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Description of the measurement object of the electromagnetic flowmeter

For the measurement object of the electromagnetic flowmeter, the premise of using the electromagnetic flowmeter is that the liquid to be tested must be electrically conductive and cannot be lower than the threshold (ie, the lower limit). If the conductivity is lower than the threshold, measurement error will be generated until it can not be used. If the threshold value exceeds the threshold, it can be measured even if the change is small. The threshold value of the general-purpose electromagnetic flowmeter is 10-4~(5×10-6)S/cm. Between, depending on the model. It also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The length of the signal line corresponding to the conductivity is usually specified in the manufacturer's instruction manual. Non-contact capacitive coupling large-area electrodes can measure liquids with conductivity as low as 5 × 10-8 S / cm.
The conductivity of industrial water and its aqueous solution is greater than 10-4 S/cm, and the conductivity of acid, alkali and salt liquid is between 10-4 and 10-1 S/cm. There is no problem in use, and low-grade distilled water is 10-5 S/ There is no problem with cm. Petroleum products and organic solvents cannot be used if their conductivity is too low. Table 1 lists the conductivity of several liquids. It is found from the data that some pure liquids or aqueous solutions have low conductivity and are considered to be unusable. However, in practice, examples can be used because they contain impurities, and such impurities are advantageous for increasing conductivity. For aqueous solutions, the conductivity in the data is measured in the laboratory using pure water ratio. The actual aqueous solution may be used in industrial water ratio, and the conductivity will be higher than that obtained, which is also beneficial for flow measurement.

Table 1 Conductivity of several liquids at 20 ° C
Liquid name conductivity
Liquid name conductivity
Liquid name conductivity
Petroleum (3~5)×10-13
Acetone (2~6)×10-8
Pure water, highly distilled water 4×10-8
Benzene 7.6×10-8
Liquid ammonia 1.3×10-7
Methanol (4.4 to 7.2) × 10-7
Drinking water ≈10-4
Sea water ≈ 4×10-2
Sulfuric acid (5% to 99.4%) (2.1×10-1) to (8.5×10-3)
Ammonia water (4% to 30%) (1×10-3)～(2×10-4)
Sodium hydroxide (4% to 50%) (1.6 × 10-1) ~ (8 × 10-2)
Saline solution (2.5%) 2×10-1

Depending on the experience of use, the actual conductivity of the liquid is preferably at least an order of magnitude greater than the threshold specified by the instrument manufacturer. Because the lower limit specified by the manufacturer's instrument specification is the lowest value that can be measured under various conditions of good use. It is subject to some use conditions, such as conductivity uniformity, connecting signal lines, external noise, etc., otherwise output sloshing will occur. We have encountered measurement of low-grade distilled water or deionized water many times, and its conductivity is close to the threshold of 5×10-6S/cm, and output sloshing occurs when used.

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