The Doppler flowmeter for drainage networks has a lower limit for accurate velocity measurement.

　　The application areas of flow measurement in small-diameter drainage pipe networks are:

　　Typically at low flow rates, and sometimes at high flow rates; flow velocity and flow rate vary significantly. If the flow velocity is less than 0.1 m/s, Doppler flow meters are generally not suitable.

　　Typically a low-level sensor, but with significant fluctuations in the liquid level, it may experience full‑pipe conditions.

　　The bottom typically contains accumulations.

　　Selection of Flow Measurement Devices for Small-Diameter Drainage Networks

　　Below, we analyze instruments that are likely to be used for flow measurement in large-diameter drainage networks:

　　Ultrasound Doppler flowmeter for drainage networks

　　Ultrasonic Doppler flowmeters for drainage networks are commonly used instruments for measuring flow in sewer systems.

　　A Doppler flowmeter operates based on the Doppler effect. Doppler flowmeter for drainage networks Continuous ultrasonic waves are emitted into the water; when these waves encounter suspended particles, they are reflected. The Doppler flowmeter detects the frequency shift of the reflected waves. It then analyzes this frequency change and, using the Doppler effect, calculates the particle velocity at that location. Finally, by applying a pre‑established hydrodynamic model, it determines the average flow velocity across the entire cross‑section.

　　The impact of the Nyquist limit on Doppler flowmeters is that the flow velocity in the pipe is inversely proportional to the penetration depth of the Doppler flowmeter’s signal.

　　The penetration depth of Doppler flowmeter signals typically ranges from 0.3 to 0.5 meters. When the flow velocity in the pipeline is very high, the ultrasonic Doppler flowmeter’s signal penetration depth becomes insufficient, leading to increased measurement errors.

　　More importantly, Doppler flowmeters have a lower limit for velocity measurement. If the flow velocity in the pipeline is less than 0.1 m/s, Doppler flowmeters typically cannot detect the velocity.

　　Therefore, when assessing the total flow in large-diameter drainage networks, it is essential to consider the impact of low flow velocities within the pipes on Doppler flow meters.

　　Electromagnetic flowmeter

　　An electromagnetic flowmeter is a type of flowmeter widely used for measuring flow in various types of pipelines.

　　Electromagnetic flowmeters apply Faraday’s fundamental law of magnetic induction to measure the flow rate of liquids in pipelines. In such devices, an electromagnetic field is generated and penetrates the liquid flowing through the pipe. According to Faraday’s law, as the conductive liquid moves through this electromagnetic field, a voltage signal is induced and collected by electrodes mounted on the inner wall of the flow tube. The faster the fluid flows, the greater the induced voltage. The magnitude of this voltage is directly proportional to both the fluid velocity and the volumetric flow rate Q.

　　However, electromagnetic flowmeters also have certain drawbacks, as detailed below:

　　More suitable for liquids with a conductivity greater than 5 µS;

　　The measured flow velocity is approximately 0.5 m/s; when it falls below 0.5 m/s, a low‑flow removal method is typically selected.

　　The protective coating on the electrodes can also introduce data errors.

　　It cannot be installed during system operation, meaning it cannot be assembled in a humid environment.

　　It is generally used for full‑pipe flow; at present, non‑full‑pipe electromagnetic flowmeters are also available. For low‑level instruments, the liquid level should not fall below one‑third of the pipe’s diameter. Similarly, they are not suitable for applications involving large fluctuations in the liquid level within drainage networks.