Redefining the Kilogram

Topics: Kilogram, Mass, International System of Units Pages: 18 (5719 words) Published: October 11, 2013




It is only the Kilogram in the International System (SI) that is defined by a monument: the International kilogram prototype (IPK). The feasible changes in International prototype Kilogram (IPK) may result in imbalances in international system (SI) unit as some other units in the SI depend on the kilogram for their definition and therefore, a distinct definition of the kilogram became a necessity on the agenda of the International metrology. A meaningful attempt is been proposed to redefine the kilogram centred on basic constants, and a major agreement presently thrives to define the kilogram in relation to the Planck Constant “h”. To accomplish this redefinition, a consistent realization of the kilogram centred on several analysis is necessary. At the moment, measurement of the Planck Constant based on the Silicon Sphere Avogadro and Watt balance approaches have been concluded, but there exists certain disagreements in the results which mar the achievement within limit of allowable uncertainty of “2 X 10-8”.The Proposed Joint Report Project need to improve on these analysis with the objective of resolving the disagreements in results of the various experiments, hence, allowing the redefinition of the Kilogram.


Currently, the Kilogram is the only International System of Units (SI) been defined on a monument. Way back 1889, mass metrology had depended on the International Prototype of the Kilogram (IPK) which is defined as mass of absolutely 1kg. Large changes in the International Prototype Kilogram (IPK) and both the official and national prototypes exist, over the past 120 years, this drift is found to be as much as 50µg. A meaningful international work is been proposed to redefine the kilogram centred on basic constants, it’s now generally agreed that the redefinition of kilogram shall be centred on an established numerical value of Planck Constant “h”. Two distinct experimental methodologies are being developed, fundamentally, the watt balance approach is centred on the similarity between fundamental mechanical and electrical powers and defines kilogram to a basic constant by the quantum electrical definitive and on the other hand, the X-ray crystal mass approach (Avogadro Project) that defines Avogadro Constant (NA) by measuring both apparent and atomic densities of a distinct spherical silicon crystal. The necessary factor to achieve this new definition is a consistent realization of the kilogram through distinctive experiments. This new definition should not bring further uncertainties into the series nor alter the current accuracy system of the mass standard as contained in EMRP call 2011. The Comite Consultatif pour la Masse et les Grandeurs Apparentees (CCM) sanctioned that the listed requirements be met if a redefinition of the kilogram is to be adopted: 1. Not less than 3 separate experiments be performed including from Watt balance and Avogadro Projects which gives values of Planck Constants with respective uncertainties not more than 5 X 10-8. 2. Not less than 95% consistency in the rate of the Planck Constant obtained in each experiment. 3. Provision of a system of traceability of the BIPM prototypes to various national prototype of the International prototype Kilogram. At the moment, the level of consistency from different Watt balance and Avogadro experiment and the uncertainties in each result are not adequate to meet the conditions above and efforts should be geared towards resolving the discrepancies and making the uncertainties minimum.


A definition for the gram was decreed in France in 1795 and this stated that “the total weight of a volume of water equals to the cube of the hundredth part of the meter, at the temperature of...
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