The inlet of the piping system of the process industry is often connected to a container or a high-level tank, and the inlet of a public utility piping system is often connected to a pool or a river channel. The inlet must have a distance of 2-5 times the diameter of the inlet pipe below the liquid surface.
If the distance is too close, the suction port will produce a vortex, and the air drawn into the liquid-air interface will enter the pipeline with the vortex, which will affect the normal measurement.
