The pipeline and flow meter should be thoroughly inspected before the operation of the electromagnetic meter, including pipeline debris cleaning and electrode drying treatment. It also involves the inspection of the electrical circuit. First of all, we need to detect the grounding of the line to ensure that the grounding is reliable. Testings of the insulation resistance and grounding resistance should also be taken.
The electromagnetic meter has put into operation normally for more than one year without any fault basically. However, a year later, it can not work properly with the measurement value becomes larger or smaller, or fluctuates constantly. The possibility of pipe dissatisfaction, media gas, and other phenomena has been ruled out by on-site inspection. It is found that such problems may be related to the fault of cable. As the flow meter with the pipeline buried under the ground, the sensor which has IP68 protection structure and the converter which is installed in the instrument box or indoor are connected through the cable. due to scene changes such as ground subsidence and the humid mountainous environment. The moisture-proof (waterproof and so on) of the cable joints of sensors and converters is not completely done, And the moisture will intrusion into the cable connectors when coinciding with the joint in a humid environment such as instrumentation wells, cable trenches and so on, which has the possibility to cause the following situations:
(1) The signal line to ground insulation decreased with signal attenuation, which leads to a smaller final result;
(2) The contact resistance of the signal cable becomes larger with a smaller measurement value. The measurement value is not stable and the interference is easy to be introduced if the contact resistance is unstable.
(3) The field insulation of the excitation coil decreases, resulting in a smaller measurement result;
(4) The signal line excitation line to the ground insulation performance decreases, that will make the measurement results far greater than the normal data. The impact on the instrument changes and then appear fluctuations if this kind of disturbance is unstable;
(5) The contact resistance of the excitation circuit cable becomes larger, which makes the excitation circuit of the converter is in the non-constant current working area. And the measurement result is smaller with the reduce of the excitation current. The measurement value would fluctuate if the contact resistance is unstable.
Since this kind of unusual situation occurs in the signal loop, it has the certain difficulty when judging this kind of situation. The only in-depth scene, we should check that whether there are any problems with the cable after eliminating another possibility of the breakdown of the flow meter. On-site instrument maintenance personnel only need to compare the field excitation circuit resistance measurement with the data provided by the manufacturer, test grounding resistance, and determine the problem gradually. Then take measures, or replace the whole cable, or to keep the cable joint drying.
