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Considerations when using electromagnetic flowmeters - Focus on microfluidic control technology

1. The electromagnetic flowmeter has a wide range of applications.
Large-caliber instruments are mostly used in water supply and drainage projects. Medium and small calibers are often used for solids, liquids, and difficult-to-measure fluids or high-demand locations such as pulp and black liquor in paper industry, pulp in non-ferrous metallurgical industry, coal slurry in coal preparation plants, strong corrosive liquids in chemical industry, and cooling water control in the blast furnace of steel industry. And leakage, long-distance pipeline coal hydraulic transmission flow measurement and control. Small calibers and micro calibers are commonly used in places where there are hygiene requirements, such as the pharmaceutical industry, food industry, and bioengineering.

2, electromagnetic flowmeter accuracy level and function
The performance of general-purpose electromagnetic flowmeters in the market is quite different. Some have high accuracy, many functions, and some have low accuracy and simple functions. The basic error of the high-precision instrument is (±0.5%~±1%)R, and that of the low-precision instrument is (±1.5%~±2.5%)FS. The price difference between the two is 1 to 2 times. Sites with low requirements for measurement accuracy (eg, non-trade accounting only for control purposes, seeking high reliability and excellent repetitive locations) are not economical when using high-precision meters. Some models claim higher accuracy and the basic error is only (±0.2%～±0.3%)R. However, there are strict installation requirements and reference conditions. For example, the ambient temperature is 20-22°C, and the straight pipe length requirements before and after are greater than 10D, 3D (usually 5D, 2D) proposed that the flow sensor should be aligned with the front and rear straight pipes to make a real flow calibration on the standard device to reduce the impact of improper clamping. When comparing multiple models, do not simply look at high indicators. Read the manufacturer's samples or instructions carefully for comprehensive analysis.
Electromagnetic Flowmeter

3, electromagnetic flow meter flow rate, full flow, range and diameter
The caliber of the selected instrument is not necessarily the same as the diameter of the tube. It depends on the flow rate. The process industry transports water and other liquids with different viscosities. The flow rate of the pipeline is generally an economic flow rate of 1.5 to 3 m/s. EMF uses such pipes, the sensor diameter and diameter can be the same. The flow rate of the liquid at the full flow rate of EMF can be selected within the range of 1 to 10 m/s, and the range is relatively wide. The upper limit flow rate is in principle unlimited, and it is generally recommended not to exceed 5 m/s. The lining material can withstand the flow scouring. The practical application rarely exceeds 7 m/s, and more rarely than 10 m/s. The lower limit of the full flow rate is generally 1m/s, and for some types of instruments it is 0.5m/s. Some new construction projects have low flow rate or low flow rate at the initial stage of operation. From the point of view of measurement accuracy, the diameter of the instrument should be changed to smaller than the pipe diameter and connected with different diameter pipes. It is used for fluids with easy adhesion, deposition, scaling, etc., with a flow rate of not less than 2m/s, preferably 3 to 4m/s or more, which functions as self-cleaning and adhesion deposition. It is used for strong abrasion fluid such as pulp, and the common flow rate should be lower than 2～3m/s to reduce wear on lining and electrode. Measure near-threshold low-conductivity liquids, select the lower flow rate (less than 0.5 to 1 m/s) as possible, because the flow velocity increases the flow noise will increase, and output shaking phenomenon occurs. EMF range is relatively large, usually not less than 20, with a range of automatic switching function instrument, can exceed 50 to 100. Domestic can provide stereotyped product diameter from 10mm to 3000mm, although the actual application of mostly small and medium caliber, but compared with most other principle flow meters (such as positive displacement, turbine, vortex street or Coriolis quality, etc.) Large-diameter instruments account for a large proportion. In a company's nearly 10,000 Taiwan meters, small caliber below 50mm, 65 ～ 250mm medium caliber, 300 ～ 900mm large caliber, large caliber above 1000mm accounted for 37%, 45%, 15% and 3% respectively.

4, electromagnetic flow meter liquid conductivity
The use of electromagnetic flowmeters is based on the premise that the liquid to be measured must be electrically conductive and must not fall below the threshold (ie, the lower limit value). Conductivity below the threshold will produce measurement error until it can not be used, the change in the threshold value can also be measured, the indication error does not change much, universal EMF threshold 10-4 ~ (5 × 10-6)S/cm, depending on the model Different. When used, it also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The non-contact capacitive coupling large area electrode meter can measure the conductivity as low as 5×10-8 S/cm. Industrial water and its aqueous solution conductivity greater than 10-4S/cm, acid, alkali, salt liquid conductivity 10-4 ~ 10-1S/cm, use no problem, low distilled water is 10-5S/cm problem . Petroleum products and organic solvents cannot be used because their conductivity is too low. Table 1 lists several liquid conductivity. From the data, it was found that some pure liquids or aqueous solutions had low conductivity, and they were considered unusable. In actual work, they encountered instances where impurities could be used, and such impurities were favorable for increasing the conductivity. Aqueous solution, the conductivity of the data is measured with pure water ratio laboratory, the actual use of aqueous solution may use industrial water ratio, conductivity will be higher than the investigation, but also conducive to flow measurement. The use of experience, the actual application of liquid conductivity is preferably at least an order of magnitude greater than the instrument manufacturer specified threshold. The lower limit of the manufacturer's instrument specification is the lowest value that can be measured under various conditions of use. In the selection is subject to some restrictions on the use of conditions, such as conductivity uniformity, connecting signal lines, external noise, etc., otherwise there will be output shaking phenomenon. We encountered repeated measurements of low-level distilled or deionized water, whose conductivity was close to the threshold of 5×10-6 S/cm, and output shaking occurred during use.

5, electromagnetic flowmeter and fluid contact parts material selection
The components contacting the fluid are lining (or measuring material made of insulating material), electrodes, connecting rings and gaskets, and their material corrosion resistance, wear resistance and upper limit of use temperature affect the instrument's adaptability to fluids. With few parts, simple shape, flexible material selection, and electromagnetic flow sensor adaptability to fluids.
(1) Electromagnetic flow meter lining materials (or measuring tubes directly in contact with the medium) Commonly used lining materials include fluoroplastics, urethane rubber, neoprene rubber and ceramics. In recent years, there are 999.7% AI2O3) high-purity alumina linings, but only small and medium caliber sensors. Neoprene and FRP are used for non-corrosive or weakly corrosive liquids, such as industrial water, waste water, and weak acids and bases, which are the cheapest. Fluoroplastics have excellent chemical resistance but poor abrasion resistance and cannot be used to measure mineral slurries. The earliest application of fluoroplastics is PTFE, which can only be used for pressure piping because of the pressure and adhesive force between the measuring tubes, and can not be used for negative pressure pipelines. Afterwards, various modified varieties are developed to achieve injection molding, which is comparable to measuring tubes. Strong binding force can be used for negative pressure, polyurethane rubber has excellent wear resistance, but poor acid and alkali corrosion resistance. Its abrasion resistance is equivalent to natural rubber 10 times, suitable for coal slurry, pulp, etc.; medium temperature is lower than 40 ~ 60/70 °C. Alumina ceramics have excellent abrasion resistance and abrasion resistance to strong acids and bases. The wear resistance is about 10 times that of polyurethane rubber. It is suitable for corrosive pulps; but brittle, negligible and fragile when installed and clamped. At higher temperatures (120-140/180°C), but to prevent drastic changes in temperature, such as steam sterilization, the general temperature change can not be greater than 100 °C, heating 150 °C to have 10min.
(2) Electromagnetic Flowmeter Electrode and Ring Material The electrode is the first choice for selecting the material to measure the corrosion resistance of the medium, and secondly, whether it will produce surface effects such as passivation and noise formed.

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