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Mechanical Engineering: Measurement and Instrumentation
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Mechanical Engineering: Measurement and Instrumentation

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Questions and Answers

What is the definition of accuracy in mechanical engineering?

  • The consistency of repeated measurements under unchanged conditions
  • The smallest change in the measured quantity that the instrument can detect
  • The closeness of the measured value to the true value (correct)
  • The ratio of the change in the output signal to the change in the input quantity
  • What is the term for the smallest change in the measured quantity that the instrument can detect?

  • Sensitivity
  • Linearity
  • Resolution (correct)
  • Precision
  • What is the importance of high precision in mechanical engineering?

  • To increase the sensitivity of the instrument
  • To ensure reliable and consistent measurements (correct)
  • To detect small changes in the measured quantity
  • To ensure accurate measurements
  • What is the definition of sensitivity in mechanical engineering?

    <p>The ratio of the change in the output signal to the change in the input quantity</p> Signup and view all the answers

    What is the characteristic that ensures the instrument's response is predictable and straightforward to interpret?

    <p>Linearity</p> Signup and view all the answers

    What type of characteristics describe how measuring instruments behave under steady-state conditions?

    <p>Static characteristics</p> Signup and view all the answers

    What is the term for the consistency of repeated measurements under unchanged conditions?

    <p>Precision</p> Signup and view all the answers

    Why is high accuracy essential in mechanical engineering?

    <p>To ensure measurements reflect the actual value of the quantity being measured</p> Signup and view all the answers

    What is the range of an instrument?

    <p>The minimum and maximum limits within which the instrument can accurately measure the input quantity</p> Signup and view all the answers

    What is hysteresis in the context of measurement instruments?

    <p>The difference in output when the input quantity is increased and then decreased</p> Signup and view all the answers

    What type of error is caused by human mistakes during measurement or recording?

    <p>Gross error</p> Signup and view all the answers

    What is the process by which errors in individual components affect the overall system error?

    <p>Error propagation</p> Signup and view all the answers

    What is the combination of errors in a measurement system?

    <p>Assumes that all component errors combine in the most unfavorable way</p> Signup and view all the answers

    What type of instrument is characterized by an instantaneous response to changes in the input?

    <p>Zero-order instrument</p> Signup and view all the answers

    What is the importance of understanding dynamic characteristics of measurement instruments?

    <p>To ensure the integrity of measurement systems</p> Signup and view all the answers

    What is the general mathematical model of a zero-order instrument?

    <p>The output is directly proportional to the input</p> Signup and view all the answers

    What is the primary purpose of using telemetry systems in strain gauge measurement?

    <p>To transmit strain data wirelessly from the rotating shaft</p> Signup and view all the answers

    What is a critical consideration when selecting a strain gauge for measurement?

    <p>Material compatibility</p> Signup and view all the answers

    What is the purpose of the Wheatstone bridge circuit in a strain gauge pressure cell?

    <p>To convert the strain gauge resistance changes into a measurable voltage output</p> Signup and view all the answers

    What type of gauge is used to measure extremely high pressures in scientific research and industrial applications?

    <p>Bridgeman type gauge</p> Signup and view all the answers

    What is the principle of operation of a McLeod gauge?

    <p>Measures low pressures by compressing a known volume of gas and measuring the resulting increase in pressure</p> Signup and view all the answers

    What is the function of a diaphragm in a strain gauge pressure cell?

    <p>To deform under pressure and cause strain in the strain gauges</p> Signup and view all the answers

    What is a critical step in the installation process of strain gauges?

    <p>Bonding the strain gauge to the specimen</p> Signup and view all the answers

    What is the application of strain gauge pressure cells?

    <p>Industrial processes, hydraulic systems, and fluid mechanics experiments</p> Signup and view all the answers

    What is the primary application of strain gauges in mechanical engineering?

    <p>All of the above</p> Signup and view all the answers

    What type of strain gauge is commonly made from constantan, nickel-chromium alloys, or similar materials?

    <p>Metallic Foil Strain Gauges</p> Signup and view all the answers

    What is the purpose of the calibration process in strain measurement?

    <p>To ensure accurate strain measurements</p> Signup and view all the answers

    What is the advantage of optical fiber strain gauges?

    <p>Immunity to electromagnetic interference</p> Signup and view all the answers

    What is the purpose of the dummy gauge method in temperature compensation?

    <p>To compensate for temperature-induced changes in the strain gauge</p> Signup and view all the answers

    What type of strain gauge circuit is simplest and least expensive but less sensitive?

    <p>Quarter-Bridge Circuit</p> Signup and view all the answers

    What is the purpose of the calibration curve in strain measurement?

    <p>To plot the known loads against the electrical output</p> Signup and view all the answers

    What type of strain gauge is primarily used in pressure sensors?

    <p>Piezoresistive Strain Gauges</p> Signup and view all the answers

    What is the primary function of a manometer in pressure measurement?

    <p>To measure the increase in pressure</p> Signup and view all the answers

    Which of the following is a characteristic of a Knudsen Gauge?

    <p>Measures pressure based on the thermal transpiration effect</p> Signup and view all the answers

    What is the purpose of the electron source in an Ionization Gauge?

    <p>To ionize the gas molecules</p> Signup and view all the answers

    What is the principle behind Thermal Conductivity Gauges?

    <p>Measures pressure based on the thermal conductivity of the gas</p> Signup and view all the answers

    What is the application of Pirani Gauges?

    <p>Industrial processes and mechanical engineering</p> Signup and view all the answers

    What is the purpose of the porous plug in a Knudsen Gauge?

    <p>To separate two chambers at different temperatures</p> Signup and view all the answers

    What is the application of Ionization Gauges?

    <p>Semiconductor manufacturing and high vacuum systems</p> Signup and view all the answers

    What is the importance of understanding the principles and operations of pressure measurement methods?

    <p>To allow for the proper selection and utilization of pressure measurement instruments</p> Signup and view all the answers

    Study Notes

    General Performance Characteristics of Measuring Instruments

    • Static characteristics: describe the behavior of measuring instruments under steady-state conditions
      • Accuracy: closeness of the measured value to the true value
      • Precision: consistency of repeated measurements under unchanged conditions
      • Resolution: smallest change in the measured quantity that the instrument can detect
      • Sensitivity: ratio of the change in the output signal to the change in the input quantity
      • Linearity: degree to which the output signal is directly proportional to the input quantity
      • Range: minimum and maximum limits within which the instrument can accurately measure the input quantity
      • Hysteresis: difference in output when the input quantity is increased and then decreased
    • Types of errors: errors that can arise in measurements
      • Gross errors: human errors or mistakes during measurement or recording
      • Systematic errors: consistent, predictable errors that can often be corrected
        • Instrumental errors: due to imperfections or miscalibration of the instrument
        • Environmental errors: caused by external conditions like temperature, humidity, and pressure
        • Observational errors: errors due to the observer, such as parallax errors
      • Random errors: unpredictable variations in the measurement process that cannot be easily corrected
    • Combination of component errors: errors in individual components can propagate and affect the overall accuracy of a measurement system

    Dynamic Characteristics of Instruments

    • Dynamic characteristics: describe how an instrument responds to changes in the measured variable over time
    • General mathematical models: used to describe the behavior of zero-order, first-order, and second-order instruments
    • Response to different inputs: instruments can respond to various inputs, including step, ramp, impulse, and frequency
    • Zero-order instruments: characterized by an instantaneous response to changes in the input
    • First-order instruments: characterized by a response that changes over time
    • Second-order instruments: characterized by a response that changes over time and is influenced by damping and natural frequency

    Strain Measurement

    • Strain gauges: used to measure strain on various structures
    • Types of strain gauges:
      • Metallic foil strain gauges: most common type, consisting of a grid of thin metallic foil bonded to an insulating backing material
      • Semiconductor strain gauges: made from silicon or germanium, higher sensitivity but more temperature-sensitive
      • Bonded and unbonded strain gauges: differ in how they are attached to the test specimen
      • Optical fiber strain gauges: use changes in light transmission to measure strain
      • Piezoresistive strain gauges: used in pressure sensors, utilize the change in electrical resistance of a piezoresistive material
    • Strain gauge circuits:
      • Quarter-bridge circuit: simplest and least expensive, uses a single active strain gauge and three fixed resistors
      • Half-bridge circuit: uses two strain gauges, can measure bending strains and provide some temperature compensation
      • Full-bridge circuit: uses four strain gauges, provides maximum sensitivity and temperature compensation
    • Calibration: process of ensuring accurate strain measurements
      • Applying known loads: apply known mechanical loads or strains to the test specimen and record the corresponding electrical output
      • Generating calibration curve: plot the known loads against the electrical output to create a calibration curve
      • Deriving calibration factor: calculate the calibration factor, which relates the electrical output to the actual strain
    • Temperature compensation: methods to compensate for temperature changes that can affect strain gauge readings
      • Using temperature-compensated strain gauges: gauges designed with materials having matching thermal expansion coefficients
      • Dummy gauge method: uses a non-stressed dummy gauge in the same thermal environment as the active gauge
      • Quarter-bridge temperature compensation: uses an additional strain gauge in the bridge that is not subject to strain but experiences the same temperature changes
    • Use of strain gauges on rotating shafts: special considerations for using strain gauges on rotating shafts
      • Wireless data transmission: use telemetry systems to transmit strain data wirelessly from the rotating shaft to a stationary data acquisition system
      • Slip rings: use slip rings to maintain electrical connections between the rotating strain gauge and the stationary measurement equipment
      • Careful installation: ensure strain gauges are securely bonded and the wiring is protected against centrifugal forces
    • Selection and installation of strain gauges: critical for obtaining accurate and reliable strain measurements
      • Selection criteria: strain range, gauge factor, environmental conditions, and material compatibility
      • Installation process: surface preparation, bonding, wiring, and protection

    Pressure Measurement

    • Basic methods of pressure measurement:
      • Strain gauge pressure cell: uses the deformation of a diaphragm or other elastic element under pressure to measure strain
      • High pressure measurement: uses the Bridgeman type gauge, which measures high pressures by using the change in electrical resistance of a wire or material under pressure
      • Low pressure measurement: uses various methods, including McLeod, Knudsen, ionization, and thermal conductivity gauges
    • Strain gauge pressure cell: components and operation
      • Diaphragm: a flexible membrane that deforms under pressure
      • Strain gauges: bonded to the diaphragm to measure deformation
      • Wheatstone bridge circuit: converts the strain gauge resistance changes into a measurable voltage output
    • High pressure measurement - Bridgeman type gauge: principle, components, and operation
      • Principle: measures high pressures by using the change in electrical resistance of a wire or material under pressure
      • Components: pressure vessel, wire or material, and electrical circuit
      • Operation: measures the change in resistance of the wire or material under pressure
    • Low pressure measurement methods:
      • McLeod gauge: measures low pressures by compressing a known volume of gas and measuring the resulting increase in pressure
      • Knudsen gauge: measures pressure based on the thermal transpiration effect, where gas molecules flow through a porous plug due to a temperature gradient
      • Ionization gauge: measures very low pressures by ionizing the gas and measuring the resulting ion current
      • Thermal conductivity gauges: measures pressure based on the thermal conductivity of the gas, which varies with pressure

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