The prerequisite for using an electromagnetic flowmeter is that the liquid being measured must be conductive and cannot be lower than the threshold (i.e. lower limit).
The prerequisite for using an electromagnetic flowmeter is that the liquid being measured must be conductive and cannot be lower than the threshold (i.e. lower limit). If the conductivity is lower than the threshold, measurement errors will occur or even cannot be used. If the conductivity exceeds the threshold, it can be measured even if it changes. The indication error does not change much. The threshold of a general-purpose electromagnetic flowmeter is 10-4 ~ (5×10-6) S/cm vary depending on the model. The use also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The manufacturer's instruction manual usually stipulates the length of the signal line corresponding to the conductivity. Instruments with non-contact capacitively coupled large-area electrodes can measure liquids with conductivity as low as 5×10-8S/cm.
The working principle of the electromagnetic flowmeter is based on the fact that the measured medium is conductive, which is called conductivity. The conductivity of the medium is also closely related to the temperature and the doping degree of impurities in the medium. Temperature
Conductivity has a strong correlation with temperature. The electrical conductivity of metals decreases as temperature increases. The conductivity of semiconductors increases with temperature. Over a range of temperature values, conductivity can be approximated as being proportional to temperature. In order to compare the conductivity of substances at different temperatures, a common reference temperature must be set. The correlation between conductivity and temperature can often be expressed as the slope of a graph of conductivity versus temperature.
The doping level of solid-state semiconductors can cause large changes in electrical conductivity. Increasing the level of doping results in increased conductivity. The conductivity of an aqueous solution depends on the concentration of solute salts or other chemical impurities that will decompose into electrolytes. The conductivity of water samples is an important indicator for measuring the salt content, ionic content, impurity content, etc. of water. The purer the water, the lower its conductivity (higher resistivity). The conductivity of water is often recorded as conductivity; conductivity is the conductivity of water at a temperature of 25°C.
The conductivity of industrial water and its aqueous solution is greater than 10-4S/cm, and the conductivity of acid, alkali, and salt solutions is between 10-4 and 10-1S/cm. There is no problem in use. Low There is no problem if distilled water has a temperature of 10-5S/cm. Petroleum products and organic solvents cannot be used if their conductivity is too low. It is found from the data that some pure liquids or aqueous solutions have relatively low conductivity and are considered unusable. However, in actual work, they may be used because they contain impurities. Such impurities increase the conductivity of the medium. For aqueous solutions, the conductivity in the data was measured in the laboratory using pure water proportions. The actual aqueous solution may be proportioned with industrial water. The conductivity will be higher than that measured, which is also conducive to flow measurement.
The performance of general-purpose electromagnetic flowmeters on the market varies greatly. Some have high precision and many functions, while others have low precision and simple functions. The basic error of a high-precision sanitary electromagnetic flowmeter is (±0.5%~±1%) R, while the basic error of a low-precision instrument is (±1.5%~±2.5%) FS. Both***The price differs by 1 to 2 times. Therefore, it is economically uneconomical to use precision instruments in places where measurement accuracy is not very important, such as places where non-commercial accounting is only for control purposes and only requires reliability and excellent repeatability.
Some models of sanitary electromagnetic flowmeters claim to have certain heights and a basic error of only (±0.2%~±0.3%) R, but they have strict installation requirements and reference conditions, such as an ambient temperature of 20~22 ℃, the length of the front and rear straight pipe sections is required to be greater than 10D and 3D respectively (usually 5D and 2D). It is even proposed that the flow sensor should be integrated with the front and rear straight pipes and calibrated for real flow on the flow standard device to reduce the impact of clamping. Therefore, when choosing among multiple models, don't just look at the absolute indicators, but read the manufacturer's samples or instructions in detail to make a comprehensive analysis.
The functions of electromagnetic flowmeters on the market are also very different. A simple one only measures one-way flow and only outputs an analog signal to drive the rear instrument; a multi-functional sanitary electromagnetic flowmeter can measure two-way flow and range. Switching, upper and lower limit flow alarms, empty pipe and power cutoff alarms, small signal cutoff, flow display and total calculation, automatic verification and fault self-diagnosis, communication with the host computer and motion configuration, etc. The serial digital communication function of some models of instruments can choose from a variety of communication interfaces and chips (ASIC) to connect to HART protocol systems, PROFTBUS, Modbus, CONFIG, FF fieldbus, etc.
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