Thread Content
At present, electromagnetic flowmeters have been developed into various types, with many different classification methods: 1. Classification by excitation method 1) DC excitation type. There are very few of these electromagnetic flowmeters, which are used only for measuring the flow of liquid metals, such as mercury at room temperature and liquid sodium and potassium at high temperatures. 2) AC power-frequency excitation type: Earlier electromagnetic flowmeters used 50Hz power-frequency mains electricity for excitation; due to issues such as susceptibility to electromagnetic interference and zero-point drift, they have been gradually replaced by low-frequency rectangular excitation. However, when measuring liquid-solid two-phase flows such as slurry and pulp, the low-frequency rectangular wave excitation method fails to eliminate the spike noise generated by solids rubbing against the electrode surface, whereas instruments with power-frequency AC excitation do not have this drawback; as a result, some electromagnetic flowmeters both at home and abroad still use AC excitation. 3) Low-frequency rectangular wave excitation type: It is used for low-frequency rectangular wave excitation as it has low power consumption and stable zero point; it is currently the main excitation method for electromagnetic flowmeters. Its waveform has two types: binary \"positive–negative\" and ternary \"positive–zero–negative–zero\". For some electromagnetic flowmeters, the excitation frequency can be set by the user; generally, higher frequencies are used for small-diameter instruments, while lower frequencies are used for large-diameter instruments. 4) Dual-frequency excitation type: The waveform of the excitation current is a high-frequency rectangular wave superimposed on a low-frequency rectangular wave, primarily to overcome the slurry noise and flow noise associated with binary rectangular-wave excitation, thereby improving the stability and response characteristics of the instrument. 2. Classification by the composition of sensors and converters: 1) Separate type – this is the main type of electromagnetic flowmeter. The sensors are installed on the flow pipes, while the converters are placed in the control room or near the sensors, where they can be easily installed and operated; the distance between them is generally in the range of dozens to hundreds of meters. The advantage is that the converter can be placed away from the harsh environmental conditions at the site, making it easier to inspect electronic components, make adjustments, and compare measurement parameters. 2) The integrated electromagnetic flowmeter sensor and converter are assembled together and mounted on the process pipeline to directly output a standard signal in the form of current (or frequency) that reflects the flow rate. The advantage is that it reduces the length of the signal lines and excitation wires connecting the sensor and the converter; without such external wiring, the electrical connections are simple and the cost is relatively low. However, it is restricted by the piping layout; if installed in locations that are difficult to access, maintenance becomes very inconvenient ； Furthermore, the electronic components in the converter are mounted on the pipeline and are susceptible to the influence of liquid temperature and pipeline vibration. 3. Classification by connection method: Based on the way in which the sensor is connected to the pipeline, it can be divided into flange connection, flangeless clamping connection, threaded connection, and others. 4. Classification by application: 1) General-purpose type – These are the standard electromagnetic flowmeters used in various industrial fields such as metallurgy, petrochemicals, papermaking, light textiles, water supply and drainage, sewage treatment, as well as in the pharmaceutical, food, biological, and fine chemical industries; they represent the main type of electromagnetic flowmeter. There are requirements regarding the conductivity of the medium under test; it generally must not exceed the specified upper and lower limits. 2) Explosion-proof type: Used in areas with explosive atmospheres. Due to the high energy of the excitation current, flameproof types are still predominantly used at present. In recent years, intrinsically safe electromagnetic flowmeters, namely safety-spark type electromagnetic flowmeters, have emerged abroad. Their excitation power has been significantly reduced, allowing them to be designed as integrated units that can operate entirely within hazardous areas. 3) Hygienic type: Electromagnetic flowmeters used in industries such as pharmaceuticals, food, and biochemistry must meet relevant hygiene requirements in terms of preventing bacterial growth over time and facilitating disassembly for cleaning. 4) Water-resistant type: Used for sensors installed underground, capable of withstanding short-term submersion in water. 5) Submersible type: Used to measure the flow rate during free flow at the free surface of open channels or underfilled buried channels. The sensor is located below the flow-blocking baffle in the open channel and operates underwater for extended periods. It differs from ordinary electromagnetic flowmeters in both structure and operation. 6) Insertion type: Used for electromagnetic flowmeters with large pipe diameters. Sensors are inserted radially through openings in the pipe to measure local flow velocities and thereby estimate flow rates; they offer lower accuracy but are inexpensive and suitable for control systems. For more information, please visit the company’s official website at http://www.yb1518.com/. Please retain this link when reproducing the content! http://www.yb1518.com/UploadFiles/2012717174126829.jpg