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.
The kilogram was originally defined as the weight of a certain volume of water, a convenient and readily purified liquid. That was however an inconvenient quantity to precisely replicate, so in 1799 a platinum artifact ‘Kilogram of the Archives’ was fashioned to define the kilogram. That artifact, and the later IPK, have been the standard of the unit of mass for the metric system ever since. The IPK is made of a platinum alloy known as “Pt10 Ir”, which is 90% Platinum and 10% Iridium and is machined into a right-circular cylinder of about 39 millimeters height and same diameter to minimize its surface area. The addition of 10 % iridium improved upon the all-platinum Kilogram of the Archives by greatly increasing hardness while still retaining platinum’s many virtues: extreme resistance to oxidation, extremely high density satisfactory electrical and thermal conductivities, and low magnetic susceptibility. The IPK and its six sister copies are stored at the BIPM in an environmentally monitored safe in the lower vault located in the basement of the BIPM’s Pavilion de Breteuil in Saint Cloud on the outskirts of Paris. The IPK is rarely used or handled. Copies of the IPK kept by national metrology laboratories around the world were compared with the IPK in 1889, 1948, and 1989 to provide traceability of measurements of mass anywhere in the world back to the IPK. In spite of best efforts to maintain it, the IPK has diverged from its replicas by approximately 50 micrograms since their manufacture late in the 19th century.
This led to efforts to develop measurement technology precise enough to allow replacing the kilogram artifact with a definition based directly on physical phenomena, which is now scheduled to take place on 20 May 2019. The new definition is based on the Planck’s constant, thereby fixing the value of the kilogram in terms of the second and the meter, and eliminating the need for the IPK. Since the second and meter are defined completely in terms of physical constants, the kilogram is defined in terms of physical constants only. All one needs now to have a Kibble balance to find the value of one kilogram accurately. The grand K will be just an artifact henceforth. After the kilogram’s definition is changed officially- on 20th May, 2019, most people will never notice the difference. It would not change baking ingredients on a kitchen scale or our daily purchase of grocery. National Physical Laboratory, Director D.K. Aswal said the facility to produce ‘Kibble balance’ machines to be used across the country will be ready in the next three-four years. It is expected to cost Rs 60 crore.
Dr Seemin Rubab is HoD Physics, NIT Srinagar