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The last edit to this post was made by I’m stupid and nameless on 2012-12-21 at 16:08. Electromagnetic flowmeters are classified based on whether the electrodes of the flow sensor come into contact with the liquid being measured: (1) Contact-type electrodes – The electrodes that come into contact with the liquid represent the traditional design for EMF flowmeters; usually, there is a pair of such electrodes, while larger-diameter instruments use two pairs. The non-full-pipe EMF also uses 3 pairs of motors or strip electrodes. (2) Non-contact electrodes: Large-area electrodes are attached closely to the outer surface of the lining (or insulated measuring tube) to detect flow signals via capacitive coupling. This type of electromagnetic flowmeter, which uses capacitive detection rather than contact electrodes, is simply referred to as a capacitive flowmeter; its principle is illustrated in the figure above. The preamplifier is located inside the sensor and relies solely on the electrodes; the excitation frequency is higher than that of ordinary EMFs, at 50/2 Hz, with some cases reaching over 100 Hz. Instruments of this type do not generate noise resulting from electrode surface effects such as electrode passivation, oxidation, and catalytic action; there is also little to no flow noise or slurry noise. A non-conductive layer such as grease, or a thin insulating structural layer on the inner surface of the lining, will not affect the measurement ； However, if the covering layer is a conductive film, the instrument will give no indication. Some instrument manufacturers abroad refer to this type of instrument as electrodeless electromagnetic flowmeters. Electromagnetic flowmeters are classified according to the structure of their flow sensors: (1) Short-tube type. The short-tube structure is similar to that of traditional electromagnetic flowmeters, with the flow sensor featuring a measurement tube section connected to the pipeline system. (2) Insertion type: The sensor of an insertion-type electromagnetic flowmeter is essentially an electromagnetic flow velocity sensor; the excitation coil and electrodes are assembled into a rod shape and inserted through a hole in the pipeline to be measured. The measured flow velocity is multiplied by coefficients such as the pipe area preset in the converter to calculate the flow rate. In addition to the “point flow velocity” at a single point, there is also the “radial flow velocity” at multiple side points. This type of instrument is suitable for large pipelines, as it calculates fluid flow by measuring local flow velocities; its measurement accuracy is much lower than that of instruments used for short pipes. It is typically used only for process control and is not appropriate for measurement purposes in trade accounting. But the price is relatively cheap. For application features, see also insert-type flow meters. Classified by the connection method between the flow sensor and the pipeline: (1) Flange connection. This is the traditional connection method; the sensor has connection flanges at both ends, which are fixed to the pipeline flanges using bolts, allowing for installation in one direction only. Large-diameter sensors all use this connection method. It is larger in both volume and weight compared to the clamped connection method. The length between the two flange faces of DN15～600mm electromagnetic flowmeters has been standardized by the International Organization for Standardization (ISO 13359;1998) to ensure consistency. (2) Clamping connection: A connection method that has developed in recent years; the sensor itself does not have flanges, and it is clamped between the flanges of the pipeline using long bolts to be integrated into the piping system. This type of sensor is small in size and light in weight, and it has strong adaptability to different pressure specifications as well as to the flange spacing in standard piping systems ； But it is only applicable to smaller pipe diameters (below 200 mm) and can handle lower liquid working pressures. http://www.kfllj.com/up_files/image/Article/2010/12/15/41676351.jpg (3) Sanitary connection: The example shown in the image on the right depicts a sanitary-type connection; it is connected to IDF fittings provided by the International Dairy Federation, enabling quick disassembly and assembly as well as facilitating frequent cleaning on a daily basis. http://www.kfllj.com/up_files/image/Article/2010/12/15/41676352.jpg (4) Threaded connection: The threaded connection method for small-diameter instruments is shown in the figure below. Such instruments are widely used in industries such as pharmaceuticals and food, as well as in applications involving dosage mixing in the pharmaceutical sector. Threaded connections are also widely used in industries such as petroleum and geological exploration for measuring the flow rate of high-pressure water injection or cement slurry at pressures above 16–25 MPa, with a trapezoidal thread shape.