Instrument used to measure velocity of flow at a point Which device is classically used to determine the local flow speed in a pipe or open channel?

Difficulty: Easy

Correct Answer: Pitot tube

Explanation:


Introduction / Context:
Hydraulic engineers employ various instruments to quantify flow. While Venturimeters and orifice meters infer discharge from pressure differences across constrictions, a Pitot tube directly senses local velocity through stagnation pressure.



Given Data / Assumptions:

  • Steady, incompressible flow at the measurement location.
  • Probe aligned with flow; calibration factor near unity.


Concept / Approach:
The Pitot tube measures stagnation and static pressures, whose difference relates to the dynamic pressure. Using Bernoulli’s relation, the local velocity is obtained via V = C * sqrt(2 * (p_t − p_s) / rho). In contrast, Venturimeter and orifice meter provide section discharge Q; velocity is then Q/A and not strictly a point value.



Step-by-Step Solution:

Install Pitot or Pitot-static tube at the point of interest.Record stagnation and static pressures to get dynamic pressure.Compute local velocity from the dynamic pressure.


Verification / Alternative check:
Compare with a calibrated current meter (open channels) or with PIV/LDV in labs; results align within instrument uncertainty when properly used.



Why Other Options Are Wrong:
Venturimeter and orifice meter measure discharge over a section, not point velocity. “Current meter only” ignores that a Pitot tube is the canonical point-velocity device. “None of these” is incorrect.



Common Pitfalls:
Not correcting for velocity profile when converting point velocity to average velocity, and misalignment causing under-reading.



Final Answer:
Pitot tube

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