Open channel ultrasonic flow meter for Parshall flumes application

Open channel ultrasonic flow meter involves using an ultrasonic flow meter in conjunction with a Parshall flume to measure the flow of liquid in an open channel. Here are the key aspects:

1. How it works:
   • Parshall flume: The Parshall flume is a fixed hydraulic structure that accelerates the subcritical, open channel flow. It contracts both the parallel sidewalls and has a drop in the floor at the flume throat. This allows the flow rate in the upstream section of the flume to be determined by a single level measurement at a specified location.
   • Ultrasonic flow meter: A sensor mounted above the flow stream in the Parshall flume transmits a sound pulse that is reflected by the surface of the water. The elapsed time between sending a pulse and receiving an echo determines the water level in the flume. This level measurement is then converted into a flow rate through the use of a preprogrammed equation specific to the type and size of the Parshall flume being used.
2. Advantages:
   • Accuracy: Can provide good overall accuracy for flow measurement, typically within ±8-10% depending on various factors such as flume accuracy, flow meter accuracy, installation error, and calibration error.
   • Non-contact installation: The ultrasonic sensor does not contact the liquid, which means it is not affected by factors such as grease, suspended solids, silt, and corrosive chemicals in the flow stream. It also requires minimal maintenance.
   • Wide range of applications: Parshall flumes are widely used in various applications such as irrigation channels, surface waters, industrial discharges, and wastewater treatment plants. The combination with an ultrasonic flow meter makes it a versatile solution for open channel flow measurement.
3. Disadvantages and challenges:
   • Environmental factors: Ultrasonic flow meters may be affected by wind, steam, and air temperature gradients. Changes in air temperature can affect the speed of sound, so the flow meter needs to compensate for these changes. Sunlight can also impact sensor performance by artificially heating the sensor.
   • Flow disturbances: Turbulence, foam, and floating debris in the flow can provide artificially high flow readings as the meter may detect these before it detects the true surface level of the flow stream.
   • Flume installation requirements: Parshall flumes need to be level from front to back and from side to side for accurate measurements. Although corrections have been developed for some flumes, ensuring proper installation and alignment is crucial for accurate flow measurement.