Open-channel flow measurement Which of the following instruments cannot be used to measure liquid flow rate in an open channel?

Introduction / Context:
Open-channel flows (streams, canals, effluent channels) are measured with hydraulic structures that create a known head–discharge relation under free-surface conditions. In contrast, pipe flows under pressure commonly use differential-pressure primary elements such as orifices. Distinguishing between these measurement families ensures correct device selection.

Given Data / Assumptions:

• Open-channel implies free-surface flow open to atmosphere.
• Cipolletti, rectangular, and V-notch are standard sharp-crested weirs.
• An orifice meter is a differential-pressure device for closed-conduit (pipe) flow.

Concept / Approach:
Weirs convert flow into a head over the crest; empirically validated equations (e.g., Q ∝ H^(3/2) for sharp-crested weirs) relate the measured head to discharge. Orifice meters require fully filled pipes with established upstream/downstream pressure taps. Therefore, an orifice meter is inappropriate in an open-channel setting where the flow is not pressurized within a pipe.

Step-by-Step Solution:

Verification / Alternative check:
Design manuals for irrigation and WWTPs list weirs and flumes (Parshall, Venturi flume) for open channels; orifice meters are absent from these sections.

Why Other Options Are Wrong:

• Cipolletti, rectangular, V-notch: All are standard open-channel weirs.

Common Pitfalls:
Attempting to place an orifice plate in a partially full conduit; accuracy collapses without fully developed pressurized flow.

Final Answer:
Orifice meter