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15.2 Pressure and Temperature Measurements

4 min readLast Updated on July 19, 2024

Pressure and temperature measurement devices are crucial tools in fluid mechanics. From manometers to pressure gauges, these instruments help engineers quantify fluid behavior. Understanding their principles and applications is essential for accurate data collection and analysis in fluid systems.

Temperature measurement techniques like thermocouples and RTDs provide insights into fluid thermal properties. Selecting the right device based on accuracy, environmental factors, and process conditions is key. Proper calibration and installation ensure reliable measurements, critical for effective fluid mechanics studies and applications.

Pressure Measurement Devices

Principles of pressure measurement devices

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  • Manometers balance fluid pressure against a liquid column
    • U-tube manometer uses a U-shaped tube filled with liquid (mercury or water) to determine pressure difference by measuring height difference between the two legs
    • Inclined manometer similar to U-tube but with one leg inclined provides higher resolution for small pressure differences
  • Pressure gauges measure pressure by deformation of a sensitive element
    • Bourdon tube gauge contains a curved, hollow tube that straightens when pressurized and translates movement to a pointer on a calibrated scale
    • Diaphragm gauge uses a flexible diaphragm that deflects under pressure and measures the deflection to convert it to a pressure reading
  • Pressure transducers convert pressure into an electrical signal
    • Strain gauge pressure transducer measures strain on a diaphragm or pressure-sensitive element using strain gauges that convert mechanical strain into electrical resistance change
    • Capacitive pressure transducer measures change in capacitance between a diaphragm and fixed plate that varies with pressure-induced deflection of the diaphragm
    • Piezoelectric pressure transducer uses a piezoelectric material generating an electrical charge when subjected to pressure suitable for measuring dynamic pressures and fluctuations

Temperature Measurement Techniques

Temperature measurement techniques

  • Thermocouples consist of two dissimilar metals joined at one end (hot junction)
    • Temperature difference between hot junction and reference junction (cold junction) generates a voltage (Seebeck effect)
    • Voltage is measured and converted to temperature using standard thermocouple tables or polynomial equations
    • Different types of thermocouples (Type J, K, T) suitable for various temperature ranges and environments
  • Resistance Temperature Detectors (RTDs) measure temperature by change in electrical resistance of a metal (usually platinum) with temperature
    • Resistance increases with temperature in a predictable manner
    • Wheatstone bridge circuit commonly used to measure resistance change accurately
    • Offer high accuracy and stability over a wide temperature range
  • Infrared thermometers measure temperature by detecting infrared radiation emitted by an object
    • Non-contact measurement technique suitable for measuring temperature of moving objects or in hazardous environments
    • Emissivity of the object must be considered for accurate measurements
      • Emissivity is the ratio of energy radiated by an object to that radiated by a perfect blackbody at the same temperature and varies with material, surface finish, and temperature

Accuracy factors in measurements

  • Calibration is essential to maintain accuracy and should be performed regularly against known standards traceable to national or international standards
  • Environmental factors can affect measurements
    • Temperature variations cause thermal expansion or contraction of pressure-sensing elements but compensation techniques (temperature-compensated pressure transducers) can minimize effects
    • Ambient temperature fluctuations influence temperature measurements but proper insulation and shielding of sensors can reduce the impact
  • Sensor placement and installation are crucial for representative measurements
    • Pressure sensors should avoid areas with flow disturbances (bends or valves) that cause localized pressure variations
    • Temperature sensors should be in direct contact with process fluid or installed in a thermowell with adequate immersion depth to minimize stem conduction errors
  • Extreme temperatures, high pressures, and corrosive or abrasive media can affect sensor performance and longevity so selecting suitable materials and operating ranges is crucial for reliable measurements
  • Proper shielding, grounding, signal conditioning (amplification and filtering), and digital communication protocols (HART or Fieldbus) ensure accurate, stable measurements and improve signal integrity

Selection of measurement devices

  • Consider process fluid properties
    • Chemical compatibility of sensor materials with process fluid
    • Operating temperature and pressure ranges
    • Presence of particulates, bubbles, or contaminants that may affect measurement
  • Determine required accuracy and precision and select a device with appropriate specifications for the application considering impact of measurement errors on process control and product quality
  • Evaluate process conditions
    • Ambient temperature and pressure fluctuations
    • Vibration, shock, or other mechanical stresses
    • Corrosive, abrasive, or hazardous environments may require special sensor materials or protective coatings
  • Consider installation requirements
    • Available space for sensor installation
    • Accessibility for maintenance and calibration
    • Need for remote monitoring or wireless communication
  • Assess cost and maintenance requirements
    • Initial cost of measurement device
    • Long-term maintenance and calibration costs
    • Expected sensor lifespan under given process conditions
  • Consult industry standards and guidelines
    • Refer to relevant standards (ASME PTC 19.2 for pressure measurement, ASTM E230 for temperature measurement) for best practices and recommendations
    • Consider regulatory requirements for hazardous areas or food and pharmaceutical processing
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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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