Metrology for Common Man

Dr. Seemin Rubab
20 May is celebrated as World Metrology Day to commemorate the signing of the Metre Convention in 1875. The International Bureau of Weights and Measures (BIPM) at Paris was founded as an outcome of this treaty. It is the apex body responsible for scientific metrology. Among other responsibilities, it is the custodian of International Prototype Kilogram, a globally valid standard of measuring mass or weight. Standards are objects or ideas that are designated as being authoritative for some accepted reason. Whatever value they possess is useful for comparison to unknowns for establishing an assigned value based on the standard. The design of this comparison process for measurements is metrology. It is distinct from meteorology which is the science of weather.  The execution of measurement comparisons for the purpose of establishing the relationship between a standard and some other measuring device is known as calibration. The ideal standard is independently reproducible without uncertainty.   Metrology is not, however, exclusively the domain of scientists. It is something of vital importance to all of us. The intricate network of services, supplies and communications upon which modern life is dependent rely on metrology for their efficient and reliable operation. For example the economic success of nations depends upon the ability to manufacture and trade precisely made and tested products and components. Satellite navigation systems and international time correlation make accurate location possible – allowing the networking of computer systems around the world.  Human health depends critically on the ability to make accurate diagnosis, which is impossible without reliable measurements. If all these seem outlandish to you then consider an example from your day today life. Your daily grocery and vegetable purchase is impossible without metrology. The International Prototype Kilogram ensures that wherever you are in the world one kilogram actually weighs one kilogram. Every country maintains its own metrology system. Our National Prototype Kilogram is calibrated with International Prototype Kilogram and its working standards are supplied to various states and districts. National Physical Laboratory, New Delhi is the custodian of various prototypes in our country.  In addition to standards created by national and international standards organizations, many large and small industrial companies also define metrology standards and procedures to meet their particular needs for technically and economically competitive manufacturing. Metrology is an old science, which has evolved over many centuries.  The earliest systems of weights and measures were based on human morphology and naturally occurring substances. Consequently, these units of measurement varied from place to place. Although standardization of weights and measures has been a goal of social and economic advance since very early times, it was not until the 18th century that there was a unified system of measurement. One of the first natural measures was the metre, which was defined as being equal to the ten millionth part of one quarter of the terrestrial meridian. It was specified by measurements undertaken between Dunkerque and Barcelona. The kilogram was originally defined as the weight of a certain volume of water, a convenient and readily purified liquid. Later, it was learnt that the Earth’s surface is a terrible basis for a standard. The Earth is not spherical and it is constantly changing in shape. But the special alloy meter bars that were created and accepted in that time period standardized international length measurement until the 1950s. Careful calibrations allowed tolerances as small as 10 parts per million to be distributed and reproduced in metrology laboratories worldwide, in spite of the shortfalls of the meter’s original basis. Metrologists are therefore continuously involved in the development of new measurement techniques, instrumentation and procedures, to satisfy the ever-increasing demand for greater accuracy, increased reliability and rapidity of measurements.
Metrology may be divided into three subfields: scientific, industrial and legal metrology. Scientific metrology concerns with the establishment of quantity systems, unit systems, and units of measurement, the development of new measurement methods, realisation of measurement standards and the transfer of traceability from these standards to users in society. All of us had an orientation with scientific metrology in our high school physics curriculum. Most of us may recall that there are seven fundamental physical variables viz., mass, length, time, temperature, current, luminance and amount of substance.  Their corresponding units of measurements are kilogram, metre, second, Kelvin, Ampere, Candela and Mole. All measurements of all types are based on one or more of these independent units. Two supplemental independent units radian and stradian are also recognized internationally, both dealing with angular measurements. All around the world metrologists are trying to represent the seven base units in terms of constant of nature such as velocity of light in vacuum and Plank’s constant.
 Industrial or commercial metrology deals with the application of measurement science to manufacturing and other processes and their use in society, ensuring the suitability of measurement instruments, their calibration and quality control of measurements. Legal metrology concerns with regulatory requirements of measurements and measuring instruments for the protection of health, public safety, the environment, enabling taxation, protection of consumers and fair trade. It is important therefore to have reliable and accurate measurements which are agreed and accepted by the relevant authorities worldwide.
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