A five-step method to resolve the unstable zero point issue in intelligent electromagnetic flowmeters

Thread Content

This post was last edited by yuchenchf on 2017-5-2 at 14:11. In industrial production, it is common to encounter instability in the zeroing process of intelligent electromagnetic flowmeters. There are various reasons for this issue, with the main ones being as follows: 1. The pipeline is not filled with liquid, or the liquid contains bubbles; objectively, there is no movement inside the flow sensor, but in reality, there is a slight amount of movement. 2. Prolonged use of the valve or contamination by liquids, which leads to imperfect sealing of the valve, can also cause instability in the zero point. 3. Variations or unevenness in liquid conductivity can cause shifts in the zero point when at rest, and fluctuations in the output when in motion. 4. Since the scaling on the inner and outer walls as well as the degree of electrode contamination cannot be exactly the same and symmetrical, it disrupts the equilibrium condition of the initial zero-setting. 5. Changes in the condition of electrical equipment near the flow sensor (such as an increase in leakage current) lead to changes in the grounding potential, which in turn can cause variations in the zero point of the intelligent electromagnetic flow meter. By analyzing the problems, we can identify appropriate solutions based on the corresponding phenomena. 1. Initial troubleshooting steps: Follow a systematic procedure to identify the causes of instability in the smart electromagnetic flowmeter. First, conduct a comprehensive preliminary investigation and assessment, and then proceed with detailed checks to eliminate faults one by one. The order of the inspection items in this procedure is based on the following principles: (1) Those that can be determined through observation or inquiry without requiring complex operations should be checked first, i.e., start with the easier tasks; (2) Items that have occurred frequently in past maintenance experiences and are likely to occur again should be addressed first; (3) Consider the sequence required for carrying out each inspection. If preliminary investigations indicate that several of these fault causes are likely, detailed checks can be carried out ahead of time.

2. Inadequate grounding of the smart electromagnetic flowmeter sensor, along with interference from external factors such as stray currents: External disturbances like stray currents in the pipeline can affect the proper grounding of the smart electromagnetic flowmeter. It is generally required that the grounding resistance be less than 1000, and the flowmeter should not share its grounding connection with other motors or electrical devices. In some cases, even without proper grounding, the flowmeter may function normally under favorable environmental conditions. However, once such favorable conditions are no longer present, the instrument may malfunction, and attempting to diagnose the problem at that point will lead to additional complications.

Changes in the condition of electrical equipment near the flow sensor (such as an increase in leakage current) can cause variations in the grounding potential, which in turn can result in changes in the zero point of the smart electromagnetic flowmeter.

3. Issues related to the liquid itself (such as uniformity of the liquid’s conductivity or contamination of the electrodes). Changes or unevenness in the liquid’s conductivity can cause fluctuations in the zero point when the liquid is stationary, and fluctuations in the output signal when the liquid is flowing. Therefore, the flowmeter should be placed far away from the point where the liquid is injected or from sections of the pipeline where chemical reactions occur. It is best to install the flow sensor upstream of these areas. If the liquid contains solid particles, impurities that accumulate on the inner walls of the measurement tube, scale formation on those walls, or contamination of the electrodes due to grease or other substances, it can all lead to changes in the zero point. Since the degree of scaling on the inner walls and the level of electrode contamination cannot always be identical or symmetrical, this disrupts the balance necessary for the initial zero calibration. Effective measures include removing contaminants and scale deposits; if the zero point variation is not significant, re-calibrating the instrument may also be an option. IV. Decreased insulation in the signal circuit of intelligent electromagnetic flowmeters. A decrease in the insulation of the signal circuit can lead to unstable zero points. The main cause of this issue is a decline in insulation at the terminal connections; however, it’s also possible that there is a decrease or damage to the insulation of the signal cables and their connection terminals. In some cases, the field conditions are very harsh, and even minor negligence regarding the sealing of instrument covers or wire connections can allow moisture, acidic fumes, or dust particles to enter the instrument’s wiring box or cable insulation, thereby reducing the insulation level. The insulation resistance of the signal circuit should be checked separately on both the cable side and the flow sensor side, using a megohmmeter. Since the signal cable can be checked first, the flow sensor can be inspected in two steps: first by measuring the surface contact resistance of the electrodes when the tube is filled with liquid, and then by measuring the insulation resistance of those electrodes.

V. Checking the electrode contact resistance and electrode insulation resistance is done in two steps.
(1) Measuring the liquid contact resistance on the electrode surface: Disconnect the signal cable from the flow sensor, and use a multimeter to measure the resistance between each electrode and the ground. The difference in resistance between the two electrodes should be within 10%–20%. Further details on this can be found in Section 9, “Measurement of Electrode Contact Resistance.”
(2) Measuring the electrode insulation: Empty the measurement tube, wipe its inner surface with a dry cloth, and once it’s completely dry, use an H500VDC megohmmeter to measure the resistance between each electrode and the ground. This resistance value should be above 100 MΩ. Generally, by following these five steps, the problem of unstable zero points can be resolved.