Strain Measurement in Mechanical Engineering

Strain Measurement

• Strain gauges are essential tools in mechanical engineering for measuring strain on various structures.
• Types of strain gauges:
  • Metallic Foil Strain Gauges: consist of a grid of thin metallic foil bonded to an insulating backing material.
  • Semiconductor Strain Gauges: made from silicon or germanium, with higher sensitivity but more temperature-sensitive.
  • Bonded and Unbonded Strain Gauges: bonded gauges are fixed to the surface of the test specimen, while unbonded gauges are mounted on a frame.
  • Optical Fiber Strain Gauges: use changes in light transmission to measure strain, immune to electromagnetic interference.
  • Piezoresistive Strain Gauges: primarily used in pressure sensors, utilizing the change in electrical resistance with applied strain.

Strain Gauge Circuits

• Quarter-Bridge Circuit: uses a single active strain gauge and three fixed resistors in a Wheatstone bridge configuration, simplest and least expensive.
• Half-Bridge Circuit: uses two strain gauges in the Wheatstone bridge, can measure bending strains and temperature compensation to some extent.
• Full-Bridge Circuit: uses four strain gauges in the Wheatstone bridge, provides maximum sensitivity and temperature compensation, can measure axial and bending strains accurately.

Calibration

• Calibration is essential to ensure accurate strain measurements.
• Process involves applying known loads, generating a calibration curve, and deriving a calibration factor.

Temperature Compensation

• Temperature changes can affect strain gauge readings due to thermal expansion of the specimen and the gauge itself.
• Methods for temperature compensation include:
  • Using temperature-compensated strain gauges.
  • Dummy gauge method.
  • Quarter-Bridge temperature compensation.

Use of Strain Gauges on Rotating Shafts

• Strain gauges can be used to measure torque and stress on rotating shafts.
• Key considerations include:
  • Wireless data transmission.
  • Slip rings.
  • Careful installation.

Selection and Installation of Strain Gauges

• Selection criteria:
  • Strain range.
  • Gauge factor.
  • Environmental conditions.
  • Material compatibility.
• Installation process:
  • Surface preparation.
  • Bonding.
  • Wiring.
  • Protection.

Pressure Measurements

• Basic methods of pressure measurement:
  • Strain gauge pressure cells.
  • High pressure measurement using the Bridgeman type gauge.
  • Low-pressure measurement techniques:
    • McLeod gauge.
    • Knudsen gauge.
    • Ionization gauge.
    • Thermal conductivity gauges.

Strain Gauge Pressure Cell

• Principle: measures pressure using the deformation of a diaphragm or other elastic element under pressure.
• Components:
  • Diaphragm.
  • Strain gauges.
  • Wheatstone bridge circuit.
• Operation:
  • Diaphragm deforms under pressure, causing strain in the strain gauges.
  • Resistance change in the strain gauges is measured using a Wheatstone bridge circuit.

High Pressure Measurement - Bridgeman Type Gauge

• Principle: measures high pressures by using the change in electrical resistance of a wire or material under pressure.
• Components:
  • Pressure vessel.
  • Electrical circuit.
• Operation:
  • Material's electrical resistance changes under pressure.
  • Resistance change is measured and correlated with the pressure.

Low Pressure Measurement

• McLeod Gauge:

  • Principle: measures low pressures by compressing a known volume of gas and measuring the resulting increase in pressure.
  • Components:
    • Bulb and capillary tube.
    • Manometer.

• Operation:

  • Gas is trapped in a bulb and compressed into a capillary tube.
  • Increase in pressure is measured with a manometer.

• Knudsen Gauge:

  • Principle: measures pressure based on the thermal transpiration effect.
  • Components:
    • Porous plug.
    • Thermocouples.

• Operation:

  • Gas flows through the porous plug from the hot side to the cold side.
  • Pressure difference created by this flow is measured and related to the absolute pressure.

• Ionization Gauge:

  • Principle: measures very low pressures by ionizing the gas and measuring the resulting ion current.
  • Components:
    • Ionization chamber.
    • Electron source.
    • Collector electrode.

• Operation:

  • Electrons from a hot filament or cold cathode ionize the gas molecules.
  • Ions are collected, and the ion current is measured.

• Thermal Conductivity Gauges:

  • Principle: measures pressure based on the thermal conductivity of the gas, which varies with pressure.
  • Types:
    • Pirani gauge.
    • Thermocouple gauge.

• Operation:

  • Pirani gauge: heat loss from a heated wire to the gas depends on the gas pressure.
  • Thermocouple gauge: heat loss from a heated element to the gas depends on the gas pressure.

Summary

• The selection of a pressure measurement method depends on the range and accuracy required, the environment, and the specific application.

Strain Measurement

• Strain gauges are essential tools in mechanical engineering for measuring strain on various structures.
• Types of strain gauges:
  • Metallic Foil Strain Gauges: consist of a grid of thin metallic foil bonded to an insulating backing material.
  • Semiconductor Strain Gauges: made from silicon or germanium, with higher sensitivity but more temperature-sensitive.
  • Bonded and Unbonded Strain Gauges: bonded gauges are fixed to the surface of the test specimen, while unbonded gauges are mounted on a frame.
  • Optical Fiber Strain Gauges: use changes in light transmission to measure strain, immune to electromagnetic interference.
  • Piezoresistive Strain Gauges: primarily used in pressure sensors, utilizing the change in electrical resistance with applied strain.

Strain Gauge Circuits

• Quarter-Bridge Circuit: uses a single active strain gauge and three fixed resistors in a Wheatstone bridge configuration, simplest and least expensive.
• Half-Bridge Circuit: uses two strain gauges in the Wheatstone bridge, can measure bending strains and temperature compensation to some extent.
• Full-Bridge Circuit: uses four strain gauges in the Wheatstone bridge, provides maximum sensitivity and temperature compensation, can measure axial and bending strains accurately.

Calibration

• Calibration is essential to ensure accurate strain measurements.
• Process involves applying known loads, generating a calibration curve, and deriving a calibration factor.

Temperature Compensation

• Temperature changes can affect strain gauge readings due to thermal expansion of the specimen and the gauge itself.
• Methods for temperature compensation include:
  • Using temperature-compensated strain gauges.
  • Dummy gauge method.
  • Quarter-Bridge temperature compensation.

Use of Strain Gauges on Rotating Shafts

• Strain gauges can be used to measure torque and stress on rotating shafts.
• Key considerations include:
  • Wireless data transmission.
  • Slip rings.
  • Careful installation.

Selection and Installation of Strain Gauges

• Selection criteria:
  • Strain range.
  • Gauge factor.
  • Environmental conditions.
  • Material compatibility.
• Installation process:
  • Surface preparation.
  • Bonding.
  • Wiring.
  • Protection.

Pressure Measurements

• Basic methods of pressure measurement:
  • Strain gauge pressure cells.
  • High pressure measurement using the Bridgeman type gauge.
  • Low-pressure measurement techniques:
    • McLeod gauge.
    • Knudsen gauge.
    • Ionization gauge.
    • Thermal conductivity gauges.

Strain Gauge Pressure Cell

• Principle: measures pressure using the deformation of a diaphragm or other elastic element under pressure.
• Components:
  • Diaphragm.
  • Strain gauges.
  • Wheatstone bridge circuit.
• Operation:
  • Diaphragm deforms under pressure, causing strain in the strain gauges.
  • Resistance change in the strain gauges is measured using a Wheatstone bridge circuit.

High Pressure Measurement - Bridgeman Type Gauge

• Principle: measures high pressures by using the change in electrical resistance of a wire or material under pressure.
• Components:
  • Pressure vessel.
  • Electrical circuit.
• Operation:
  • Material's electrical resistance changes under pressure.
  • Resistance change is measured and correlated with the pressure.

Low Pressure Measurement

• McLeod Gauge:

  • Principle: measures low pressures by compressing a known volume of gas and measuring the resulting increase in pressure.
  • Components:
    • Bulb and capillary tube.
    • Manometer.

• Operation:

  • Gas is trapped in a bulb and compressed into a capillary tube.
  • Increase in pressure is measured with a manometer.

• Knudsen Gauge:

  • Principle: measures pressure based on the thermal transpiration effect.
  • Components:
    • Porous plug.
    • Thermocouples.

• Operation:

  • Gas flows through the porous plug from the hot side to the cold side.
  • Pressure difference created by this flow is measured and related to the absolute pressure.

• Ionization Gauge:

  • Principle: measures very low pressures by ionizing the gas and measuring the resulting ion current.
  • Components:
    • Ionization chamber.
    • Electron source.
    • Collector electrode.

• Operation:

  • Electrons from a hot filament or cold cathode ionize the gas molecules.
  • Ions are collected, and the ion current is measured.

• Thermal Conductivity Gauges:

  • Principle: measures pressure based on the thermal conductivity of the gas, which varies with pressure.
  • Types:
    • Pirani gauge.
    • Thermocouple gauge.

• Operation:

  • Pirani gauge: heat loss from a heated wire to the gas depends on the gas pressure.
  • Thermocouple gauge: heat loss from a heated element to the gas depends on the gas pressure.

Summary

• The selection of a pressure measurement method depends on the range and accuracy required, the environment, and the specific application.

Strain Measurement

• Strain gauges are essential tools in mechanical engineering for measuring strain on various structures.
• Types of strain gauges:
  • Metallic Foil Strain Gauges: consist of a grid of thin metallic foil bonded to an insulating backing material.
  • Semiconductor Strain Gauges: made from silicon or germanium, with higher sensitivity but more temperature-sensitive.
  • Bonded and Unbonded Strain Gauges: bonded gauges are fixed to the surface of the test specimen, while unbonded gauges are mounted on a frame.
  • Optical Fiber Strain Gauges: use changes in light transmission to measure strain, immune to electromagnetic interference.
  • Piezoresistive Strain Gauges: primarily used in pressure sensors, utilizing the change in electrical resistance with applied strain.

Strain Gauge Circuits

• Quarter-Bridge Circuit: uses a single active strain gauge and three fixed resistors in a Wheatstone bridge configuration, simplest and least expensive.
• Half-Bridge Circuit: uses two strain gauges in the Wheatstone bridge, can measure bending strains and temperature compensation to some extent.
• Full-Bridge Circuit: uses four strain gauges in the Wheatstone bridge, provides maximum sensitivity and temperature compensation, can measure axial and bending strains accurately.

Calibration

• Calibration is essential to ensure accurate strain measurements.
• Process involves applying known loads, generating a calibration curve, and deriving a calibration factor.

Temperature Compensation

• Temperature changes can affect strain gauge readings due to thermal expansion of the specimen and the gauge itself.
• Methods for temperature compensation include:
  • Using temperature-compensated strain gauges.
  • Dummy gauge method.
  • Quarter-Bridge temperature compensation.

Use of Strain Gauges on Rotating Shafts

• Strain gauges can be used to measure torque and stress on rotating shafts.
• Key considerations include:
  • Wireless data transmission.
  • Slip rings.
  • Careful installation.

Selection and Installation of Strain Gauges

• Selection criteria:
  • Strain range.
  • Gauge factor.
  • Environmental conditions.
  • Material compatibility.
• Installation process:
  • Surface preparation.
  • Bonding.
  • Wiring.
  • Protection.

Pressure Measurements

• Basic methods of pressure measurement:
  • Strain gauge pressure cells.
  • High pressure measurement using the Bridgeman type gauge.
  • Low-pressure measurement techniques:
    • McLeod gauge.
    • Knudsen gauge.
    • Ionization gauge.
    • Thermal conductivity gauges.

Strain Gauge Pressure Cell

• Principle: measures pressure using the deformation of a diaphragm or other elastic element under pressure.
• Components:
  • Diaphragm.
  • Strain gauges.
  • Wheatstone bridge circuit.
• Operation:
  • Diaphragm deforms under pressure, causing strain in the strain gauges.
  • Resistance change in the strain gauges is measured using a Wheatstone bridge circuit.

High Pressure Measurement - Bridgeman Type Gauge

• Principle: measures high pressures by using the change in electrical resistance of a wire or material under pressure.
• Components:
  • Pressure vessel.
  • Electrical circuit.
• Operation:
  • Material's electrical resistance changes under pressure.
  • Resistance change is measured and correlated with the pressure.

Low Pressure Measurement

• McLeod Gauge:

  • Principle: measures low pressures by compressing a known volume of gas and measuring the resulting increase in pressure.
  • Components:
    • Bulb and capillary tube.
    • Manometer.

• Operation:

  • Gas is trapped in a bulb and compressed into a capillary tube.
  • Increase in pressure is measured with a manometer.

• Knudsen Gauge:

  • Principle: measures pressure based on the thermal transpiration effect.
  • Components:
    • Porous plug.
    • Thermocouples.

• Operation:

  • Gas flows through the porous plug from the hot side to the cold side.
  • Pressure difference created by this flow is measured and related to the absolute pressure.

• Ionization Gauge:

  • Principle: measures very low pressures by ionizing the gas and measuring the resulting ion current.
  • Components:
    • Ionization chamber.
    • Electron source.
    • Collector electrode.

• Operation:

  • Electrons from a hot filament or cold cathode ionize the gas molecules.
  • Ions are collected, and the ion current is measured.

• Thermal Conductivity Gauges:

  • Principle: measures pressure based on the thermal conductivity of the gas, which varies with pressure.
  • Types:
    • Pirani gauge.
    • Thermocouple gauge.

• Operation:

  • Pirani gauge: heat loss from a heated wire to the gas depends on the gas pressure.
  • Thermocouple gauge: heat loss from a heated element to the gas depends on the gas pressure.

Summary

• The selection of a pressure measurement method depends on the range and accuracy required, the environment, and the specific application.