Short notes on branches of metrology

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 the use of unfair or inaccurate instruments is 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 the 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 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 and elastic deformation of the instrument). In this approach, measurement of the process largely replaces measurement of each part. It is used in high-accuracy work such as CMMs, error-compensated CNC machines and gauge block calibration, where the part is treated as a known geometrical form and the measurement is automated.

SI base units