Open-channel metering — small channels: For measuring the liquid flow rate in a small open channel, which primary measuring device is the most appropriate?

Introduction / Context: Measuring flow in open channels requires devices that relate upstream head to discharge under free-surface conditions. For small channels in civil/chemical process settings—lab flumes, plant trenches—simplicity, accuracy, and ease of installation are key considerations. This item compares common instruments and selects the best match for a small open-channel scenario.

Given Data / Assumptions:

• Flow is free-surface (open channel), not pressurised pipe flow.
• Channel is small; permanent head loss and cost should be modest.
• Steady, subcritical flow is assumed near the device.

Concept / Approach: Weirs (sharp-crested, V-notch, rectangular) are standard for small-channel measurement. The discharge Q relates to upstream head h via empirical formulae, allowing practical calibration. Pitot tubes, venturimeters, or orifice plates are designed for closed, pressurised conduits; vane meters likewise suit pipelines, not open channels. Therefore, a weir is the preferred choice here.

Step-by-Step Solution:

Verification / Alternative check: Standard hydraulics texts list V-notch weirs as highly sensitive for low flows, exactly the use case for small channels.

Why Other Options Are Wrong:

Common Pitfalls: Confusing “small channel” with “small pipe” and defaulting to orifice/venturi meters, which are inappropriate for free-surface flow.

Final Answer: Weir