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I. Classification of electromagnetic flowmeters by excitation current method: (1) DC excitation. DC-excited electromagnetic flowmeters are used to measure the flow rate of liquid metals, such as mercury at room temperature, and liquid sodium, lithium, potassium, etc. at high temperatures. In the primary loop of fast neutron breeder nuclear reactors, instruments used to measure the flow rate of molten sodium at temperatures above 500 degrees Celsius have been reported to be used in piping systems with a diameter of DN300 mm. The accuracy of the dry-method verification flow rate can reach 3% compared to wet-method (actual flow) verification. In addition to sensors with secondary fields generated by electromagnetic coil excitation, there are also smaller-diameter sensors that utilize the magnetic field of permanent magnets. The use of this type of instrument in China’s conventional process industries is still in the exploratory stage; it was successfully applied to measure the flow rate of pumps in electrolyzers at normal temperatures during the 1960s and 1970s ； The non-ferrous metallurgy industry also attempted to use it for measuring the flow rate of molten zinc, but it was not successful due to issues such as scaling on the inner walls of the measurement tube and zinc oxide formation. (2) AC excitation Early electromagnetic flowmeters were excited by 50Hz mains electricity, generating a sinusoidal alternating magnetic field ； The reason for using AC excitation is to avoid polarization phenomena on the electromagnetic surface that occur with DC excitation; however, due to susceptibility to the accumulation of various induction noises correlated with the flow signal caused by mains interference, leading to issues such as zero drift, it has now gradually been replaced by low-frequency rectangular wave excitation. These overlapping induction noises are caused by the following reasons. 1) Eddy currents in the liquid generate noise in phase with the flow signal, as well as noise caused by electrode contamination and zero-point drift ； 2) The iron loss in the magnetic circuit shifts the phase between the excitation current and the magnetic field, resulting in the same type of noise being generated, as a result of the orthogonal (90°) noise phase shift that was originally caused by the transformer effect in the signal circuit ； 3) Electrostatic induction noise between the excitation coil and the signal line, as well as between the excitation coil and the fluid. However, magnetic induction intensity in AC excitation flow sensors is usually relatively high, offering the advantages of a large signal electromotive force (about 1 mV per 1 m/s, while rectangular wave excitation yields only 0.2–0.3 mV) and a high signal-to-noise ratio. Furthermore, when measuring fluids such as slurry with a solid-liquid two-phase structure, the low-frequency rectangular wave excitation method generates slurry noise in the form of spikes as the solids scrape against the electrode surface, causing fluctuations in the output signal; electromagnetic flowmeters driven by power-frequency AC do not have this drawback. Therefore, some instrument manufacturers at home and abroad continue to supply electromagnetic flowmeters driven by power-frequency or other frequency AC currents. (3) Low-frequency rectangular wave excitation Since its introduction in 1975, the low-frequency rectangular wave excitation method has seen rapid development due to advantages such as low power consumption, stable zero point, and minimal impact from electrode contamination, and it has become the primary excitation method to date. The low-frequency rectangular wave excitation waveforms can be binary with ‘positive-negative’ values, or ternary with ‘positive-zero-negative-zero’ values, as shown in b and c of the figure above to the right. The three-value excitation uses signals during the non-excitation period to improve zero-point stability. The flow signal is sampled over intervals during which the signal voltage remains essentially constant. The excitation frequency is 1/32 to 1/2 of the mains frequency, with values between 1/8 and 1/4 being commonly used. Lower frequencies are used for large-diameter instruments, while higher frequencies are used for small-diameter instruments; the manufacturer sets this based on the diameter of the sensor used. Abroad, there are also meters whose frequency can be set by the user, such as the commercially known \"Programmable Keyed Storage\" electromagnetic flowmeter. Although rectangular-wave alternating excitation is not affected by slow changes in polarization voltage, the use of direct current for a short period to stabilize the magnetic field, combined with rapidly changing polarization voltage, still results in sharp changes in output. Furthermore, when solid particles in the fluid collide with the electrodes, their surface oxide films are damaged, causing changes in the surface potential that result in spike-like noise, leading to significant fluctuations in the output. This phenomenon is known as slurry noise; it occurs in high-concentration slurries or liquids containing sediment particles, and it affects the use of low-frequency rectangular-wave electromagnetic flowmeters. Reasons why high-frequency power-frequency excitation provides superior resistance to slurry noise compared to low-frequency rectangular-wave excitation ； It can also be explained using the frequency characteristics of slurry noise. An example of the noise spectrum of pulp slurry: the noise level is high in the low-frequency range and low in the high-frequency range, which is the so-called 1/√f characteristic. Therefore, with a higher excitation frequency, the noise level is low, allowing for signals with a higher S/N ratio to be obtained. (4) Dual-frequency excitation: Dual-frequency excitation involves superimposing a high-frequency rectangular wave on a low-frequency rectangular wave. As shown in the figure on the right, sampling is performed at two frequencies, followed by the use of low-pass and high-pass filters to obtain high-frequency and low-frequency signals. By combining these signals, a flow rate signal is generated. This approach overcomes the slurry noise and flow noise associated with low-frequency rectangular-wave excitation, thereby improving the stability and responsiveness of the instrument. Fast response capability can reflect the flow conditions of piston pumps at frequencies up to 1 Hz. 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