The effect of grounding and shielding wires in electromagnetic flowmeters

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　　The electromagnetic flowmeter is a very important industrial control system in industrial production, and it holds significant significance for industrial processes. Currently, electromagnetic flowmeters are widely used in various industrial processes, and they are highly effective for measuring liquid flow rates. This article will briefly explain the role of grounding and shielding wires in electromagnetic flowmeters. 　　The high accuracy and good stability of electromagnetic flowmeters have led to their increasing use in the water supply industry. In the water supply industry, the supplier and the user rely on flow meters to settle water fees. Only instruments with high precision and good stability can accurately reflect water consumption, thereby fostering trust between supply and demand parties and ensuring fairness and equity. 　　How to maintain electromagnetic flowmeters: Detailed explanation of the functions of grounding and shielding wires for electromagnetic flowmeters. Under normal use, the maintenance required for electromagnetic flowmeters is not much. However, in the south, especially in Dongguan City located in the Pearl River Delta, there are many thunderstorms during the summer, and lightning strikes thus pose the greatest threat to the safe use of electromagnetic flowmeters. In the first few years after we started using electromagnetic flowmeters, thunderstorms posed a real problem for us; every year, half of the flowmeter converters were damaged by lightning strikes ； In several flowmeters located in areas with frequent lightning strikes, the converter at one site was damaged by lightning as many as five times per year at its peak. Later, taking into account the characteristics of electromagnetic flowmeters, we installed lightning protection devices; as a result, the electromagnetic flowmeters in our company are now rarely damaged by lightning. During the use of electromagnetic flowmeters, some common fault phenomena may occur, but their causes are not easy to determine. Here is an example I encountered at work. 　　In Case 1, our company installed a DN120 electromagnetic flowmeter for a customer, and the customer themselves installed another flowmeter of the same brand and diameter about thirty meters behind our flowmeter (for comparison purposes). A few days after water flow was initiated, a problem arose: the relative error between the two meters was 2.8%, with the meter at Gao She Shui Si showing a lower reading; Gao She Shui Si requested that the flow meter be inspected. On site, we measured that the installation position of the valve in front of the flow meter was basically within the required range, at around 6 meters. The parameters of the converter were set correctly. It was observed that the instantaneous flow rate of the flow meter was only about 100 m3/h; for a DN1200 electromagnetic flow meter, such a low flow rate results in too low a water velocity, which affects the accuracy of the flow meter and prevents it from achieving an accuracy of 0.5%. So we asked the users to open the valves wider, increasing the water flow to 1500 m3/h; after operating in this manner for a few days, the relative error of the meter dropped to 2.4%. This situation indicates that there are other reasons. When we inspected the flow meter sensor well at Gao Shui Si, we found that the sensor’s grounding wire was not connected to the grounding ring, nor was there any grounding connection; therefore, we ensured a proper connection between the sensor’s grounding wire and the grounding ring. After several days of use, the relative error between the two meters was 0.7%, indicating that the electromagnetic flowmeter met the required performance standards; it seems that the problem was caused by a lack of proper grounding. 　　It is very important to ground the sensor of an electromagnetic flowmeter, especially when there is stray current interference in the pipeline; only with proper grounding can the electromagnetic flowmeter provide accurate measurements. Perhaps in an environment free from such interference, not grounding the device does not affect the flow meter’s ability to provide accurate measurements. However, once the environment changes, it prevents the flow meter from functioning accurately and stably. Therefore, it is best to ensure proper grounding of the sensor during installation, in order to create a favorable working environment for the electromagnetic flow meter. 　　The signal generated by the sensor of an electromagnetic flowmeter is very weak, usually only a few millivolts, and it is prone to interference that can prevent it from functioning properly. Therefore, during installation, not only must the shielding wire of the signal line be in good condition, but it should also be kept as far away as possible from power cables. It is advisable to wrap the signal line in multiple layers of metal shielding to reduce external interference and ensure better performance of the electromagnetic flowmeter. In Case 3, during the long-term use of electromagnetic flowmeters, unexpected damage can occur; for example, rats may chew through the signal wires, or the signal wires might be damaged during road construction work. Under these circumstances, there was no choice but to leave a connection in place, which also created potential risks. If the insulation of the connector is poor, its insulating strength will decrease over time, resulting in inaccuracies in the flow meter’s measurements. Our company has a user table for DN600 electromagnetic flowmeters. There is a repair shop nearby with a high volume of traffic, and as a result the signal wires of the flowmeters were damaged. Since we couldn’t replace them with new wires, we had no choice but to reconnect the wires and continue using them. Last summer, we noticed that the cumulative flow measured by the flow meter was lower than usual for several days, dropping from around 6,000 m3 per day to just over 3,000 m3. We checked the flow meter converter and found it to be in good condition; the sensor’s excitation coil was also functioning properly, and the electrode contact resistance was within normal limits. Therefore, no fault was identified. In the following days, the cumulative traffic per day becomes even smaller. We visited the users, and their water consumption remained unchanged. We thought of the connector that used to be on the signal cable, as the ground was quite damp due to continuous rainy days. We dug out the signal wires; there was water inside the conduit, and the joints were also damp. We redid the joints, dried them out, and took measures to prevent moisture from entering, after which the flow meter’s measurement function returned to normal. Now, for the sections of the signal wires where there are breaks, we try to replace them with whole wires. If that’s not possible, then the joints must be properly handled. Solder at the joints using a soldering iron to prevent long-term oxidation of the joints, which could lead to poor contact. Insulation and moisture resistance are even more important; when the signal wires get wet, it directly affects the operation of the flow meter. Now we first seal each connector with heat-shrink tubing, and then wrap it in two layers of waterproof tape to prevent it from getting damp. For the inspection of signal wires, it is possible to measure the electrode contact resistance value after each flow meter starts to be used, and record this value for archiving. This allows for comparison during subsequent maintenance, enabling identification of any changes in resistance values and thus making it easier to diagnose faults.