Introduction and Working Principle of Open Channel Flow Meters

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　　Industrial wastewater discharge includes pressure pipelines and open channels, as well as pressure-free discharge methods such as non-pressure pipelines. It is relatively convenient to measure the flow rate of wastewater discharged through pressure pipes; electromagnetic, laser, and venturi differential pressure type pipe flow meters can be used for this purpose. At present, wastewater discharge in our country mainly takes the form of open-channel discharge without pressure, and wastewater flow meters in open channels were designed to address the measurement issues associated with this type of wastewater discharge. 　　1. Working principle and structure of open-channel flow meters Open-channel sewage flow is commonly measured using Pitot tubes and thin-plate weirs; both rely on throttling flow measurement techniques. The direct parameter measured in these methods is the water level difference between the upstream and downstream sections of the throttling device, and based on this difference, the instantaneous flow rate at the flow cross-section can be calculated. 　　An open-channel sewage flow meter consists of a flow measurement weir (flow channel), a level sensor, and secondary instruments. It performs tasks such as generating head (standard flow), measuring water level height, and calculating flow rate, respectively. 　　2. Types and application ranges of weirs and channels 2.1 Flow measurement weirs Any structure that can block water flow and allow it to overflow from its top is referred to as a weir. There are many types of weirs: based on thickness, they can be classified as thin-walled weirs (with wall thickness less than 0.67 times the geometric head), practical weirs, and wide-crested weirs; based on cross-sectional shape, they include triangular weirs, trapezoidal weirs, equal-width weirs, etc.; based on the way the jet connects with the water level below, they can be divided into submerged weirs and non-submerged weirs. 　　Commonly used ones are thin-walled triangular weirs, thin-walled trapezoidal weirs, and thin-walled rectangular weirs. 　　Under certain conditions, for a specific thin-walled weir, the flow rate has a definite functional relationship with the geometric head height, which can be used to measure the wastewater flow rate. 　　When measuring flow using a water weir, head loss is high; it is suitable for measuring wastewater with low levels of suspended solids and no fibrous substances. 　　2.2 Flow measuring channel A wide-crested weir with lateral contraction; when the height of the weir crest is zero, it is referred to as a channel. It represents a special variation of a weir, and includes types such as short-throat channels (Pascal channels), channels without a throat (Sun channels), and long-throat channels. 　　The Pitot tube is commonly used for measuring flow in open channels. 　　Under certain conditions, for a specific Barreling tank, the flow rate has a definite functional relationship with the upstream water level, which can be used to measure the wastewater flow rate. 　　When using a flow channel for flow measurement, the head loss is low and it can operate under a wide range of water quality conditions. When used to measure wastewater, it is necessary to promptly remove aquatic organisms from the channel walls to keep them smooth. 　　3. Level sensors 　There are various methods for measuring water level, including ultrasonic level sensors, pressure-type level sensors, capacitive level sensors, and float-type level sensors. These level sensors each have their own characteristics and different ranges of application. 　　3.1 Ultrasonic level sensors Ultrasonic sensors represent a practical application of ultrasonic ranging technology; they allow for flow measurement without any physical contact, and they have a wide range of applicable scenarios. However, they require the liquid surface to reflect ultrasonic waves effectively. The presence of foam, debris, biological sludge, or other substances that cause abnormal reflections can lead to significant errors. In addition, ultrasound is also affected by weather factors such as temperature, air pressure, and wind speed. Although these effects can be compensated for, the instruments involved are complex in structure and costly. 　　3.2 Pressure-type level sensors 　　Their advantage is simple installation. However, it requires the specific gravity of the liquid being tested to remain essentially constant, which limits its use in wastewater with large variations in specific gravity. Additionally, when using it in wastewater, care must be taken to ensure that the pressure measurement ports are unobstructed. 　　3.3 Capacitive hydraulic sensors These sensors have no moving parts, but it is difficult to balance the mechanical strength and dielectric properties of the insulating medium. Moreover, changes in capacitance due to temperature effects reduce the measurement accuracy; in addition, when used in sewage, the surface of the electrodes needs to be cleaned regularly. 　　3.4 Float-type level sensors These sensors are characterized by their low cost, immunity to the effects of water quality, and wide range of applicability. However, they require a relatively constant specific gravity of the water, and attention must be paid to the impact of deposits on the surface of the float on its specific gravity. 　　4. Secondary instruments A secondary instrument is an automatic device that can receive the voltage (current) signals from level sensors, and use these signals to calculate the flow rate of wastewater; it can provide both the instantaneous flow rate and the cumulative flow rate. It can be connected to a recorder to monitor changes in flow rate, and it can also be connected to a microcomputer to enable automatic flow rate monitoring. 　　Flow measurement is an important basis for corporate economic accounting. Accurate and reliable flow measurement is a prerequisite for ensuring the normal operation of enterprises, and it is one of the key means to achieve optimal economic benefits.