Flow is an important parameter in industrial production, in the process of industrial production, many raw materials, semi-finished products, and finished products are presented in a fluid state. The flow rate of the fluid has become the key to determining the composition and quality of the product, and it is also an important basis for production cost accounting and rational use of energy. Therefore, for industrial production, it is often necessary to detect the right and wrong flow rate.
According to the state of the fluid, the flow sensor can be divided into gas flow sensor, liquid flow sensor and solid flow sensor, among which gas flow sensor and liquid flow sensor are used a lot.
According to different operating principles, coupled with the integration of microelectronics skills, computer skills and communication network skills, the variety of flow sensors is increasing, and different flow sensors are used in different scenarios.
1. Volumetric type
The positive displacement flow sensor was presented earlier, and its structure is relatively simple, which is equivalent to using a precise specification container to measure the measured fluid continuously. When the measured fluid flow expires, the rotor is pushed to rotate, and the two drive gears change the master-slave drive connection with each other to make a continuous, unequal speed rotation motion without dead center. Knowing the rotational speed of the rotor, the flow rate of the fluid can be calculated. Theoretically, the measurement accuracy of this type of flowmeter has nothing to do with the type, viscosity, density and other properties of the fluid. The measurement error is generally ± (0.2%-O.5%)R, which can be used as a standard appearance for industrial flow measurement. However, when the diameter of the measured pipe is large, the appearance of the body appears too coarse.
2. Turbine type
The turbine flow sensor is a speed measurement appearance developed in the past 30 years. The operating principle is that the turbine is placed in the measured fluid, and the liquid activity impacts the turbine blades to roll, and the speed of the turbine is proportional to the flow rate of the fluid. The speed of the turbine is converted into the corresponding electrical signal output through the magnetoelectric conversion device.
The turbine flow sensor has the advantages of high measurement accuracy and wide measurement range; However, since it is necessary for the turbine to be installed in the pipeline, the cleanliness of the tested fluid is required. The temperature, viscosity and density of the fluid have a great influence on the measurement accuracy. Rolling parts can cause wear and tear on the bearings, which can affect the service life of the sensor.
3. Differential pressure type
Differential pressure flow sensors have a long pre-production history and are widely used, and the production has been standardized and there are many types. Such as: orifice plates, sonic nozzles, average velocity tubes, venturi tubes and other flow sensors. The principle of operation of the differential pressure flow sensor is that when the fluid flows through the saving part built into the pipe, a pressure difference related to the flow rate is presented before and after the fluid flows through the pipe, and the flow value can be obtained by measuring the pressure difference. It is characterized by high machining accuracy of saving parts, strict installation requirements, and it is necessary to have long enough straight pipes before and after it to ensure the stability of the fluid flow state; Large fluid pressure loss; For low-velocity fluids, the differential pressure generated is small, and the error is increased; Fluid measurement that is not suitable for pulsation.
4. Momentum
The most typical of the momentum flowmeter is the target flowmeter, which is a product developed in the 60s of the 20th century, and is commonly used to measure the flow rate of higher viscosity oils. Its main body is a disc-shaped target, when the liquid is active, the active particle impact on the target, so that the target produces a small displacement, this displacement reflects the size of the flow.
5. Variable area type
Rotaflow sensors were introduced earlier, but they are widely used in industrial measurement in recent decades. It has the advantages of high sensitivity, simple structure, intuitiveness, small pressure loss, large measurement range, and low price.
It consists of a conical tube and a rotor placed in a conical tube that can move up and down freely, the sensor is vertically installed on the measuring pipe, and the measured fluid moves from the bottom to the top, propelling the rotor, and the height of the rotor suspension is a measure of the flow rate.
6. Fluid vibration type
The Kármán vortex street flow sensor is a measurement surface based on the principle of fluid vibration developed in the 70s of the 2O century, and has developed rapidly in recent years. It is characterized by low fluid pressure loss; It can be used for the measurement of liquid and gas, and can measure the flow rate and mass flow; The flow regime is required to be stable, the pipeline conditions are strict, and it is necessary to have a straight pipe section of a certain length before and after the vortex generator, and the price is relatively high.
7. Electromagnetic type
Electromagnetic flow sensor is a new type of flow surface developed with the use of electronic skills, and is now widely used in the field of flow measurement of various conductive liquids.
According to Faraday's law of electromagnetic induction, the conductive liquid measures its surface activity, which is equivalent to the movement of a conductor through a magnetic field to cut magnetic field lines, thereby inducing an electromotive force E, which is proportional to the uniform flow velocity. In principle, the electromagnetic flowmeter is not affected by the temperature, pressure, density and viscosity of the fluid, and there are no blocking parts and moving parts inside the pipeline, so the original state of the fluid will not be changed. Flow rates range from 0 to 102 m/s. It is suitable for flammable, explosive and corrosive media. But it also has limitations in some ways; The measured medium must be a conductive liquid, and the conductivity is greater than 10-3S/m; It cannot be used to measure the flow rate of ferromagnetic media; The signal is susceptible to interference from external magnetic fields.
8. Peltier type
The Peltier low-flow gas sensor (flow rate greater than 2×10-2m/s) is based on the principle of Boltier electromotive force and the principle of thermoelectric electromotive force, and the temperature of the sensor itself is only 15K, and there is no thermal disorder. The sensitive element is coated with a protective film, with strong anti-pollution and corrosion ability, long life, because its own temperature is not high, so the gas temperature requirements are not strict, and it can measure high-temperature fluids and has a wide operating temperature range.
9. Optical fiber type
The optical fiber flow sensor is a combination of optical fiber technology and flow sensor, so that the flow information is converted into optical signal at the optical fiber transmitter head after passing through the sensor. Then it is transmitted to the follow-up processing system through the optical fiber to complete the long-distance transmission, which is convenient for the completion of the sensor network management, and one microcomputer can handle and monitor multiple sensors. In this way, the advantages of the original sensor are maintained, and the advantages of anti-interference of optical fiber transmission lines are integrated.
10. Ultrasonic type
The ultrasonic flow sensor is based on the principle that the ultrasonic wave will carry the fluid flow velocity information when it is transmitted in the fluid, and is suitable for the measurement of two-phase flow fluid, requiring the measured fluid to contain a certain amount of medium that can reverse ultrasonic, that is, there are solid particles or bubbles and other two-phase media in the fluid.
11. Mass formula
Coriolis mass flow sensors are widely used, liquid and gas measurement can be applied, produced in United States in the 70s of the 20th century, using the principle of fluid mechanics, to establish a functional relationship between fluid mass flow and fluid force. It is mainly suitable for liquid measurement, and the gas measurement is required to be under high pressure to ensure that the mass flow is within the measurement range, and is suitable for the measurement under the condition that the pipe diameter is less than 200mm.
12. Laser type
Doppler sensors, which introduce laser skills into flow measurement, are combined with Doppler flow sensors. It can measure low-flow fluids, with strong anti-interference ability and high accuracy, but it is necessary to inject reflective particles into the fluid, which limits its measurement range.
13. Hotline type
Thermal flow sensors revolutionize flow metering and provide direct measurement of fluid mass flow. It uses the theory of heat transfer and fluid mechanics, selects the principle of thermal equilibrium, and establishes the functional relationship between the heat loss of the thermal element and the fluid flow rate and mass flow, so as to obtain the fluid flow rate and flow rate. Thermal flow sensors mainly include hot wire type, thermistor type, semiconductor integrated circuit type, etc., and the flow rate measurement is completed according to the connection between the heat dissipation of the heat element in the pipeline and the flow rate and quality; The external thermal resistance type is to put the heat source on the outside of the pipe, heat the fluid in the pipe, and calculate the mass flow rate by measuring the change of fluid heat.
Although due to the rapid development of electronic skills and the continuous improvement of various compensation skills, the accuracy of the hot-wire flow sensor has been greatly improved and the measurement range has been expanded, the consistency of the hot-wire flow sensor is very poor, and it is difficult to produce in batches: when the low-flow rate fluid is measured, the thermal disorder is very large, the hot-wire has poor anti-pollution and corrosion ability, the price is high, and it is easy to be damaged. There is electronic noise in the measurement, which causes it to slow down its response speed.
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