# Discussion on the application of the hottest elect

2022-10-23
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Discussion on relevant problems in the application of electromagnetic flowmeter

first, the error caused by non axisymmetric flow

when the flow velocity of the fluid in the pipe is axisymmetric distribution, and in a uniform magnetic field, the magnitude of the electromotive force generated on the electrode of the flowmeter has nothing to do with the flow velocity distribution of the fluid, and is proportional to the average flow velocity of the fluid. When the flow velocity distribution is non axisymmetric, that is, the geometric position of each flow particle relative to the electrode is different, The magnitude of the induced electromotive force generated by the opposite electrode is also different. The closer it is to the electrode, the greater the induced electromotive force generated by the particle with higher velocity. Therefore, it is necessary to ensure that the fluid flow rate is axisymmetric. If the velocity in the pipe is non axisymmetric, it will cause errors. Therefore, when installing the electromagnetic flowmeter, the requirements of the straight pipe section should be ensured as much as possible to reduce the error caused by it

second, the problem of fluid conductivity

the reduction of fluid conductivity will increase the output impedance of the electrode and produce errors due to the load effect caused by the input impedance of the converter. Therefore, the lower limit of fluid conductivity in the application of electromagnetic flowmeter is specified according to the following principles

The output impedance of the electrode determines the input impedance required by the converter, and the output impedance of the electrode can be controlled by the conductivity of the fluid and the size of the electrode

in theoretical analysis, taking the electrode as a point electrode, the size can be ignored. In fact, the electrode has a certain size. When the circular plate electrode with diameter D contacts the semi infinite broadened fluid with conductivity K, its widening resistance is 1/2kd. Therefore, if the pipe diameter d>> D, the output impedance of the electrode is the sum of the two widening resistances, which is equal to 1/kd

generally, the lower limit of the measured fluid conductivity is 5 s/? 10 s/? Cm. Therefore, if the electrode diameter is 1 cm, the output impedance of the electrode is 1/kd=100k Ω ~ 200K Ω. In order to limit the influence of the output impedance to less than 0.1%, the input impedance of the converter should be about 200m Ω

III. Influence of electrode lining attachment

when measuring the fluid with attached sediment, the electrode surface will be polluted, often causing zero point changes, so we must pay attention to it

it is difficult to conduct quantitative analysis on the relationship between the zero point change and the degree of electrode pollution, but it can be said that the smaller the electrode diameter is, the less it will be affected. In use, pay attention to the cleaning of the electrode to prevent adhesion

error when sediment is attached to the lining Δε， If the attached thickness is the same, it can be determined by the formula:

Δε Convert the elongation into tension value =/[1+ (k ω/Kf）+（1- K ω/Kf ） × (t/d) 2] calculation, where k ω、 KF is the conductivity of the attachment and the measuring fluid respectively. The thickness of the attachment is t and the diameter is d

if where, K ω If it is equal to KF, there is no error. When the conductivity of the attachment is low, the above formula is also true, but it is limited because it will increase the output impedance of the electrode, such as the insulating sediment immersed in the fluid. On the contrary, if metal powder is attached, the induced potential is short circuited due to the attachment layer with high conductivity, resulting in low electrode output and negative deviation

when measuring the fluid with sediment attachments, in addition to selecting the lining that is difficult to adhere to sediment, such as glass or polytetrafluoroethylene, its flow rate should also be increased. If bubbles are uniformly contained in the fluid, the volume flow including bubbles is measured, and the measured flow value is unstable, resulting in errors

to sum up, when selecting flowmeter, especially large-diameter electromagnetic flowmeter, we should consider the maintenance of sensor electrode and lining in the future. For example, the scraper electrode or replaceable electrode of Shanghai Guanghua aimet Instrument Co., Ltd. is selected, or a deformation precision access hole for cleaning is preset at the appropriate position upstream or downstream of the sensor, so as to clean the sensor in the future

IV. the problem of the length of the signal transmission cable

the transmission assay starts from the friction and wear experiment. The shorter the connecting cable between the machine sensor (i.e. electrode) and the converter, the better. However, some sites are limited by the location of the installation environment, and the distance between the converter and the sensor is far. At this time, the maximum length of the connecting cable should be considered. The maximum length of the connecting cable between the sensor and the converter is determined by the distributed capacitance of the cable and the conductivity of the measured fluid

in practical use, when the conductivity of the measured fluid is within a certain range, it determines the maximum length of the cable between the electrode and the converter. When the cable length exceeds the maximum length, the load effect caused by the cable distributed capacitance becomes a problem. To prevent this from happening, a double core two-layer shielded cable is used, and the converter provides a low impedance voltage source so that the inner shield and the core wire get the same voltage to form a shield. Even if there is a distributed capacitance between the core wire and the shield, but the core wire and the shield have the same potential, there is no current between them, and there is no load effect of the cable, so the maximum length of the signal cable can be extended. In addition, a special signal transmission cable can be used to extend the maximum length between the converter and the sensor

v. technical problems of excitation

excitation technology is one of the key technologies for measuring the performance of electromagnetic flowmeter. The excitation mode can be divided into AC sine wave excitation, non sine wave AC excitation and DC excitation in practical application

AC sine wave excitation, when the AC power supply voltage (sometimes frequency in 2013) is unstable, the magnetic field intensity will change, so the induced electromotive force generated between electrodes will also change. Therefore, the signal corresponding to the calculated magnetic field intensity must be taken from the sensor as the standard signal. This excitation method is easy to cause zero point change and reduce its measurement accuracy

non sine wave AC excitation adopts square wave or triangular wave excitation lower than the industrial frequency, which can be considered as a way to generate constant DC and periodically change the polarity. Because this excitation power supply is stable, it is not necessary to calculate to remove the change of magnetic field intensity

the main problem of AC excitation mode is the serious induced noise

DC excitation mode makes the polarization potential on the electrode an important obstacle. Therefore, the DC excitation method with a certain value is only applicable to the measurement of non electrolyte (such as liquid metal) liquids

when measuring tap water, source water and other aqueous solutions, periodic intermittent DC excitation is generally used. The intermittent period should be an integral multiple of the cycle of the AC power supply, which can eliminate the noise of the frequency of the AC power supply and eliminate the eddy current of the AC magnetic field and the polarization interference of the DC magnetic field

the excitation frequency is reduced, and the zero point stability can be improved, but the anti-low-frequency interference ability of the instrument is weakened, and the response speed is slow. If the excitation frequency is high, the anti-low-frequency interference ability is enhanced, but the

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