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3.3 Manometry and Pressure Instruments

3 min readLast Updated on July 19, 2024

Manometers and pressure measurement devices are crucial tools in fluid mechanics. They allow us to quantify and analyze pressure differences in various systems. Understanding their working principles is key to accurately measuring and interpreting pressure data.

From simple U-tube manometers to advanced pressure transducers, these instruments play a vital role in engineering applications. Mastering problem-solving techniques with manometers and analyzing pressure measurements are essential skills for fluid mechanics students and professionals alike.

Manometry and Pressure Measurement

Working principle of manometers

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  • Simple manometer consists of U-shaped tube filled with liquid (mercury or water)
  • Measures pressure difference between two points in fluid system
  • Works based on hydrostatic pressure
    • Pressure difference causes change in liquid level in manometer
    • Height difference (hh) between two liquid columns proportional to pressure difference (ΔP\Delta P)
      • ΔP=ρgh\Delta P = \rho g h, where ρ\rho is density of manometer liquid and gg is acceleration due to gravity
  • Commonly used to measure pressure in closed containers (tanks, pipelines), determine pressure drop across flow restrictions (orifices, valves), and calibrate other pressure measurement devices

Problem-solving with manometers

  • Calculate pressure using manometer readings by determining height difference (hh) between two liquid columns
  • Use manometer equation: ΔP=ρgh\Delta P = \rho g h
    • Consider density (ρ\rho) of manometer liquid
    • Account for acceleration due to gravity (gg)
  • Convert pressure to desired unit (Pascal, bar, psi)
  • Analyze pressure measurements in various configurations:
    1. Open manometer: One end exposed to atmospheric pressure
      • Absolute pressure = Atmospheric pressure + Gauge pressure
    2. Differential manometer: Measures pressure difference between two points
      • Pressure difference = ρgh\rho g h
    3. Inclined manometer: Used for measuring small pressure differences
      • Pressure difference = ρghsinθ\rho g h \sin \theta, where θ\theta is angle of inclination

Pressure Measurement Devices

Types of pressure measurement devices

  • Bourdon tube consists of curved, hollow metal tube with oval cross-section
    • One end fixed and connected to fluid system
    • Other end sealed and connected to pointer mechanism
    • Works based on deformation of tube
      • Pressure inside tube causes it to straighten or uncoil
      • Deformation proportional to applied pressure
      • Pointer indicates corresponding pressure on calibrated scale
  • Pressure transducers convert pressure into electrical signal
    • Various types based on different working principles:
      • Piezoelectric transducers utilize piezoelectric effect of certain materials (quartz, tourmaline)
        • Pressure causes change in electric charge generated by material
      • Strain gauge transducers use strain gauges to measure deformation caused by pressure
        • Deformation alters electrical resistance of strain gauge
      • Capacitive transducers measure change in capacitance due to pressure-induced displacement
    • Electrical output can be processed, displayed, or recorded for analysis

Analysis of pressure measurements

  • Interpret manometer readings by:
    1. Identifying reference point (usually atmospheric pressure)
    2. Determining direction of pressure difference (higher or lower than reference)
    3. Calculating pressure using appropriate manometer equation
  • Analyze pressure measurements from various instruments:
    • Bourdon tube gauges
      • Read pressure directly from calibrated scale
      • Consider range and accuracy of gauge
    • Pressure transducers
      • Convert electrical output to pressure value using calibration data
      • Account for any signal conditioning or amplification
  • Compare and correlate pressure measurements from different devices
    • Ensure consistent units and reference points
    • Validate measurements by comparing results from multiple instruments
    • Identify and address any discrepancies or sources of error
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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.
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