Open Channel Flow Meters: Types and Working Principles

Open Channel Flow Meters differ from other closed full-tube flow meters as they are designed for measuring flow in non-full-tube conditions or open channels with natural free surface flow. The open channel principle makes open channel flow meters cost-effective and easy to use, leading to their widespread adoption in open channel flow measurements. Open channel flow meters, categorized by their measurement principles, include ultrasonic open channel flow meters, Doppler open channel flow meters, and multi-channel open channel flow meters, all of which are system monitoring devices for measuring fluid flow in open channels or canals. The open channel flow meter monitoring system is suitable for measuring flows in rectangular and trapezoidal open channels and culverts in reservoirs, rivers, water conservancy projects, urban water supply, agricultural irrigation, wastewater treatment, and water resources management. Applications include urban water supply conduits, power plant cooling water intake and discharge channels, wastewater treatment inflow and outflow channels, industrial wastewater discharge, and water conservancy and agricultural irrigation channels. They are available in fixed and portable models, with portable models further divided into standard and comparative measurement types.

Working Principle:

Ultrasonic Open Channel Flow Meter:

Utilizing open-channel technology, based on the velocity-water level algorithm, flow rate is determined by measuring the fluid level height, combined with the geometric dimensions of standard weirs, slope coefficients, channel accuracy, hydraulic gradients, and vertical plane velocity correction coefficients. After internal microprocessor calculations, the flow rate is obtained. As a non-contact measurement, the open-channel flow meter can be applied in harsh environments. Under microcomputer control, the open-channel flow meter emits and receives ultrasonic waves, calculates the distance to the measured liquid surface based on the transmission time, thereby determining the liquid level height. Due to the proportional relationship between the liquid level and flow rate, the liquid flow rate Q can be derived from the calculated formula.

Doppler Open Channel Flow Meter:

By employing a contact measurement method, the sensor is placed at the bottom of the channel. Utilizing the velocity-area method, between the two probes, one transmitting and one receiving, the system calculates the flow velocity through the Doppler time effect. Then, by using the cross-sectional area of the region where the sensor is located, the instantaneous flow rate is computed based on the formula.

Three: Multi-channel Open Channel Flow Meters

Also known as the Time-Difference Open Channel Flow Meter, it is an online measurement device. It utilizes the ultrasonic time-of-flight method to measure water flow velocity. Ultrasonic time-of-flight flow sensors are installed on both sides of the channel to measure the average water velocity, the water level height is measured using an ultrasonic water level gauge, and the instantaneous flow of the open channel is calculated through the flow meter integrator and hydraulic model calibration.