Yokogawa electromagnetic flowmeter encountered common problems

Problems with fluid conductivity The decrease in fluid conductivity will increase the output impedance of the electrode and the load caused by the input impedance of the converter Therefore, according to the following principles, the lower limit of the conductivity of the fluid in the application of electromagnetic flowmeter is specified. The output impedance of the electrode determines the input impedance required by the converter, and the electrode output impedance can be considered to be dominated by the conductivity of the fluid and the size of the electrode. The influence of deposits on the electrode lining. When measuring fluids with deposits attached, the electrode surface will be contaminated, often causing zero point changes, so attention must be paid. It is difficult to quantitatively analyze the relationship between the zero point change and the degree of electrode contamination, but it can be said that the smaller the diameter of the electrode, the less affected it will be. During use, attention should be paid to cleaning the electrode to prevent adhesion. When measuring fluids with sedimentation, in addition to selecting a lining such as glass or polytetrachlorethylene that is difficult to adhere to sedimentation, the flow rate should also be increased. If bubbles are uniformly contained in the fluid, the volumetric flow rate including the bubbles will be measured, and the measured flow value will be unstable, thereby introducing errors. The problem of signal transmission cable length: The shorter the connecting cable between the sensor (ie 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 relatively long. In this case, the long length of the connecting cable must be considered. The maximum length of the connecting cable between sensor and converter is in turn determined by the distributed capacitance of the cable and the conductivity of the measured fluid. In actual use, when the conductivity of the fluid being measured is within a certain range, the maximum length of the cable between the electrode and the converter is determined. When cable lengths exceed maximum lengths, loading effects caused by cable distributed capacitance become a problem. To prevent this from happening, use a double-core, two-layer shielded cable. 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 distributed capacitance between the core wire and the shield, However, if the core wire and the shield are at the same potential, there will be no current passing between them, and there will be no load effect on the cable. Therefore, the signal cable can be extended to a large length. In addition, special signal transmission cables can be used to extend large distances between converter and sensor. Technical issues of excitation Excitation technology is one of the key technologies for electromagnetic flowmeter measurement performance. In practical applications, excitation methods can be divided into AC sine wave excitation, non-sinusoidal AC excitation and DC excitation methods. AC sine wave excitation, when the AC power supply voltage (sometimes frequency) is unstable, the magnetic field strength will change, so the induced electromotive force generated between the electrodes will also change. Therefore, the signal corresponding to the calculated magnetic field strength must be taken out from the sensor as Standard signal. This excitation method can easily cause zero point changes and reduce the measurement accuracy. Non-sinusoidal AC excitation is a square wave or triangular wave excitation method that is lower than the industrial frequency. It can be considered to generate a constant DC and change the polarity periodically. Since this excitation power supply is stable, it does not need to be used to remove the magnetic field strength. carried out with changes.

Errors caused by non-axisymmetric flow The flow velocity of the fluid in the tube is the axisWhen the distribution is symmetrical and in a uniform magnetic field, the magnitude of the electromotive force generated on the flowmeter electrode has nothing to do with the flow velocity distribution of the fluid and is proportional to the average flow velocity of the fluid. However, when the flow velocity distribution is asymmetric, that is, each flow particle is relative to Depending on the geometric position of the electrode, the size of the induced electromotive force generated by the counter electrode is also different. The closer to the electrode, the greater the induced electromotive force generated by the particle with a high speed. Therefore, it is necessary to ensure that the fluid flow rate is axially symmetrical. If the flow velocity in the pipe is non-axisymmetrically distributed, errors will occur. Therefore, when selecting an electromagnetic flowmeter, it is necessary to ensure the requirements of the straight pipe section as much as possible to reduce the error caused by it.