Static and Dynamic Characteristics in Mechanical Engineering
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Questions and Answers

What is the primary importance of high accuracy in measurements?

  • To ensure high precision in measurements
  • To reflect the actual value of the quantity being measured (correct)
  • To achieve high sensitivity in the instrument
  • To detect very small changes in the measured quantity

    • What is the definition of resolution in measurement instruments?

  • The ratio of the change in the output signal to the change in the input quantity
  • The consistency of repeated measurements under unchanged conditions
  • The degree to which the output signal is directly proportional to the input quantity
  • The smallest change in the measured quantity that the instrument can detect (correct)

    • Which of the following is NOT a component of precision?

  • Repeatability
  • Sensitivity (correct)
  • Reproducibility
  • Accuracy

    • What is the importance of high linearity in measurement instruments?

    <p>To ensure the instrument's response is predictable and straightforward to interpret</p> Signup and view all the answers

    What is the difference between static and dynamic characteristics in measurement instruments?

    <p>Static characteristics describe behavior under steady-state conditions, while dynamic characteristics describe behavior under changing conditions</p> Signup and view all the answers

    What is the expression of accuracy in measurement instruments?

    <p>Often expressed as a percentage of the full-scale reading or as an absolute error value</p> Signup and view all the answers

    What is the importance of high sensitivity in measurement instruments?

    <p>To ensure that even small changes in the input quantity produce noticeable changes in the output signal</p> Signup and view all the answers

    What is the definition of precision in measurement instruments?

    <p>The consistency of repeated measurements under unchanged conditions</p> Signup and view all the answers

    What is the primary importance of a wide range in an instrument?

    <p>To allow the instrument to measure a broad spectrum of values without requiring adjustments</p> Signup and view all the answers

    What type of error is caused by external conditions like temperature, humidity, and pressure?

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

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

    <p>Error Propagation</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 term for the difference in output when the input quantity is increased and then decreased?

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

    What is the result of combining component errors in the most unfavorable way?

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

    What type of error is due to imperfections or miscalibration of the instrument?

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

    Which of the following is NOT a type of systematic error?

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

    What is the primary concern when using strain gauges on rotating shafts?

    <p>Centrifugal forces</p> Signup and view all the answers

    What is the primary application of piezoresistive strain gauges?

    <p>Measuring pressure in industrial applications</p> Signup and view all the answers

    What is the purpose of a slip ring in measuring torque on a rotating shaft?

    <p>To maintain electrical connections between the rotating strain gauge and stationary measurement equipment</p> Signup and view all the answers

    What is the key factor in selecting a strain gauge for measuring torque on a rotating shaft?

    <p>Strain range</p> Signup and view all the answers

    Which type of strain gauge is most immune to electromagnetic interference?

    <p>Optical fiber strain gauge</p> Signup and view all the answers

    What is the primary principle of operation for a strain gauge pressure cell?

    <p>The deformation of a diaphragm under pressure causes a change in electrical resistance</p> Signup and view all the answers

    What is the main advantage of a full-bridge circuit in strain gauge measurements?

    <p>It provides maximum sensitivity and temperature compensation</p> Signup and view all the answers

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

    <p>To place a non-stressed gauge in the same thermal environment as the active gauge</p> Signup and view all the answers

    What is the primary application of a Bridgeman type gauge?

    <p>Measuring extremely high pressures in industrial applications</p> Signup and view all the answers

    What is the principle of operation for a McLeod gauge?

    <p>The compression of a known volume of gas increases the pressure</p> Signup and view all the answers

    Which type of strain gauge has a higher sensitivity (gauge factor) compared to metallic gauges?

    <p>Semiconductor strain gauge</p> Signup and view all the answers

    What is the main purpose of calibration in strain gauge measurements?

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

    What is the primary consideration when installing strain gauges on a rotating shaft?

    <p>Carefully routing and securing the wiring to prevent damage</p> Signup and view all the answers

    What is the advantage of using temperature-compensated strain gauges?

    <p>They are designed with materials having matching thermal expansion coefficients as the test specimen</p> Signup and view all the answers

    What is the primary advantage of using telemetry systems in measuring torque on a rotating shaft?

    <p>Wireless transmission of strain data from the rotating shaft</p> Signup and view all the answers

    What is the primary application of quarter-bridge circuits in strain gauge measurements?

    <p>Measuring bending strains and temperature compensation</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

    Which type of gauge is commonly used in semiconductor manufacturing?

    <p>Ionization Gauge</p> Signup and view all the answers

    What is the principle of operation of a Thermal Conductivity Gauge?

    <p>Thermal conductivity of the gas varying with pressure</p> Signup and view all the answers

    Which component is used to collect the ions in an Ionization Gauge?

    <p>Collector Electrode</p> Signup and view all the answers

    What is the purpose of the thermocouple in a Thermocouple Gauge?

    <p>To measure the temperature of a heated element</p> Signup and view all the answers

    What is the principle of operation of a Pirani Gauge?

    <p>Measuring the resistance change of a heated wire</p> Signup and view all the answers

    What is the main factor in selecting a pressure measurement method?

    <p>The range and accuracy required</p> Signup and view all the answers

    Which gauge is used in vacuum systems and laboratories for very low pressure measurements?

    <p>Manometer</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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    Description

    This quiz covers the static and dynamic characteristics of measuring instruments in mechanical engineering, including types of errors and mathematical models of zero, first, and second order instruments.

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