1. What is Measurement ?
Measurement is the process of comparing an unknown quantity with a known standard. In engineering, accurate measurement ensures quality, interchangeability, safety, and reliability of components.
Standards of measurement are defined references used to maintain uniformity and accuracy.
The science of measurement is called Metrology.
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2. Need for Standards of Measurement
Standards are essential because:
- To ensure uniformity and consistency in measurement.
- To achieve accuracy and precision in manufacturing.
- To enable interchangeability of parts.
- To maintain quality control in industries.
- To support scientific research and development.
- To facilitate international trade and communication.
3. International System of Units (SI Units)
The globally accepted system is maintained by the International Bureau of Weights and Measures.
It defines base units such as:
| Quantity | SI Unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Temperature | kelvin | K |
| Electric current | ampere | A |
| Luminous intensity | candela | cd |
| Amount of substance | mole | mol |
4. Classification of Standards of Measurement
Standards are classified in a hierarchical order to maintain accuracy.
4.1 International Standards
These are the highest standards and are maintained at the international level.
Features:
- Maintained by international organizations.
- Used for comparison and calibration of national standards.
- Highly accurate.
Example:
- Standard metre, kilogram, etc.
They are maintained in laboratories such as in Sèvres (France).
4.2 Primary Standards
These are the most accurate standards within a country.
Features:
- Used to calibrate secondary standards.
- Maintained in national laboratories.
- Rarely used in routine measurements.
In India, these are maintained by the National Physical Laboratory.
4.3 Secondary Standards
These are calibrated using primary standards.
Features:
- Used for calibration in industrial laboratories.
- Less accurate than primary standards.
4.4 Tertiary or Reference Standards
These are used in industries for calibration.
Features:
- Compared with secondary standards.
- Used in workshop laboratories.
4.5 Working Standards
These are used for day-to-day measurements in industries.
Features:
- Frequently used.
- Less accurate.
- Subject to wear and need regular calibration.
Example:
Vernier calipers, micrometers, gauges.
5. Types of Measurement Standards
5.1 Line Standard
Measurement is done by comparing lengths marked on the instrument.
Example:
- Scale, ruler.
Advantages:
- Simple and quick.
Disadvantages:
- Less accurate.
5.2 End Standard
Measurement is made between two flat surfaces.
Example:
- Slip gauges.
Advantages:
- Highly accurate.
Disadvantages:
- Time-consuming.
6. Characteristics of Good Standards
A good measurement standard should:
- Be accurate and precise.
- Be stable and durable.
- Not change with temperature, humidity, or time.
- Be easy to reproduce.
- Be universally accepted.
- Be accessible for calibration.
7. Calibration
Calibration is the process of comparing a measuring instrument with a standard.
Purpose:
- To maintain accuracy.
- To reduce measurement errors.
- To ensure reliability.
8. Traceability
Traceability means linking a measurement result to a national or international standard through an unbroken chain.
This ensures:
- Accuracy
- Reliability
- Standardization
9. Importance in Engineering
Standards of measurement are important in:
✔ Manufacturing industries
✔ Quality control
✔ Machine design
✔ Aerospace and automobile sectors
✔ Construction and civil engineering
✔ Research laboratories