How does a weir differ from a venturi meter in flow measurement?

How does a weir differ from a venturi meter in flow measurement?

• A. A venturi uses a pressure drop due to a constriction to determine flow rate
• B. A weir measures flow by head over a crest; a venturi uses a pressure drop due to a constriction to determine flow rate
• C. A weir uses a magnetic sensor to estimate flow
• D. A venturi measures flow by visual observation of water level only

Answer: (B)

The key idea is how each device translates flow into a measurable signal in different channel setups. A weir sits in an open channel and its discharge is determined by the water height above the crest—the head. A venturi meter sits in a closed conduit and creates a constriction; as the fluid speeds up through the throat, the pressure drops, and that differential pressure is used to compute flow rate via Bernoulli/continuity. So, describing both together as in the statement—that a weir measures flow by head over a crest and a venturi uses a pressure drop due to a constriction to determine flow rate—captures the fundamental difference. The weir relies on the water surface level in an open channel, while the venturi relies on pressure changes in a closed path. This is why the other ideas don’t fit: one option mentions only the venturi principle without describing the weir, another incorrectly suggests a magnetic sensor or visual observation for a venturi, which aren’t how these devices operate.

The key idea is how each device translates flow into a measurable signal in different channel setups. A weir sits in an open channel and its discharge is determined by the water height above the crest—the head. A venturi meter sits in a closed conduit and creates a constriction; as the fluid speeds up through the throat, the pressure drops, and that differential pressure is used to compute flow rate via Bernoulli/continuity.

So, describing both together as in the statement—that a weir measures flow by head over a crest and a venturi uses a pressure drop due to a constriction to determine flow rate—captures the fundamental difference. The weir relies on the water surface level in an open channel, while the venturi relies on pressure changes in a closed path.

This is why the other ideas don’t fit: one option mentions only the venturi principle without describing the weir, another incorrectly suggests a magnetic sensor or visual observation for a venturi, which aren’t how these devices operate.