（简称EMF）是利用法拉第电磁感应定律制成的一种测量导电液体体积流量的仪表。 Electromagnetic flowmeter (EMF for short) is a meter that measures the volumetric flow of conductive liquids by using Faraday's law of electromagnetic induction. In the early 1950s, EMF realized industrial application. In recent years, EMF production has accounted for about 5% to 6.5% of the total number of industrial flow meters in the world. Especially since 2000, this value has been increasing year by year. As a kind of industrial flow measuring instrument, electromagnetic flowmeter has some tips and precautions in its selection. Here I will share some personal experience and discuss with you, hoping to contribute to the correct selection of electromagnetic flowmeter.
Selection of electromagnetic flowmeter
应用领域非常广泛。 The application field of electromagnetic flowmeter is very wide. According to the application, there are large, medium and small calibers, small calibers and micro calibers. Among them, large caliber electromagnetic flow meters are mostly used in water supply and drainage projects. Paper industry pulp and black liquor, non-ferrous metallurgy pulp, coal slurry of coal preparation plant, strong corrosive liquid of chemical industry, cooling water control of blast furnace tuyere of iron and steel industry, flow measurement and control of hydraulic transportation of long-distance pipeline coal, etc. The small and small calibers are often used in places with health requirements such as the pharmaceutical industry, food industry, and biological engineering. 2. Precision level and function. The performance of general-purpose EMFs on the market is quite different. Some have high accuracy and many functions, while others have low accuracy and simple functions. The basic error of a high-precision meter is (± 0.5% to ± 1%) R, and the low-precision meter is (± 1.5% to ± 2.5%) FS. The price difference between the two is 1 to 2 times. Therefore, in places where measurement accuracy is not very high (for example, non-trade accounting is only for control purposes, and only places with high reliability and excellent repeatability), it is not economical to choose high-precision instruments. Some models claim to have higher accuracy, and the basic error is only (± 0.2% ～ ± 0.3%) R, but there are strict installation requirements and reference conditions, such as ambient temperature of 20 ～ 22 ℃, and the length of the straight pipe section before and after Greater than 10D and 3D (usually 5D and 2D), it is even proposed that the flow sensor should be integrated with the front and rear straight pipes to perform solid flow calibration on the flow standard device to reduce the impact of clamping. Therefore, when selecting and comparing multiple models, do not just look at high indicators, but read the manufacturer's samples or instructions for comprehensive analysis. The functions of EMF in the market are also very different. The simple one is to measure the unidirectional flow and only output the analog signal to drive the rear instrument. The multi-function instrument has bidirectional flow measurement, range switching, upper and lower limit flow alarm, air pipe and power cut off alarm. , Small signal removal, flow display and total calculation, automatic check and fault self-diagnosis, communication with upper computer and motion configuration, etc. The serial digital communication function of some models of instruments can select multiple communication interfaces and application-specific chips (ASICs) to connect HART protocol systems, PROFTBUS, Modbus, CONFIG, FF fieldbus, etc. The flow rate, full-scale flow, range and caliber, and the selected instrument caliber may not necessarily be the same as the pipe diameter, and should depend on the flow rate. The process industry conveys liquids with different viscosities, such as water, and the flow velocity of the pipeline is generally 1.5 to 3 m / s. EMF is used in such pipes, and the diameter of the sensor can be the same as the pipe diameter.
The liquid flow velocity of the electromagnetic flowmeter can be selected in the range of 1 ~ 10m / s when the full-scale flow rate is reached, and the range is relatively wide. The upper limit flow rate is not limited in principle, however, it is generally not recommended to exceed 5m / s, unless the lining material can withstand the erosion of the liquid flow. Practical applications rarely exceed 7m / s, and more than 10m / s is more rare. The lower limit of full-flow velocity is generally 1m / s, and some models are 0.5m / s. For some new construction projects, the flow rate is low or the flow rate is low at the beginning of the operation. From the perspective of measurement accuracy, the diameter of the instrument should be changed to be smaller than the diameter of the pipe and connected with a reducer. For fluids with substances that are easy to adhere, deposit, scale, etc., select a flow rate of not less than 2m / s, and preferably increase to 3 to 4m / s or more, to perform self-cleaning and prevent adhesion and deposition. For abrasive fluids such as slurry, the usual flow rate should be less than 2 ~ 3m / s to reduce the wear on the lining and the electrode. When measuring low-conductivity liquids that are close to the threshold (that is, the lower limit), select a lower flow rate (less than 0.5 to 1 m / s) as much as possible. As the flow rate increases, the flow noise will increase, and the output will slosh.
的范围度是比较大的，通常不低于20，带有量程自动切换功能的仪表，可超过50～100。 The range of the electromagnetic flowmeter is relatively large, usually not less than 20, and the instrument with automatic range switching function can exceed 50 to 100. The calibers that can be provided in China are from 10mm to 3000mm. Even in practical applications, they are mostly small and medium calibers, but they are similar to most other flow meters (such as volumetric, turbine, vortex, or Coriolis mass type). ) Compared with large-caliber instruments, they occupy a larger proportion. Among the nearly 10,000 meters of an enterprise, small diameters below 50mm, medium diameters from 65 to 250mm, large diameters from 300 to 900mm, and super large diameters above 1000mm account for 37%, 45%, 15%, and 3% of liquid conductivity, respectively. The measured liquid must be conductive and cannot be lower than the threshold (ie, the lower limit). If the conductivity is lower than the threshold value, it will cause measurement error or even cannot be used. Even if it exceeds the threshold value, it can be measured. The indication error does not change much. The threshold value of general EMF is between 10-4 ～ (5 × 10-6) S / cm. , Depending on the model. It also depends on the length of the flow signal line and its distributed capacitance between the sensor and the converter. The manufacturer's instruction manual usually specifies the length of the signal line corresponding to the conductivity. Non-contact capacitively coupled large-area electrodes can measure liquids with conductivity as low as 5 × 10-8S / cm. The conductivity of industrial water and its aqueous solution is greater than 10-4S / cm, and the conductivity of acid, alkali and saline solution is between 10-4 ～ 10-1S / cm. There is no problem in use. Low-distilled water is 10-5S / cm is not a problem. Petroleum products and organic solvents are too low to be used. According to the information, some pure liquids or aqueous solutions have low conductivity and are considered unusable. However, in actual work, they may be used because they contain impurities, which increase the conductivity. For the aqueous solution, the conductivity in the data is measured in the laboratory with pure water ratio. The actual aqueous solution may be used with industrial water ratio. The conductivity will be higher than that found, which is also beneficial to flow measurement.