Open channel flow meters are divided into ultrasonic open channel flow meters, Doppler open channel flow meters, and multi-channel open channel flow meters according to different measurement principles. They are all system monitoring devices for measuring fluid flow in open channels or channels. The open channel flowmeter monitoring system is suitable for measuring the flow of rectangular and trapezoidal open channels and culverts in reservoirs, rivers, water conservancy projects, urban water supply, sewage treatment, agricultural irrigation, water management and water resources.

Working principle of open channel flowmeter

Ultrasonic open channel flowmeter

Using open channel technology, based on the velocity water level calculation method, the flow rate is obtained by measuring the fluid level height, combining the geometric dimensions of the standard weir groove, slope coefficient, channel accuracy, hydraulic slope, and vertical plane correction coefficient of the flow velocity. The flow rate is then calculated by the internal microprocessor of the instrument. Due to its non-contact measurement, open channel flow meters can be applied in harsh environments. Under the control of a microcomputer, the open channel flowmeter emits and receives ultrasonic waves, and calculates the distance between the open channel flowmeter and the measured liquid level based on the transmission time, thereby obtaining the liquid level height. As there is a certain proportional relationship between the liquid level and the flow rate, the liquid flow rate can be finally obtained according to the calculation formula.

Doppler open channel flowmeter

Using the contact measurement method, the sensor is placed at the bottom of the channel. Using the velocity area method, the system calculates the water flow velocity based on the Doppler time effect between the two probes, and then calculates the instantaneous flow rate according to the formula based on the cross-sectional area of the area where the sensor is located.

Multi channel open channel flowmeter

Also known as time difference method open channel flowmeter, it belongs to online measurement. Use ultrasonic time difference method to measure water flow velocity. Install ultrasonic time difference flowmeter sensors on both sides of the channel to measure the average flow velocity in the water. Measure the water level height using an ultrasonic water level gauge and calibrate the hydraulic model using a flow meter integrator to calculate the instantaneous flow rate of the open channel.

Precautions for selecting open channel flow meters

1. The accuracy level and function are selected by the user based on measurement requirements and usage scenarios, achieving cost-effectiveness. For example, in situations such as trade settlement, product handover, and energy measurement, Doppler open channel flow meters with high accuracy levels should be selected; For process control applications, choose different accuracy levels according to control requirements; In some cases where only flow detection is needed without precise control and measurement, ultrasonic open channel flow meters with slightly lower accuracy levels can be selected;
2. If you want to measure flow velocity indicators, you need to choose a Doppler open channel flowmeter or a multi-channel open channel flowmeter; If only measuring flow is selected, an ultrasonic open channel flowmeter can be used;
3. Fluid characteristics mainly refer to the pressure, temperature, density, viscosity, compressibility, etc. of gas. As the volume of gas varies with temperature and pressure, compensation and correction should be considered.
4. Instrument performance refers to the accuracy, repeatability, linearity, range ratio, pressure loss, initial flow rate, output signal, and response time of the instrument. When selecting a flowmeter, careful analysis and comparison of the above indicators should be carried out to select an instrument that can meet the flow requirements of the measuring medium.
5. The installation conditions refer to the flow direction of gas, the direction of the pipeline, the length and diameter of the upstream and downstream straight pipes, the spatial position, and the fittings, all of which will affect the accurate operation, maintenance, and service life of the gas flow meter.
6. Economic factors refer to purchase costs, installation costs, maintenance costs, calibration costs, and spare parts, which are also affected by the performance, reliability, and lifespan of gas flow meters.

Key considerations for selecting open channel flow meters

Considering the flow measurement characteristics of urban water supply channels, industrial water supply and drainage channels, sewage treatment channels, etc., the following factors should be taken into account when selecting appropriate measurement methods.
(1) Waterway size and shape, flow velocity range, maximum and minimum flow rates;
(2) Measurement accuracy requirements;
(3) Location and environmental conditions for flowmeter installation;
(4) Liquid condition, cleanliness, solid concentration, corrosiveness;
(5) On site allowable drop (or raised water level) and channel slope;
(6) Materials of instrument components in contact with liquids;

Estimate channel flow and raise water level
For newly established units, the channel flow rate and proposed installation location can be calculated through the process flow, and then the instrument specifications can be selected. For old enterprises to purchase instruments, it is necessary to estimate the flow rate of existing channels and confirm the allowable upstream water level of the instruments; The determination of the specifications and flow range of the flow meter depends on two factors: the peak flow rate of the channel and the allowable raised water level.
(1) Estimate peak traffic volume
There are usually two methods: float method and daily emission estimation method.
(1) The float method can be used to measure and estimate flow in existing open channels. At the maximum flow rate, two people stand next to a straight channel at a distance of L (20-50m), with one person upstream placing a float (usually a wooden disc) and the other person downstream starting the timer at the moment of placing the wood chips. The timer stops when the float arrives, and the time t is obtained. The surface flow velocity of the channel is calculated as υ m/s (υ=L/t). By measuring the channel flow cross-sectional area Am2 again, the K correction factor in the flow Q equation can be estimated from equation (5). As the surface flow velocity is greater than the average flow velocity, it is generally taken as 0.84~0.90
(2) Estimating peak flow rate Qp from daily emissions Qd can be done using the float method or a newly built system if conditions permit. The actual (or designed) emissions can be used to estimate peak flow rate, and the estimation formula is Ku – non-uniformity coefficient in equations (6) or (7). For uniform continuous emissions, it can be taken as 1.1-1.2;
H —- The cumulative number of hours of centralized emissions per day. If it is uneven continuous emissions, it shall be calculated based on the hours of centralized emissions.
(2) Determine to raise the water level
Except for flow rate water level flow meters and non full pipe electromagnetic flow meters, the upstream water level must be raised after installing flow meters in the channel. For the newly designed canal system, the height of the raised water level can be determined comprehensively based on the measured flow range and surrounding environmental conditions. For existing channels, when selecting a flow meter, the impact of the upstream channel system water level rise (such as whether the water level will overflow the channel) should be considered, and then the instrument specifications should be selected based on the determined water level rise height and peak flow value.