METROLOGY
Metrology literally means science of measurements. In practical applications, it is the enforcement, verification, and validation of predefined standards. Although metrology, for engineering purposes, is constrained to measurements of length, angles, and other quantities that are expressed in linear and angular terms, in a broader sense, it is also concerned with industrial inspection and its various techniques. Metrology also deals with establishing the units of measurements and their reproduction in the form of standards, ascertaining the uniformity of measurements, developing methods of measurement, analysing the accuracy of methods of measurement, establishing uncertainty of measurement, and investigating the causes of measuring errors and subsequently eliminating them.
The word metrology is derived from the Greek word ‘metrologia’, which means measure. Metrology has existed in some form or other since ancient times. In the earliest forms of metrology, standards used were either arbitrary or subjective, which were set up by regional or local authorities, often based on practical measures like the length of an arm.
It is pertinent to mention here the classic statement made by Lord Kelvin (1824–1907), an eminent scientist, highlighting the importance of metrology: ‘When you can measure what you are speaking about and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge of it is of a meagre and unsatisfactory kind. It may be the beginning of knowledge, but you have scarcely in your thought advanced to the stage of science.’
Another scientist Galileo (1564–1642) has clearly formulated the comprehensive goal of metrology with the following statement: ‘Measure everything that is measurable and make measurable what is not so.’
Metrology is an indispensable part of the modern day infrastructure. In fact, it plays an important role in our lives, either directly or indirectly, in various ways. In this competitive world, economic success of most of the manufacturing industries critically depends on the quality and reliability of the products manufactured—requirements in which measurement plays a key role. It has become increasingly essential to conform to the written standards and specifications and mutual recognition of measurements and tests, to trade in national and international markets. This can be achieved by the proper application of measurement methods that enhance the quality of products and the productive power of plants.
Metrology not only deals with the establishment, reproduction, protection, maintenance, and transfer or conversion of units of measurements and their standards, but is also concerned with the correctness of measurement. In addition to encompassing different industrial sectors, it also plays a vital role in establishing standards in different fields that affect human beings, such as health sciences, safety, and environment. Hence, one of the major functions of metrology is to establish international standards for measurements used by all the countries in the world in both science and industry.
Modern manufacturing technology is based on precise reliable dimensional measurements. The term ‘legal metrology’ applies to any application of metrology that is subjected to national laws or regulations. There will be mandatory and legal bindings on the units and methods of measurements and measuring instruments. The scope of legal metrology may vary considerably from one country to another. The main objective is to maintain uniformity of measurement in a particular country. Legal metrology ensures the conservation of national standards and guarantees their accuracy in comparison with the international standards, thereby imparting proper accuracy to the secondary standards of the country. Some of the applications of legal metrology are industrial measurement, commercial transactions, and public health and human safety aspects.
A group of techniques employed for measuring small variations that are of a continuous nature is termed as ‘dynamic metrology’. These techniques find application in recording continuous measurements over a surface and have obvious advantages over individual measurements of a distinctive character.
The metrology in which part measurement is substituted by process measurement is known as ‘deterministic metrology’. An example of deterministic metrology is a new technique known as 3D error compensation by computer numerical control (CNC) systems and expert systems, leading to fully adaptive control. This technology is adopted in high-precision manufacturing machinery and control systems to accomplish micro and nanotechnology accuracies.
