The composition and properties of the sewage connected to the sewage treatment plant are complex. If a direct contact flow meter is used, it is inevitable that scaling or corrosion will occur inside the flow meter for a long time, which will affect the measurement. This requires the sewage flowmeter to have the following characteristics: (1) the measuring instrument for sewage discharge is preferably a non-contact measuring instrument; (2) The instrument has the characteristics of strong resistance to steam drum, easy installation, and low power consumption; (3) The sensing device is installed outside the sewage pipeline and does not require cutting off the pipeline.