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Types and characteristics of flow meters (2) Posted by : admin / Posted on : Oct 26, 2019

4. Electromagnetic type
This type of flow meter detects the flow rate by using an amount of induced voltage generated by a conductive flow through a magnetic field. Therefore it is only suitable for conductive media. In theory, this method is not affected by the temperature, pressure, density and viscosity of the fluid. The range ratio can reach 100:1, the accuracy is about 0.5%, and the diameter is from 2mm to 3m. It is widely used in water and mud. , flow measurement of pulp or corrosive media.
Due to the weak signal, the electromagnetic flowmeter usually only has 2.5~8mV at full scale, and the flow rate is very small, only a few millivolts, which is susceptible to external interference. Therefore, it is required that the outer casing of the transmitter, the shielded wire, the measuring conduit, and the pipes at both ends of the transmitter should be grounded and grounded separately. Never connect them to the common ground of the motor or electrical appliance.
5. Ultrasonic type
The most common types of flow meters are Doppler flow meters and time difference flow meters. The Doppler flowmeter detects flow based on changes in the frequency of the sound waves reflected by moving targets in the fluid being measured. This method is suitable for measuring high-speed fluids, is not suitable for measuring low-speed fluids, and has low precision, and requires high smoothness of the inner wall of the pipeline, but its circuit is simple.
The time difference flowmeter measures the flow rate based on the time difference between the downstream and countercurrent propagation of the ultrasonic wave in the injection fluid. Since the magnitude of the time difference is small, in order to ensure the measurement accuracy, the requirements for the electronic circuit are relatively high, and the cost of the meter is correspondingly increased. The time difference flow meter is generally applicable to a laminar liquid having a pure and uniform flow field. For turbulent liquids, a multi-beam time difference flow meter can be used.
6. Momentum rectangle
This type of flowmeter is based on the principle of conservation of momentum, which is rotated by a fluid to rotate the component, and the rotational speed of the rotating component is proportional to the flow velocity. The magnetic field, optical, mechanical counting, etc. are used to convert the rotational speed into an electrical signal to calculate the flow rate.
Turbine flowmeter is the most widely used and highly accurate type of such instrument. It is suitable for gas and liquid media, but it is slightly different in structure. For gas, its impeller angle is small and the number of blades is large. The turbine flowmeter has an accuracy of 0.2%-0.5%, 0.1% in a narrow range, and a turndown ratio of 10:1. The pressure loss is small and the pressure is high, but it has certain requirements on the cleanliness of the fluid, and is susceptible to the density and viscosity of the fluid. The smaller the aperture, the greater the influence, and the same as the orifice plate, it is necessary to ensure that there are enough before and after the installation point. Straight pipe section to avoid fluid rotation and change the angle of action on the blade.
7. Positive displacement type
The working principle of this kind of meter is measured according to the precise movement of the fluid by a fixed amount per revolution of the rotating body. The design of the meter is different, such as elliptical gear flowmeter, rotary piston flowmeter, scraper flowmeter and so on. The range of the elliptical gear flowmeter is relatively large, can reach 20:1, and the precision is high, but the moving gear is easily stuck by the impurities in the fluid. The rotary piston flowmeter has a large flow per unit, but due to the structural reasons, the leakage is relatively large. Large, poor precision. The positive displacement type flowmeter is basically independent of fluid viscosity and is suitable for mediums such as greases and waters, but not for mediums such as steam and air.
Each of the above-mentioned flowmeters has its own advantages and disadvantages, but even for the same type of watch, the products provided by different manufacturers have different structural properties.
flow measurement