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?

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

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

Linearity Signup and view all the answers

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

Static characteristics Signup and view all the answers

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

Precision Signup and view all the answers

Why is high accuracy essential in mechanical engineering?

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

What is the range of an instrument?

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

What is hysteresis in the context of measurement instruments?

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

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

Gross error Signup and view all the answers

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

Error propagation Signup and view all the answers

What is the combination of errors in a measurement system?

Assumes that all component errors combine in the most unfavorable way Signup and view all the answers

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

Zero-order instrument Signup and view all the answers

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

To ensure the integrity of measurement systems Signup and view all the answers

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

The output is directly proportional to the input Signup and view all the answers

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

To transmit strain data wirelessly from the rotating shaft Signup and view all the answers

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

Material compatibility Signup and view all the answers

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

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

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

Bridgeman type gauge Signup and view all the answers

What is the principle of operation of a McLeod gauge?

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

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

To deform under pressure and cause strain in the strain gauges Signup and view all the answers

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

Bonding the strain gauge to the specimen Signup and view all the answers

What is the application of strain gauge pressure cells?

Industrial processes, hydraulic systems, and fluid mechanics experiments Signup and view all the answers

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

All of the above Signup and view all the answers

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

Metallic Foil Strain Gauges Signup and view all the answers

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

To ensure accurate strain measurements Signup and view all the answers

What is the advantage of optical fiber strain gauges?

Immunity to electromagnetic interference Signup and view all the answers

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

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

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

Quarter-Bridge Circuit Signup and view all the answers

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

To plot the known loads against the electrical output Signup and view all the answers

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

Piezoresistive Strain Gauges Signup and view all the answers

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

To measure the increase in pressure Signup and view all the answers

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

Measures pressure based on the thermal transpiration effect Signup and view all the answers

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

To ionize the gas molecules Signup and view all the answers

What is the principle behind Thermal Conductivity Gauges?

Measures pressure based on the thermal conductivity of the gas Signup and view all the answers

What is the application of Pirani Gauges?

Industrial processes and mechanical engineering Signup and view all the answers

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

To separate two chambers at different temperatures Signup and view all the answers

What is the application of Ionization Gauges?

Semiconductor manufacturing and high vacuum systems Signup and view all the answers

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

To allow for the proper selection and utilization of pressure measurement instruments 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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Test your knowledge of mechanical engineering concepts related to measurement and instrumentation, including accuracy, precision, sensitivity, and more.