure loss, large range, high accuracy, good repeatability, and almost no influence of parameters such as fluid density, pressure, temperature, viscosity, etc. when measuring volume flow. No moving mechanical parts, so high reliability and low maintenance. Instrument parameters can be stable over a long period of time. The vortex flowmeter uses the fluid oscillation principle to measure the flow. When the fluid passes through the vortex flow transmitter in the pipeline, two columns of vortices proportional to the flow velocity are alternately generated above and below the vortex generator of the triangular column. The frequency of release of the vortex is The average velocity of the fluid flowing through the vortex generator is related to the feature width of the vortex generator. The following formula can be used: f = (St?v)/d
Where: f - the frequency of release of the vortex, Hz;
v - the average velocity of the fluid flowing through the vortex generator, m/s;
d—Variation width of vortex generator, m;
St-Stohala number, dimensionless, its value range is 0114 ~ 0127.
St is a function of the Reynolds number, St=f(1/Re). When the Reynolds number Re is in the range of 102 to 105, the St value is about 012. Therefore, in the measurement, the Reynolds number of the fluid should be as large as 102 to 105, and the vortex frequency f=0.12 v/d. It can be seen that the average velocity v of the fluid flowing through the vortex generating body can be calculated by measuring the vortex frequency, and the flow q can be found by the equation q=vA, where A is the cross-sectional area of the fluid flowing through the vortex generator.
The vortex flowmeter is a new type of flowmeter for measuring the fluid flow of a closed pipe according to the Karmen vortex principle. Because of its good media adaptability, it can directly measure the volumetric flow rate of steam, air, gas, water and liquid without temperature and pressure compensation. It is equipped with temperature and pressure sensors to measure the standard volume flow and mass flow. The disadvantage is that the installation pipeline is generally required to be a straight pipeline. It is required that the forward and backward straight pipeline sections must meet the requirements of the vortex flowmeter. The inner diameter of the distribution pipeline must also be the same as the inner diameter of the vortex flow transmitter, and should be far away from the vibration source and the strong electromagnetic interference. .
3 Turbine flow meter
Turbine flowmeters consist of turbines, bearings, preamplifiers, and display instruments. The measured fluid impinges on the turbine blades and causes the turbine to rotate. The rotational speed of the turbine changes with the flow rate, that is, the flow rate is large, and the rotational speed of the turbine is also large. Then, the rotational speed of the turbine is converted into an electrical pulse of a corresponding frequency by the magnetoelectric conversion device. After the preamplifier is amplified, it is sent to the display meter for counting and display. Instantaneous flow and cumulative flow can be obtained based on the number of pulses per unit time and the cumulative number of pulses. The working principle of the turbine transmitter is when the fluid flows along the axis of the pipeline and strikes the turbine blades.
Q=fk
Among them: Q - flow through the transmitter, L / s;
f - electric pulse frequency;
K—meter factor, times/litre.
The force of the fluid in the pipeline acts on the blades and pushes the turbine to rotate. While the turbine is rotating, the blades periodically cut the magnetic lines of force generated by the electromagnets and change the magnetic flux of the coils. According to the principle of electromagnetic induction, a pulsating potential signal will be induced in the coil. The frequency of this pulsating signal is proportional to the flow rate of the measured fluid. k is an important characteristic parameter of the turbine transmitter and it represents the flow rate per cubic meter. Pulses, or several pulses per liter of flow. Different instruments have different k, and change with the long-term wear of the instrument. Although the vortexmeters have the same design dimensions, the actually processed turbine geometry parameters will not be exactly the same. Therefore, the meter constant k of each turbine transmitter is not exactly the same. It is usually used by the manufacturer to clean the room temperature. Water is calibrated. The pulse signal output by the turbine transmitter is amplified by the preamplifier and sent to the display
Table, can realize the flow measurement [5]. Turbine flowmeter features high accuracy, good repeatability, zero drift, and high range ratio. Turbine flowmeters have high-quality bearings and specially designed deflectors, which greatly reduce wear, are insensitive to peak values, and can provide reliable measurement variables under severe conditions. Disadvantages: (1) Measurement of gas, liquid miscible or viscous fluids can cause large errors; (2) The measured particles containing fluids need to be filtered in advance to prevent the turbine from being jammed.
4 float flow meter
Floater flowmeter, also known as rotameter, is a volumetric flowmeter for flow measurement in which the floater moves up and down in a vertical conical tube as the flow changes and changes the flow-through annulus area formed between them [6]. Float flowmeter can be divided into glass tube float flowmeter, plastic tube floater flowmeter and metal tube float flowmeter according to different materials. The glass tube floater flowmeter and the plastic tube floater flowmeter are simple in structure, and the float position is clearly visible, easy to read and low in cost, and is often used to measure normal temperature, normal pressure, transparent and corrosive media. Such as air, gas, ammonia and so on. It is convenient for on-the-spot visual inspection, and is mostly used for the ratiometric measurement of industrial raw materials. Although the glass tube floater flowmeter has many advantages, it is only suitable for local instructions, the signal can not be transmitted, the glass tube strength is not
It is not enough to measure high temperature, high pressure and opaque fluids, so more metal tube float meters are used in industrial production. The flow detection element of the float flow meter consists of a vertical conical tube that expands from bottom to top and a float group that moves up and down along the cone axis. The principle of operation is that when the fluid to be measured passes through the annulus formed by the funnel and the float from the bottom up, a differential pressure is generated at the upper and lower ends of the float to form a rising force of the float. When the lifting force of the float is greater than the weight of the float immersed in the fluid, the float rises. As the annulus area increases, the fluid velocity at the annulus decreases immediately, the differential pressure at the upper and lower ends of the float decreases, and the lifting force acting on the float also decreases. Until the rising force equals the weight of the float immersed in the fluid, the float stabilizes. At a certain height. There is a correspondence between the height of the float in the cone and the flow rate through it.
Floater flowmeter has many advantages such as simple structure, reliable operation, low pressure loss and stability, measurable low flow medium and so on. The float flowmeter has a wide range of flow rates, typically 101, zui is 51, and zui is 251. The output of the flow detection element is close to linear. Lower pressure loss. However, measuring liquids containing particulate solids or liquids containing liquid droplets is generally not suitable, and flow indication correction should be performed before use.
5 digital target flow meter
The target flowmeter was applied to industrial flow measurement in the 1960s. It was mainly used to solve the flow measurement of high-viscosity, low-Reynolds-number fluids. It has undergone two major development stages: pneumatic and electric meters. The composition and working principle of the digital target flow meter: It mainly consists of measuring tube, force element (target piece), sensing element (capacitive force sensor, pressure sensor, temperature sensor), transmission part, integrator and its display and The output part is composed. When the medium flows in the measuring tube, due to its own kinetic energy and the pressure difference due to the flow block, a force acts on the spoiler, and its effect is proportional to the square of the flow velocity of the medium. Advantages include: the entire instrument structure is solid and no moving parts, plug-in structure, easy removal; the instrument has built-in self-test program, high precision, good repeatability, high temperature resistance; disadvantages are: the higher the fluid requirements, such as: The fluid to be measured must be a Newtonian fluid, a measuring tube in which the fluid must fill the flow meter, etc.
6 electromagnetic flowmeter
Electromagnetic flowmeter has two types of integrated type and split type. The output stage adopts electric isolation, which can be conveniently matched with the rear position meter to achieve the functions of recording, controlling and adjusting the flow rate. At the same time, the flowmeter is equipped with an RS-485 communication interface that can be interconnected with a computer. The working principle of electromagnetic flowmeter is Faraday's law of electromagnetic induction. Conductive liquids flow in the magnetic field to cut the lines of magnetic force, producing an induced potential. Expression: E=KBLv, where B is the magnetic induction intensity; L is the distance between the measurement electrodes; v is the average velocity of the fluid under test in the magnetic field; K is the proportionality constant. Electromagnetic flowmeter is mainly composed of flow sensor and signal converter. The sensor converts the flow of the fluid to be measured to the corresponding induced potential. The function of the signal converter is to amplify and convert the millivolt-level voltage signal output by the electromagnetic flow sensor proportional to the flow rate into a standard direct current, voltage or pulse signal output that can be received by an industrial instrument to cooperate with the instrument and regulator. Implement traffic indications, records, and calculations. Electromagnetic flowmeters have the following advantages: (1) Measurable with solid particles, suspended solids, or acids, alkalis, salt solutions, and other liquids with a certain amount of conductivity; can also be measured in two directions. (2) There are no moving parts and bluff body in the measuring tube, so there is no pressure loss, no mechanical inertia, and the reaction is very sensitive. (3) Large measurement range. (4) Electromagnetic, the volumetric flow rate measured by the flowmeter is virtually unaffected by changes in fluid density, viscosity, temperature, pressure, and electrical conductivity (as long as it is above a certain threshold). Disadvantages are: (1) Electromagnetic flow meters cannot measure liquids with very low electrical conductivity. (2) Gas, vapor, and liquids containing large bubbles are not measured. (3) Cannot be used for higher temperature liquids.
7 Ultrasonic flowmeter
Ultrasonic flowmeter consists of three parts: ultrasonic transducer, electronic circuit, and flow display and accumulation system. It is a speed-type flow meter that uses ultrasonic pulses to measure fluid flow. Ultrasonic emission transducer converts electrical energy into ultrasonic energy and transmits it to the fluid to be measured. The ultrasonic signal received by the receiver is amplified by the electronic circuit and converted into an electrical signal representing the flow rate. It is supplied to the display and integrating instrument for display. And totalization, so that the flow detection and display [7]. Ultrasonic flowmeters are roughly divided into: plug-in ultrasonic flowmeters, pipe segmented ultrasonic flowmeters, and external clamped ultrasonic flowmeters. Ultrasonic flowmeter advantages: (1) Ultrasonic flowmeter is non-contact measurement, suitable for large diameter, large flow measurement, and is not affected by the fluid temperature, viscosity, density and other parameters. (2) Ultrasonic flowmeter can measure various media of water, gas or oil. (3) Ultrasonic flowmeters consume very little energy and can be easily powered for many years. In addition, advanced intelligent hosts can easily perform wireless network communications, and their application prospects are even broader. Disadvantages: The current disadvantage of ultrasonic flowmeters is that the temperature range of the measurable fluid is limited by the degree of temperature resistance of the coupling material between the ultrasonic transducer and the transducer and the pipeline, and the transmission speed of the measured fluid at high temperature. The original data is incomplete.

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