8.1 Metrology and Measurement

8.1 Metrology and Measurement

1. Measurement Principles

1. Definition of Measurement:

   • Quantitative comparison of unknown quantity with predefined standard.

   • Process of obtaining magnitude of physical quantity.

2. Fundamental Elements:

   • Measurand: Quantity to be measured.

   • Standard: Reference for comparison.

   • Instrument: Device for comparison.

   • Observer: Person/system performing measurement.

3. Basic Requirements:

   • Standardization: Using accepted references.

   • Traceability: Chain of comparisons to national/international standards.

   • Reproducibility: Same results under same conditions.

4. Measurement Methods:

   • Direct: Value obtained directly from instrument.

   • Indirect: Value calculated from other measurements.

   • Comparative: Comparison with known standard.

   • Null Method: Balancing to achieve zero difference.

2. Errors and Calibration

1. Error Types:

   • Systematic Errors:

     • Consistent, predictable deviations.

     • Due to instrument defects, environmental factors, observer bias.

     • Correctable through calibration.

   • Random Errors:

     • Unpredictable, statistical variations.

     • Due to unknown causes, environmental fluctuations.

     • Reduced by averaging multiple readings.

   • Gross Errors:

     • Large, obvious mistakes.

     • Due to misreading, incorrect recording.

     • Eliminated by careful technique.

2. Calibration Process:

   • Comparing instrument with higher accuracy standard.

   • Establishing correction factors.

   • Documenting instrument performance.

3. Calibration Terms:

   • Standards Hierarchy: Primary → Secondary → Working standards.

   • Calibration Interval: Time between calibrations.

   • Calibration Certificate: Documented results.

4. Error Reduction Techniques:

   • Regular calibration.

   • Environmental control.

   • Proper training.

   • Statistical analysis.

3. Accuracy and Precision

1. Accuracy:

   • Closeness to true value.

   • Measures correctness.

   • Affected by systematic errors.

2. Precision:

   • Repeatability of measurements.

   • Measures consistency.

   • Affected by random errors.

3. Key Differences:

   • High Accuracy, Low Precision: Average correct but scattered.

   • Low Accuracy, High Precision: Consistent but offset.

   • High Accuracy, High Precision: Ideal measurement.

   • Low Accuracy, Low Precision: Poor measurement.

4. Related Terms:

   • Resolution: Smallest detectable change.

   • Sensitivity: Output change per input change.

   • Linearity: Constant sensitivity over range.

   • Hysteresis: Different readings for increasing/decreasing inputs.

5. Quantification:

   • Accuracy: Often expressed as ± percentage of full scale.

   • Precision: Measured by standard deviation.

4. Inspection and Acceptance Testing

1. Inspection Purpose:

   • Verify conformance to specifications.

   • Ensure quality standards.

   • Prevent defective products.

2. Inspection Types:

   • 100% Inspection: Every item checked.

   • Sampling Inspection: Statistical sample checked.

   • First-article Inspection: First production item.

   • In-process Inspection: During manufacturing.

   • Final Inspection: Before shipment.

3. Acceptance Testing:

   • Formal verification against requirements.

   • Often involves destructive/non-destructive testing.

   • Decision to accept/reject batch.

4. Statistical Quality Control:

   • Control charts for process monitoring.

   • Acceptance sampling plans.

   • AQL (Acceptable Quality Level) determination.

5. Measurement Uncertainty:

   • All measurements have associated uncertainty.

   • Must be reported with measurement results.

   • Affects acceptance decisions.

6. Documentation:

   • Inspection reports.

   • Test certificates.

   • Non-conformance reports.

   • Corrective action records.