（1） Core technical parameters
1. Measurement range: including flow measurement range, water level measurement range, and flow velocity measurement range, which should be reasonably selected based on the actual maximum flow rate, minimum flow rate, and water depth variation range of the open channel to ensure that the equipment is in the optimal measurement range under normal operating conditions.
2. Measurement accuracy: Flow measurement accuracy is usually expressed in relative error, and there are significant differences among different types of equipment. For example, the accuracy of ultrasonic open channel flow meters is generally ± 1.5%~± 3% FS, while the accuracy of weir type flow meters can reach ± 0.5%~± 1% FS. When selecting, the accuracy requirements of the application scenario should be taken into account (such as environmental monitoring requiring high accuracy, and industrial process monitoring can be appropriately reduced).
3. Work environment: including environmental temperature, humidity, air pressure, dust concentration, etc. Equipment that is suitable for the on-site environment should be selected. For example, high-temperature sensors should be selected for high-temperature environments, and outdoor environments should have waterproof, dustproof, and lightning protection functions.
4. Output methods: Common output methods include analog output (4~20mA, 0~10V), digital output (RS485, Modbus protocol), pulse output, etc. The matching output method should be selected according to the requirements of the data acquisition system to ensure stable data transmission.
5. Power supply methods: divided into AC power supply (220V AC), DC power supply (12V DC/24V DC), and solar power supply. For outdoor scenarios without power supply conditions, solar power supply or battery power supply equipment can be selected.
（2） Key selection points
1. Clarify the working conditions: Firstly, it is necessary to determine the cross-sectional shape (rectangular, trapezoidal, U-shaped, irregular), size (width, depth), water flow state (whether there are eddies, backflow), water quality conditions (sediment concentration, suspended solids content, conductivity, corrosiveness), etc. of the open channel, providing a basic basis for selection.
2. Matching measurement accuracy requirements: High precision equipment (such as weir type and multi-channel ultrasonic flow meters) should be selected for environmental monitoring, hydrological station networks, and other scenarios; Industrial process monitoring, agricultural irrigation and other scenarios can choose more cost-effective ordinary precision equipment (such as monaural ultrasonic flow meters).
3. Consider installation and maintenance costs: Non contact equipment (such as ultrasonic flow meters) is easy to install, has low maintenance costs, and is suitable for the renovation of existing open channels; Contact devices such as electromagnetic flow meters and flow meters require complex installation and regular maintenance of sensors, making them suitable for new open channels or long-term fixed monitoring scenarios.
4. Environmental adaptability: Equipment with high protection level (such as IP67 or above), lightning protection, and anti-interference function should be selected for outdoor harsh environments; Sensors with specialized materials such as 316L stainless steel and PTFE coating should be selected for high temperature and high corrosion environments.