Measurement and Instrumentation – Full Notes

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1. Measurement and Instrumentation

Measurements

Measurement is the process of determining the magnitude of a physical quantity by comparing it with a standard unit. It provides the base for science, technology, and engineering decisions.

Significance of Measurements

Measurement is important for accuracy, quality control, system performance evaluation, and research. It supports innovation and ensures reliable, repeatable outcomes.

Methods of Measurements

There are two primary methods:

  • Direct Method: The value is obtained by direct comparison with a standard.
  • Indirect Method: Several measurements are taken and a formula is used to calculate the final result.

Direct Method

This method involves comparing the unknown quantity directly with a standard (e.g., using a ruler to measure length).

  • Simple and quick
  • Less accurate for complex measurements

Indirect Method

In this method, values are calculated using mathematical relationships. For example, power is measured indirectly using voltage and current.

  • Suitable for complex systems
  • Requires formulas and instruments

Instruments

Instruments are devices used to measure physical quantities. They convert physical quantities into readable signals.

Mechanical, Electrical, Electronics Instruments

  • Mechanical Instruments: Use mechanical parts to measure (e.g., pressure gauge)
  • Electrical Instruments: Use electrical quantities like voltage and current (e.g., voltmeter)
  • Electronic Instruments: Use semiconductors and digital processing (e.g., digital multimeter)

Classification of Instruments

  • Based on output: Indicating, Recording, Integrating
  • Based on type: Analog, Digital
  • Based on function: Absolute, Secondary

Analog and Digital Mode of Operation

  • Analog Instruments: Show output on a continuous scale
  • Digital Instruments: Show readings in numbers for better accuracy and readability

Functions of Instruments and Measurement Systems

  • Display, record, and control variables
  • Help analyze performance, detect faults, and ensure product quality

Applications of Measurement Systems

  • Used in industries, laboratories, healthcare, transportation, and communication systems
  • Help in automation, quality assurance, and scientific discovery

Elements of Generalized Measurement Systems

A basic measurement system includes:

  1. Primary sensing element – Detects the physical quantity
  2. Transducer – Converts physical quantity into an electrical signal
  3. Signal conditioning – Filters, amplifies, or modifies the signal
  4. Display or recording unit – Shows or stores the result
  5. Control system (optional) – Uses feedback to control processes

1.1 Purpose and Significance of Measurement

  • Measurement is a fundamental process in science and engineering. It involves determining the quantity, dimensions, or extent of something using standard units.
  • The main purpose of measurement is to gather data that helps in understanding, analyzing, and improving systems, products, and processes.
  • In industries, accurate measurement is essential for quality control, safety, and efficiency.
  • Engineers and technicians use measurements to design and maintain equipment, ensure specifications are met, and reduce errors.
  • In laboratories, measurement enables comparison with theoretical predictions, verification of results, and scientific discovery.
    • Significance of measurement includes:
    • Ensuring products meet required standards.
    • Supporting automation and process control.
    • Enhancing customer satisfaction by delivering precise and reliable performance.
    • Enabling repeatability and reproducibility in manufacturing.
    • Providing essential data for research, development, and innovation.
  • Without proper measurement, decision-making in engineering, manufacturing, and research would be based on assumptions rather than facts.

1.2 Definitions and Explanations

Range: The minimum and maximum values that an instrument can measure.

Sensitivity: The smallest change in input which causes a noticeable change in output.

True Value: The actual or ideal value of the quantity being measured.

Indicated Value: The value shown by the measuring instrument.

Errors: The difference between the measured value and the true value.

Limiting Error: The maximum expected error under specified conditions.

Resolution: The smallest change in quantity that an instrument can detect.

Accuracy: How close a measured value is to the true value.

Precision: The consistency or repeatability of measurements.

Instrument Efficiency: A measure of the effectiveness and performance of an instrument in terms of power consumption, output, and accuracy.

1.3 Classification of Instrument Systems

1.3.1 Null and Deflection Type Instruments

  • Null Type: Measurement is done by balancing two opposing forces. E.g., Wheatstone Bridge.
  • Deflection Type: Measurement is based on the amount of deflection shown by a pointer or indicator.

1.3.2 Absolute and Secondary Instruments

  • Absolute Instruments: Measure quantity directly from physical constants. E.g., Tangent galvanometer.
  • Secondary Instruments: Require calibration against a standard. E.g., ammeter, voltmeter.

1.3.3 Analog and Digital Instruments

  • Analog Instruments: Show output in a continuous scale. E.g., analog voltmeter.
  • Digital Instruments: Show output in numerical form. E.g., digital multimeter.

1.3.4 Static and Dynamic Characteristics; Types of Errors

1. Static and Dynamic Characteristics

  • Static Characteristics: Measured under steady conditions — accuracy, precision, resolution, sensitivity, etc.
  • Dynamic Characteristics: Measured under changing conditions — speed of response, fidelity, lag, etc.

2. Types of Errors:

  • Gross Errors: Human mistakes.
  • Systematic Errors: Due to instrument or environment.
  • Random Errors: Due to unknown or unpredictable factors.

1.4 Calibration of Instruments: Necessity and Procedure

  • Necessity:
  • Ensures accuracy.
  • Minimizes errors.
  • Maintains consistency and reliability.
  • Procedure:
  • Compare the instrument with a known standard.
  • Record and adjust the instrument reading.
  • Repeat for various values in the operating range.
  • Mark corrected readings or create a calibration chart.

1.5 Classification of Measuring Instruments

  • Indicating Instruments: Show the instantaneous value of the quantity being measured (e.g., ammeter).
  • Recording Instruments: Automatically record values over time (e.g., chart recorder).
  • Integrating Instruments: Measure the total quantity over a period (e.g., energy meter).

1.5.1 Essential Requirements of Indicating Instruments

  • Accuracy: Must provide close-to-true readings.
  • Sensitivity: Should detect small changes.
  • Clear Scale: Easy to read and interpret.
  • Quick Response: Should respond promptly to changes.
  • Durability: Should withstand environmental and mechanical stress.

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