Introduction to Metrology

• Metrology is defined as the scientific study of measurement
• The objectives of Metrology include:
  • Accurate measurements at minimum cost
  • Ensuring measurement stability and comparability
  • Determining process capabilities

Types of Metrology

• Scientific Metrology
  • Development of new measurement units
  • Development of measurement unit systems
  • Development of measurement methods
  • Creation and maintenance of measurement standards
• Legal Metrology
  • A specific subset of metrology that legally protects consumers
  • Protects the environment
  • Ensures fair trade practices
• Applied Metrology
  • Known as industrial metrology
  • Covers the application of measurement to manufacturing or other industrial processes, and their utilization in society

Characteristics of instruments

• Static characteristics
• Dynamic Characteristics

Least Count of Measuring Instruments

• The least count of an instrument is the smallest increment it can measure with accuracy
• Least Count = (Value of one scale division) / (Number of divisions on vernier scale)

Example

• Vernier Caliper has a smallest division on main scale as 1mm
• Total number of division on vernier scale is 10
• Least Count = (1 mm) / (10 divisions) = 0.1 mm
• Micrometer having smallest division on main scale of 0.5 mm
• Total number of division on Thimble (vernier scale) is 50
• Least Count = (0.5 mm) / (50 divisions) = 0.01 mm

Other Characteristics of Measuring Instrument

• Range: Specified by lower and upper limits in which it is designed to operate, indicate, or to record the measured variable
  • Example: 0-150mm
• Span: The algebraic difference between upper range value and lower range value
  • Span = (150-0) = 150mm

Accuracy and Precision

• Accuracy: How close a measured value is to the true value. An accurate instrument yields results that are close to the actual value of what is being measured.
  • If measuring the length of a standard 10 cm ruler and the instrument reads 9.8 cm, the measurement is close to the true value (10 cm), indicating accuracy
• Precision: A precise instrument gives similar results every time the same measurement is taken.
  • If you measure the length of the same object several times and get readings like 9.9 cm, 9.9 cm, and 9.9 cm, the instrument is precise because the measurements are consistent, even though they may not be exactly 10 cm

Calibration

• Calibration of measuring instruments is a crucial process to ensure accurate and reliable measurements
• Calibration helps ensure instruments provide correct and reliable measurements close to the true value
• Regular calibration ensures consistent and repeatable measurements
• Helps meet most industry standards and regulatory requirements
• Calibration avoids costly mistakes due to incorrect measurments
• It helps maintain instrument performance and detect issues early
• Accurate measurements ensure trustworthy data for research and production
• Regular calibration helps identify malfunctions or drift in instrument performance

Dynamic Characteristics of Instruments

• Speed of Response: How fast the instrument reacts to changes
• Fidelity: The ability to replicate the input signal accurately
• Overshoot: When the output exceeds the target value before stabilizing
• Dynamic Errors: Errors caused by the instrument's inability to keep up with fast changes
• Measuring Lag: Delay between a change in the measured quantity and the instrument's response

Standards of Measuring Instruments

• Line Standard: Measures distance or length along a line, as with rulers or tapes
• End Standard: Measures specific lengths, such as gauge blocks, often used for calibration or comparison
• Wavelength standard: Refers to a known reference source used to calibrate instruments that measure the wavelength of electromagnetic radiation, especially in optical and spectroscopic applications

Measurement Instruments

• Vernier Calliper
• Micrometer
• Vernier Height Gauge
• Vernier Depth Gauge

Errors in Measurements

• Systematic Errors: Consistent and reproducible errors due to inherent flaws in the measurement system, instruments, or environmental conditions.
• Random Errors: Unpredictable variations due to slight fluctuations in instruments, human error, or environmental factors.
• Gross Errors: Large, avoidable mistakes caused by human error, such as misreading instruments or incorrect data recording.
• Instrumental Errors: Due to limitations or defects in the measuring instrument itself.

Metrology is the scientific study of measurement. Its objectives include ensuring accuracy, stability, and comparability while minimizing costs. Metrology includes scientific, legal, and applied (industrial) branches, each serving distinct purposes from developing standards to ensuring fair trade.