Introduction to Metrology

Metrology is the science of measurement. It covers the theory and practice of measuring physical quantities (length, angle, mass, time, temperature and so on) with the required accuracy, together with the standards, instruments and methods used.

Scope

• Selection, design and calibration of measuring instruments and standards.
• Establishing units and their traceability to national and international standards.
• Study of measurement errors, uncertainty and the capability of instruments.
• Inspection and testing of dimensions, form, surface finish, screw threads and gears.
• Checking the geometry and accuracy of machine tools.
• Supporting quality control, interchangeable manufacture and legal trade.

Objectives

1. To evaluate the performance of a new product or design against specification.
2. To determine the process capability and to select a suitable process or machine.
3. To decide the accuracy needed so that the instrument is neither inadequate nor needlessly costly.
4. To inspect raw material, components and finished products so that they are within limits and interchangeable.
5. To maintain the accuracy of measuring instruments by periodic calibration.
6. To minimise the cost of inspection and rejection by early detection of defects.
7. To standardise methods of measurement and to prepare gauges, drawings and specifications.
8. To find the causes of errors and rejected parts so that the process can be corrected.

Legal metrology

It deals with the units of measurement, the measuring instruments and the methods of measurement that are covered by law, to protect the public in trade, health, safety and the environment. Examples are weighing scales in shops, petrol pump meters, electricity meters and taxi meters. A government body verifies, stamps and periodically re-inspects such instruments, and unfair or inaccurate instruments are penalised.

Dynamic metrology

It is the measurement of quantities that change rapidly with time, using instruments that respond fast, so that the variation of the quantity is recorded continuously rather than as one fixed value. Examples are vibration, pressure pulses in an engine cylinder, surface roughness traced while the stylus moves, and measurement of dimensions of a component on a moving production line.

Deterministic metrology

Here the measurement is carried out in a fully controlled and described way so that the result of the process can be predicted from a mathematical model of the measuring system. Instead of relying only on repeated trials, errors are calculated and corrected (for example thermal expansion, elastic deformation of the instrument). It is used in high-accuracy work such as CMMs and gauge block calibration, where the part is treated as a known geometrical form and the measurement is automated.

SI base units