Open channel flow meters measure fluid flow through open channels. Open channel flow measurement is difficult because of non-uniform channel dimensions and variations in velocities across the channel. Some open channel flow sensors measure the displacement of a vane or piston that is pushed by the fluid. A non-contacting open flow sensor avoids flow disturbances and fouling. Doppler flow meters transmit acoustic waves into the flow and use the frequency shifts of reflected waves to determine the flow velocity. Ultrasonic open channel flow sensors mount a non-contacting sensor above the flume or weir through which fluid passes. An ultrasonic flow sensor transmits an ultrasonic pulse and measures the time elapsed when an echo is received. In this way, ultrasonic flow sensors can measure the water level or "head" accurately. An open channel flow monitor can continuously monitor, display, totalize and log flow information.
Specifications for open channel flow meters include monitoring type (continuous or totalized), viscosity and fluid purity. An open channel flow meter can display information at the meter, or transmit it electronically to a remote location. Additional specifications to consider are minimum and maximum flow rate for open channel flow meters, the ability to measure flow under ice, and the chemical compatibility of wetted parts. When measuring the flow of compressible fluids, open channel flow meters must be able to handle variation in volumetric flow data unless density (and sometimes also viscosity) is constant. When the velocity (volumetric flow) of incompressible liquids is measured, the presence of suspended bubbles will cause error; therefore, air and gas must be removed before the fluid reaches the meter.
Approvals and Certifications
Open channel flow meters differ in terms of approval and certifications. Technical Committee 113 (hydrometry) of the International Organization for Standardization (ISO) Technical Committee 113 (hydrometry) publishes standards that apply to an open channel flow meter. ISO/TS 24154 applies to the measurement of river velocity with acoustic Doppler profilers. Open channel flow meters that meet the requirements of Institute of Electrical and Electronic Engineers (IEEE) Standard 1451 can transmit electronic data to a data acquisition system.