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?

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

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

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

What is the expression of accuracy in measurement instruments?

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

What is the importance of high sensitivity in measurement instruments?

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

What is the definition of precision in measurement instruments?

The consistency of repeated measurements under unchanged conditions Signup and view all the answers

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

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

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

Environmental Error Signup and view all the answers

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

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

Hysteresis Signup and view all the answers

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

Error Combination Signup and view all the answers

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

Instrumental Error Signup and view all the answers

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

Random Error Signup and view all the answers

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

Centrifugal forces Signup and view all the answers

What is the primary application of piezoresistive strain gauges?

Measuring pressure in industrial applications Signup and view all the answers

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

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

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

Strain range Signup and view all the answers

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

Optical fiber strain gauge Signup and view all the answers

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

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

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

It provides maximum sensitivity and temperature compensation Signup and view all the answers

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

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

What is the primary application of a Bridgeman type gauge?

Measuring extremely high pressures in industrial applications Signup and view all the answers

What is the principle of operation for a McLeod gauge?

The compression of a known volume of gas increases the pressure Signup and view all the answers

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

Semiconductor strain gauge Signup and view all the answers

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

To ensure accurate strain measurements Signup and view all the answers

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

Carefully routing and securing the wiring to prevent damage Signup and view all the answers

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

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

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

Wireless transmission of strain data from the rotating shaft Signup and view all the answers

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

Measuring bending strains and temperature compensation 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

Which type of gauge is commonly used in semiconductor manufacturing?

Ionization Gauge Signup and view all the answers

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

Thermal conductivity of the gas varying with pressure Signup and view all the answers

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

Collector Electrode Signup and view all the answers

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

To measure the temperature of a heated element Signup and view all the answers

What is the principle of operation of a Pirani Gauge?

Measuring the resistance change of a heated wire Signup and view all the answers

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

The range and accuracy required Signup and view all the answers

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

Manometer 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.