In open channel flow metering, the primary element is a _______.

• A. Measuring Device
• B. Weir Plate
• C. Venturi Tube
• D. Measuring Structure

Answer: (D)

In open-channel flow metering, what matters is the component that directly interacts with the water to create a known height difference that can be related to flow. That component is the measuring structure, the primary element that imposes the hydraulic constraint and defines how discharge relates to the head loss across it. A separate measuring device is what reads that head to compute the flow. Specific shapes like a weir plate or a Venturi tube are examples of the kind of primary element, but the general term that covers them all is the measuring structure. So, the measuring structure best captures the essential part of the meter that governs the flow–head relationship.