Goodbye Le Grand K

All of us had a brush 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, meter, 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 constants of nature such as velocity of light in vacuum and the  Planck’s constant. It is important therefore to have reliable and accurate measurements which are agreed and accepted by the relevant authorities worldwide.

Every year, May 20 is celebrated as World Metrology Day to commemorate the signing of the Meter 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  Le Grand K or the International Prototype Kilogram (IPK)  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. 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. 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. 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. 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. Our daily grocery and vegetable purchase is impossible without metrology. The International Prototype Kilogram ensures that wherever we are in the world one kilogram actually weighs one kilogram. Every country maintains its own metrology system. The National Physical Laboratory, India is the custodian of National Prototype of the Kilogram, copy No. 57 (NPK-57), which was provided by the BIPM in 1958 after its first calibration in 1955. The NPK-57 has been recalibrated in 1985, 1992, 2002 & 2012 so far at the BIPM. It is served as primary standard in India. The mass value of NPK-57 is disseminated to NPL mass standards from 1 mg to 2,000 kg, solid density standards and other derived parameters.