Page 65 - Science India August 2022
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          system (French metric system) on 20 May which is now celebrated as the World Metrology Day. In 1939, the Central Leg- islature passed the Standards of Weights Act applicable to the whole of British In- dia and established the standard grain in terms of the platinum iridium 1b cylinder placed in the custody of the Mint Master Bombay. The act was enforced from 1 July 1942 and the Indian Standard Insti- tute (ISI) was established in 1946.
After Independence, in 1955, India accepted the International System of Units (SI) officially and the national pro- totype no. 57 of mass standard taken as 1 kilogram made of platinum-iridium is preserved at the CSIR-National Physi- cal Laboratory, the National Metrology Institute (NMI) of India by the Act of Parliament in 1957. The original proto- type copy is kept at the International Bu- reau of Weights and Measures (BIPM), Paris, and is by definition taken as a unit of mass.
Now, seven (7) units, namely metre (m) for length, second (s) for time, kilo- gram (kg) for mass, kelvin (K) for tem- perature, ampere (A) for electric current, mole (mol) for amount of substance, and candela (cd) for luminous intensity, are defined as basic standard units and all the other units are derived from these basic units. The responsibility of defin- ing units lies on the International Com- mittee of Weights and Measurements (CIPM) and is accepted during the Gen-
eral Conference on Weights and Mea- sures (CGPM). Thereafter, the units are realised at the national level by National Metrology Institutes (NMIs) of the re- spective countries. The national standard copies and primary instruments need to be matched with different NMIs at fixed intervals of time to maintain these pri- mary value units in the country. The issue with the prototype based unit definition is that variations in them occur and er- ror/uncertainty in its values arises with the passage of time. So, a need was felt to redefine more precise and constant values for standard units. Therefore, dur- ing the 26th CGPM meeting in 2018, a new definition of four basic units (mass, ampere, kelvin and mole) were accepted, based on universal physical constants, namely, the Planck’s constant (h), electric charge (e), Boltzmann constant (kB) and Avogadro constant (NA), and enforced from 20 May 2019.
Each nation has its own National Me- trology Institute or simply NMI. The re- sponsibilities of the NMIs are to develop, upgrade and maintain national measure- ment standards and disseminate SI units and if non-existent yet then to other in- ternationally agreed references. It has to aid international recognition of national measurement standards and associated measurement capabilities (CMCs). The
NMIs have to continuously sharpen/ strengthen themselves to improve their CMCs by sharing through a metrologi- cal valid measurement data by a valid international laboratory comparison programs called key comparison, and then disseminate to the consumer. These NMI participate in CGPM meetings and international comparison of measure- ments to maintain the same standard as the rest of the world. Besides being custodian of standards and maintaining them, NMIs also provide various ser- vices pertaining to metrology, such as calibration, production of certified refer- ence materials, and assigning value to in- house reference materials of customers, among others. Besides being the custo- dian of national standards, the CSIR- NPL has attained about 239 CMCs and disseminated more than 100 indigenous certified reference materials known as Bharatiya Nirdeshak Dravya (BND) in different categories for various param- eters. These are the primary standards to ensure reliability and comparability of the measurements as a benchmark for quality assurance achieved through in- ternational networking. BNDs produced by CSIR-NPL are (i) in-house and (ii) in association with Reference Material Pro- ducers (RMPs) across the country as per ISO 17034 &17035. These standards are being produced through international key comparison, proficiency/round-robin testing, pilot study in various sectors like physico-mechanical, physico-chemical, foods, feeds, biomedical, environmental, health-care, agricultural etc.
Now when the units are all defined and established, the question arises on how to ensure dissemination of these units and thereby provide accurate and pre- cise measurements to the stakeholder. To measure ‘1 m’, one takes a metre rod, for ‘1 L’ of liquid, one takes a measuring cylinder. But how to ensure that ‘1 m’ or ‘1 L’ is exactly the same? The answer to this question is ‘calibration’.
Calibration is defined in Internation- al Vocabulary of Metrology as operation performed on a measuring instrument or a measuring system that, under speci-
 Traceability Pyramid at the National Physical Laboratory, New Delhi

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