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Introduction: In the current market, devices used for monitoring flow in drainage networks can be classified into two types based on the measurement method: contact-based measurement and non-contact measurement. Common monitoring devices used in contact-based measurement methods in drainage network systems include ultrasonic Doppler flow meters, ultrasonic time-difference flow meters, and electromagnetic flow meters ； The non-contact measurement method is mainly the radar flow meter. 01 Ultrasonic Doppler Flow Meter (contact type) The ultrasonic Doppler flow meter utilizes the acoustic Doppler principle; it employs ultrasonic transducers to measure the flow velocity of water, and then calculates the flow rate using relevant formulas. Advantages: High measurement accuracy, wide range ； It has good fluid adaptability and is suitable for small and medium-sized pipelines ； It can measure full and empty tubes. They can all be used in natural water bodies, as well as channels and pipes, to measure water flow velocity and flow rate. There is no need to cut the pipeline to install a tubular sensor as with electromagnetic flowmeters, nor is there any need to use shutoff devices, let alone install fixed weirs to control the flow of water through a fixed outlet. It is not necessary to cut off the water supply; a bracket can be lowered from the top of the pipe for fixation, allowing it to be installed in the center. The sensor is fixed to the bottom of the pipe using a bracket, with its face facing the direction of the water flow. Defect: Since this type of device needs to be placed in water bodies for monitoring, the water environments of drainage networks contain silt, oil, floating debris, etc. These substances can cover the sensor probes, preventing the device from functioning properly and thus preventing the acquisition of accurate monitoring data. 02 Ultrasonic time-difference flow meter (contact type): An ultrasonic time-difference flow meter utilizes the fact that ultrasonic waves take different amounts of time to travel the same distance in water, either in the direction of flow or against it. This difference in travel time is related to the flow velocity of the water; by measuring this time difference, the flow velocity of the water can be determined, and the cross-sectional flow rate can be calculated using relevant formulas. Advantages: High measurement accuracy, wide range ； High fluid adaptability ； It can measure full and empty tubes. Defect: Affected by bubbles or excessive impurities in water ； It is greatly affected by temperature and pressure, which impacts the speed measurement accuracy ； It needs to be in contact with water, and the impurities/sediments in the water can easily cover the probe, affecting measurement accuracy or causing the sensor to fail; drainage systems are precisely such an environment. Moreover, it is difficult to install such devices in drainage pipes, so they are not suitable for use in drainage system environments. 03 Electromagnetic flowmeter (contact type): The electromagnetic flowmeter operates on the principle of Faraday’s law of electromagnetic induction. Detection electrodes are installed on the pipe wall perpendicular to both the axis of the measurement tube and the magnetic field lines. When a conductive fluid moves along the axis of the measurement tube, it cuts through the magnetic field lines, generating an induced electromotive force. This induced electromotive force is detected by the detection electrodes, and its magnitude is directly proportional to the flow rate. Advantage: Strong fluid adaptability ； It can measure full and empty tubes. Defect: The measurement accuracy is average ； It needs to be in contact with water; impurities/sediments in the water can easily cover the probe, preventing measurements from being taken ； The flow velocity accuracy is affected by the conductivity of the water; changes in water conductivity influence the induced electromotive force detected by the electromagnetic probe, which in turn affects the conversion relationship between the induced electromotive force and the water flow velocity, thereby impacting the measurement accuracy ； At the same time, since the wastewater in drainage pipes is corrosive, it will corrode the sensor probe of the electromagnetic flowmeter (the probe being made of metal); therefore, such devices cannot be used in drainage network environments. 04 Radar flow meter (non-contact) The radar flow meter is a new type of non-contact flow monitoring device, consisting of a radar current meter and a radar water level gauge. Radar flow meters use the principle of the Doppler effect to measure the flow velocity at the surface of a fluid. When the radar flow meter moves relative to the water body at a velocity V, the frequency of the electromagnetic waves received by the radar flow meter differs from the frequency of the electromagnetic waves emitted by the radar itself; this frequency difference is known as the Doppler shift. The surface flow velocity of the fluid is obtained by calculating the relationship between the Doppler shift and V. Advantage: Wide range ； Low power consumption ； Its compact size does not affect monitoring ； The product is corrosion-resistant ； No need for contact with water; easy to install ； It can be installed in cross-sections where the water flow is stable, free from contamination and corrosion caused by debris in the water ； No regular maintenance is required later on. Defect: When the drainage pipe is full, this sensor is unable to collect data. Mud, water, and sewage are dirty; being exposed to sewage for a long time can easily damage the sensors, preventing them from functioning.