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Fluid Mechanics

15.1 Flow Measurement Techniques

3 min readLast Updated on July 19, 2024

Flow measurement techniques are crucial in fluid mechanics, allowing engineers to quantify and control fluid movement in various systems. This topic covers three main methods: orifice plates, Venturi meters, and Pitot tubes, each with unique principles and applications.

Understanding these techniques is essential for selecting the right tool for specific flow measurement needs. We'll explore how these devices work, their calculations, and compare their strengths and weaknesses to guide informed decision-making in engineering projects.

Flow Measurement Techniques

Principles of flow measurement techniques

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  • Orifice plates operate on the principle of creating a pressure drop across a constriction in the pipe, widely used in industrial settings for flow measurement due to their simplicity, low cost, and ease of installation, but have limitations such as high permanent pressure loss, limited accuracy, and susceptibility to wear
  • Venturi meters utilize the Bernoulli effect, measuring the pressure difference between the inlet and throat sections, suitable for clean, non-corrosive fluids in large pipes, offering high accuracy, low permanent pressure loss, and wide flow range, but are more expensive and require more space for installation compared to orifice plates
  • Pitot tubes measure the local velocity at a point in the flow using the Bernoulli equation, commonly used in air and gas flow measurement (wind tunnels, aircraft), being simple, inexpensive, and capable of measuring high-velocity flows, but only measure local velocity, are sensitive to flow direction, and require a separate device for pressure measurement

Flow rate calculations using equations

  • Bernoulli equation P1+12ρv12+ρgh1=P2+12ρv22+ρgh2P_1 + \frac{1}{2}\rho v_1^2 + \rho gh_1 = P_2 + \frac{1}{2}\rho v_2^2 + \rho gh_2 relates pressure, velocity, and elevation along a streamline, assuming steady, incompressible, and inviscid flow
  • Continuity equation A1v1=A2v2A_1v_1 = A_2v_2 relates the flow velocities and cross-sectional areas at two points in a pipe, assuming steady and incompressible flow
  • Orifice plates and Venturi meters use both the Bernoulli and continuity equations to calculate flow rates by measuring the pressure difference between two points and using the equations to determine velocity and flow rate
  • Pitot tubes use the Bernoulli equation to calculate the local velocity at a point by measuring the stagnation pressure and static pressure

Comparison of flow measurement methods

  • Accuracy: Venturi meters have the highest accuracy, followed by orifice plates and Pitot tubes, depending on the design, installation, and maintenance of the devices
  • Range: Venturi meters have a wide flow range, making them suitable for various applications, while orifice plates have a limited flow range due to the risk of cavitation at high velocities, and Pitot tubes can measure high-velocity flows but are limited to local velocity measurements
  • Pressure drop: Orifice plates have the highest permanent pressure loss due to the constriction in the pipe, Venturi meters have a lower pressure drop compared to orifice plates, and Pitot tubes do not introduce any additional pressure drop in the flow

Selection of flow measurement devices

  • Factors to consider when selecting a flow measurement device include fluid properties (viscosity, density, corrosiveness), flow conditions (temperature, pressure, velocity), required accuracy and range, allowable pressure drop, installation and maintenance requirements, and cost and space constraints
  • Examples of applications and suitable devices:
    1. Clean, non-corrosive liquids in large pipes: Venturi meters
    2. Slurries and highly viscous fluids: Orifice plates with larger bore sizes
    3. High-velocity gas flows: Pitot tubes
    4. Corrosive or high-temperature fluids: Specialized materials or designs for the flow measurement devices
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.