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The accurate measurement of flow rate is very important. The use of flow sensor to monitor and measure the flow rate of liquid or gas in the tested pipeline is widely used in the field of industrial control and civil facilities.

The flow rate of flow sensor is an important parameter in industrial production. In the process of industrial production, many raw materials, semi-finished products and finished products appear in a fluid state. The flow rate of fluid becomes the key to determine the composition and quality of products, and it is also an important basis for production cost accounting and rational use of energy. In addition, in order to ensure the reliability of fault-free detection and test results in the manufacturing industry, many processes need to keep the inflow and outflow of liquid or gas media consistent, in addition to pressure and temperature in the automatic production process, the measurement of flow is also very important. Therefore, the measurement and control of flow is an important part of production process automation.

A flow sensor is a sensor that can sense the flow of the fluid and convert it into an available output signal. The sensor is placed in the path of the fluid, and the change of the flow rate is measured by the interaction between the sensor and the sensor. According to the definition of flow rate, it is mainly applied to the detection of gas and liquid flow.

Classification of flow sensors:

Flow sensors can be divided into the following types according to different detection methods, and the work is carried out by the corresponding sensors:

Electromagnetic detection mode: electromagnetic flow sensor

Mechanical testing method:

1. Volume flow sensor

two。 Vortex flow sensor

3. Turbine flow sensor

Acoustic detection method: ultrasonic flow sensor

Throttle detection mode: differential pressure flow sensor

Let's talk about the differences between the above sensors:

Electromagnetic flow sensor: definition:

The electromagnetic flow sensor is composed of two parts: the sensor which is in direct contact with the pipeline medium and the upper signal conversion. It is based on Faraday's law of electromagnetic induction and is used to measure the flow of conductive liquid with conductivity greater than 5 μ s / cm. It is a kind of instrument for measuring the flow rate of conductive medium. In addition to measuring the flow rate of general conductive liquids, it can also be used to measure strong corrosive liquids such as strong acids and bases and liquids with liquid-solid suspension, such as mud, mineral pulp, paper pulp and so on.

Principle:

The working principle of electromagnetic flow sensor is based on Faraday's law of electromagnetic induction. In the electromagnetic flow sensor, the conductive medium in the measuring tube is equivalent to the conductive metal rod in the Faraday test, and the two electromagnetic coils at the upper and lower ends produce a constant electromagnetic field. When the conductive medium flows through, it will produce induced voltage. The induced voltage is measured by two electrodes inside the pipe. The measuring pipe is electromagnetically isolated from the fluid and the measuring electrode through a non-conductive lining (rubber, Teflon, etc.).

When the conductive liquid cuts the magnetic line of force in the magnetic field, the induced potential is generated in the conductor, and the induced potential E is:

E=KBVD

In the formula: Kmuri-instrument constant

BMMI-Magnetic Induction intensity

Vmurmuri-measure the average velocity in the section of the pipe

DMurray-measure the inner diameter of the pipe section

The magnitude of the induced potential is related to the intensity of magnetic induction, the diameter of the pipe and the velocity of the fluid.

That is:

The relationship between volume flow qv and fluid velocity v:

Available:

2. Definition of volumetric flow sensor:

Volumetric flow sensor, also known as fixed displacement flow sensor, referred to as PD flow sensor, has the highest precision in the flow meter. Its mechanical measuring element continuously divides 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 volume part of the fluid.

Principle:

The volumetric flow measurement uses a fixed small volume to repeatedly measure the volume of the fluid passing through the flow sensor. Therefore, there must be a space inside the volumetric Flowmeter sensor to form a standard volume, by calling it a "metering space" or "metering room" of the volumetric Flowmeter. This space consists of the inner wall of the instrument housing and the rotating parts of the flow sensor. The working principle of the volumetric flow sensor is: when the fluid passes through the flow sensor, there will be a certain pressure difference between the inlet and outlet of the sensor. The rotating part of the flow sensor (referred to as "rotor") rotates under the action of this pressure difference and discharges the flow from the inlet to the outlet. In this process, the fluid fills the "metering space" of the flow sensor again and again, and then is constantly sent to the outlet. Under the condition of a given flow sensor, the volume of the metering space is determined. as long as the rotation times of the rotor are measured, the cumulative value of the fluid volume passing through the flow sensor can be obtained.

Definition of Vortex flow Sensor:

The vortex flow sensor is developed based on Karman vortex street principle. If a triangular cylindrical vortex generator is set in the fluid, regular vortices are generated alternately from both sides of the vortex generator. this kind of vortex is called Carmen vortex.

Principle:

A non-streamlined vortex generator is placed in the fluid, so that the fluid is separated alternately on both sides of the generator, releasing two series of regularly staggered vortices, and the vortex separation frequency is proportional to the flow rate in a certain range. By measuring the frequency of the vortex, the flow rate of the fluid can be calculated according to the relevant formula.

Vortex flow sensor is mainly used to measure the flow rate of medium fluid in industrial pipeline, such as gas, liquid, steam and so on. It is characterized by small pressure loss, wide measuring range and high precision, and is almost not affected by fluid density, pressure, temperature, viscosity and other parameters when measuring volume flow under working conditions. There are no movable mechanical parts, so the reliability is high and the maintenance is small. The instrument parameters can be stable for a long time.

Definition of turbine flow sensor:

The turbine flow sensor is similar to the impeller water meter and is a kind of speed flow sensor. The turbine impeller, propeller and other components are placed in the fluid, and the energy conversion device is formed based on the principle that the speed of the turbine is proportional to the rate of the average volume flow and the speed of the propeller is proportional to the speed of the fluid.

Principle:

The turbine flow sensor installs a freely rotating impeller in the pipeline. the fluid flows through the impeller to make the impeller rotate. the greater the flow rate and the higher the velocity, the greater the kinetic energy and the higher the speed of the impeller. By measuring the speed or frequency of the impeller, the flow and total amount of fluid flowing through the pipe can be determined.

Features:

Turbine flow sensor is a kind of speed instrument, which has the advantages of high precision, good repeatability, simple structure, few moving parts, high pressure resistance, wide measuring range, small volume, light weight, small pressure loss, convenient maintenance and so on. it is used to measure the volume flow and total amount of low viscosity gas in closed pipe. It has extensive application value in petroleum, chemical industry, metallurgy, urban natural gas pipe network and other industries.

Definition of ultrasonic flow sensor:

Ultrasonic flow sensor is made of piezoelectric material pick lead titanate crystal, which can convert electric energy into sound energy. It is an instrument that measures volume flow by detecting the effect of fluid flow on ultrasonic beam (or ultrasonic pulse).

Principle:

When the ultrasonic beam propagates in the fluid, the flow of the fluid will cause a slight change in the propagation time, and the change of the propagation time is proportional to the velocity of the liquid, so that the velocity of the fluid can be measured and the flow rate can be calculated according to the diameter of the pipe.

Features:

At present, there are common problems in industrial flow measurement, such as the difficulty of large pipe diameter and large flow measurement, because the general flow sensor will bring difficulties in manufacture and transportation with the increase of pipe diameter, and the shortcomings such as higher cost, increased energy loss and inconvenient installation can be avoided. Because all kinds of ultrasonic flow sensors can be installed outside the tube and non-contact to measure flow, the cost of the instrument basically has nothing to do with the caliber of the pipe to be tested, while the cost of other types of flow sensors increases greatly with the increase of caliber. Therefore, the larger the caliber, the better the function-price ratio of ultrasonic flow sensors than other types of flow sensors with the same function. The flow measurement accuracy of ultrasonic measuring instruments is almost not affected by the measured fluid temperature, pressure, viscosity, density and other parameters, and can be made into non-contact and portable measuring instruments. therefore, it can solve the flow measurement problems of strong corrosive, non-conductive, radioactive, flammable and explosive media which are difficult to be measured by other types of instruments.

6. Definition of differential pressure flow sensor:

The differential pressure flow sensor is an instrument that calculates the flow rate according to the differential pressure produced by the flow detection parts installed in the pipeline, the known fluid conditions and the geometric size of the detection and pipeline.

Principle:

When the fluid filled with the pipe flows through the throttle in the pipe, the flow velocity will form a local contraction at the throttle, so the velocity increases and the static pressure decreases, so there is a pressure difference before and after the throttle. The greater the flow rate, the greater the pressure difference, so the flow rate can be measured according to the pressure difference.

With the continuous development of flow sensors, more and more types of flow sensors are gradually coming out, each of which has its own advantages and disadvantages. Users should choose the appropriate sensor according to their own needs when selecting the flow sensor.

This article is from the official account of Wechat: sensor Technology (ID:WW_CGQJS), author:-

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