A brief analysis of the reasons for abnormal operation of sewage flow meters

Thread Content

The wastewater flow meter is buried underground along with the pipeline; its sensor features an IP68 protection rating, while the converter is installed in a control box or indoors. The two are connected by cables. Due to factors such as ground subsidence and the humid conditions in mountainous areas, the cable connections between the sensor and the converter are not properly protected against moisture. Since these connections are often located in humid environments such as instrument pits or cable trenches, moisture can penetrate these connections, leading to the following problems: (1) A decrease in the insulation of the excitation coil relative to the ground, which results in lower measurement values； (2) An increase in the contact resistance at the cable connections in the excitation circuit causes the excitation circuit of the converter to operate in a non-constant current regime, resulting in a decrease in the excitation current; this in turn leads to lower measurement values. If this contact resistance is unstable, the measurement values will fluctuate. (3) A decrease in the insulation of the signal lines with respect to ground causes signal attenuation, resulting in lower measurement values ； (4) The insulation performance of the signal wires and excitation wires with respect to ground deteriorates, resulting in measurement values that are much higher than normal. If such interference is unstable, its impact on the hot water flow meter also varies, leading to fluctuations ； (5) An increase in the contact resistance at the signal cable connections results in lower measurement values. If this contact resistance is unstable, the measurement values will not be stable either, and interference is likely to occur ； Since these abnormal conditions occur in the signal circuit, it is difficult to determine the cause. Only by going to the site and ruling out other possible faults in the circulating water flow meter can one check whether there are problems with the cable. The instrument maintenance personnel on site need only compare the measured resistance of the excitation circuit with the data provided by the manufacturer, test the grounding resistance, and gradually identify the issue; thereafter, measures can be taken, such as replacing the entire cable or keeping the cable connections dry. Within the first year or so of operation, sewage flow meters generally do not develop any faults. However, after a year, abnormalities in their performance often occur, manifested as increased or decreased measurement values, or continuous fluctuations. After on-site inspections, possibilities such as incomplete filling of the pipes or air presence in the medium have been ruled out; it has been found that such problems may be related to faults in the cables.