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Flowmeter is one of the important measuring instruments in process automation instruments and devices. It can be divided into liquid flowmeter and gas flowmeter according to the physical properties of the measured medium. Among them, there are many kinds of liquid flow meters. Common liquid flow meters include vortex street flow meter, turbine flow meter, electromagnetic flow meter, ultrasonic flow meter, etc. The principles and characteristics of different types of flow meters are described below.

1. Vortex flowmeter

Vortex flowmeter is based on the famous Karman vortex street principle in fluid mechanics, that is, a cylinder with non streamline section is inserted into the flowing fluid, the fluid flow is affected, and vortex separation will occur downstream of the cylinder within a certain Reynolds number range. It can be used to study the volume flowmeter for measuring the volume flow of gas and standard condition, the volume flow or mass flow of steam or liquid. Vortex flowmeter can measure almost all the flow of gas, liquid and steam.

2. Turbine flowmeter

The turbine flowmeter here mainly refers to the liquid turbine flowmeter. The turbine flowmeter belongs to the speed flowmeter. When the fluid flows through the turbine flowmeter, the fluid rotates the impeller. The rotation speed of the impeller is related to the speed of the fluid. The flow or total amount is derived from the average flow rate of the fluid felt by the impeller. Turbine flowmeter has the advantages of high precision, wide measurement range, good repeatability, small volume, light weight, simple structure, few moving parts, small pressure loss, convenient maintenance and so on.

3. Electromagnetic flowmeter

Electromagnetic flowmeter is a kind of flowmeter that uses Faraday electromagnetic induction principle to measure the flow of conductive fluid according to the electromotive force induced by conductive fluid passing through an external magnetic field. Advantages of electromagnetic flowmeter: simple structure, no movable parts and flow blocking parts, no pressure loss, can measure corrosive medium, and is not affected by the temperature, viscosity, density and conductivity of the measured medium in a certain range. Disadvantages: only the flow of conductive liquid can be measured.

4. Ultrasonic flowmeter

Ultrasonic flowmeter is based on the principle of "velocity difference" to measure the liquid flow in the pipe. The ultrasonic flowmeter can be used for non-contact measurement. There are no obstacles and movable parts in the detection parts, and the measurement accuracy is hardly affected by the temperature, viscosity, density, pressure and other parameters of the measured liquid. Ultrasonic flowmeter is suitable for measuring the flow of flammable, explosive, highly corrosive, large-diameter or inaccessible medium.