Thread Content
The study of fluid noise in electromagnetic flowmeters, exploring its causes and identifying methods to reduce it, is of great significance for improving the signal-to-noise ratio of these sensors. This is particularly important for the development of electromagnetic water meters and two-wire electromagnetic flowmeters with weak excitation currents, as well as for expanding the measurement range to low flow rates (below 0.1 m/s) and high flow rates (above 15 m/s). The causes of fluid noise include the following situations: 1. Fluid friction between the lining and the electrodes, as well as the separation of positive and negative ions in the fluid from the electrolyte fluid. The rougher the lining and electrode surfaces, the higher the concentration of free ions. Under the influence of the electric field generated by the electrode signal, some ions move toward the electrode, resulting in noise voltage; this type of noise is known as drift noise. Flow noise is more prominent during low conductivity measurements. Flow noise is related to the external electric field strength; at higher flow rates, the induced signal becomes larger, and so does the noise amplitude, resulting in unstable output. 2. Sudden changes in fluid conductivity and pH value can also generate flow noise; unstable measurements resulting from the addition of chemicals upstream of the flow meter are a typical example of this. The reason is that when different media are mixed unevenly, positive and negative ions tend to separate in the fluid; under the influence of the electric field generated by the electrode signals, some of these ions move toward the electrodes, resulting in flow noise voltage and thus instability in the output. 3. Due to the high flow velocity, the thickness of the laminar boundary layer near the lining and electrodes becomes very thin; the roughness of the lining and electrodes exceeds the thickness of this laminar boundary layer. When the fluid impacts this rough surface, the flow velocity diverges and changes abruptly. There are flow velocity components that are in the same direction (or opposite direction) as the central axis of the measuring tube; due to the effect of the signal weighting function, these components have a significant impact on the electrode signals, resulting in large positive errors. This is what constitutes high-frequency flow velocity noise.