It is known from the fluid mechanics that when liquid flows in a pipe, the velocity of each point on the Cross section of the pipe is not equal, but the velocity distribution can become axisymmetric after a certain distance of straight pipe, no matter Laminar or turbulent flow The velocity is maximum at the centre of the Pipe Axis and zero at the pipe wall, and the average velocity is v

As long as the velocity distribution is symmetric with respect to the central axis of the measuring tube, the size of the electromotive force produced on the electrode is independent of the velocity distribution at each point and proportional to the average velocity of the liquid being measured. Therefore, the axial symmetry of flow velocity distribution is one of the working conditions of the electromagnetic flow meter with a uniform magnetic field

When the velocity distribution is asymmetric with respect to the central axis of the tube, the total flow rate is the same, but the electromotive force near the electrode is large, so the measured signal is larger than the actual flow rate. In contrast, at 90 degrees electromotive force the electrode, the resulting signal is smaller than the actual flow rate, resulting in measurement errors. Therefore, in order to make the velocity distribution axisymmetric, it is necessary to add a straight pipe in front of the flow meter