Stability of the Mass Values of National Prototype Kilogram

It is well known fact that out of seven SI base units, only "kilogram" is still an artifact. Since kilogram has no link with any Physical Constants, measurement of International Prototype of the Kilogram (IPK) is not possible and only considered equal to exactly "1 kg" immediately after cleaning and washing by the method used at the International Bureau of Weights and Measures (BIPM). Although a lot of R & D is going on throughout the world to link the kilogram to physical constant like Plank Constant, Avogadro Constant etc. The National Physical Laboratory, India (NPLI) is the custodian of National Prototype of the Kilogram (NPK) No. 57, which was provided by the BIPM in 1958 after its first calibration in 1955. The NPK has been recalibrated in 1985, 1992 & 2002 so for at the BIPM. Beside this, other derived parameters like Force and Hardness Standards, Pressure and Vacuum Standards, Fluid Flow Standards etc. are directly related to Mass Standards. Now-a-days Chemical Metrology also requires traceability of Mass Standards. Therefore, stability of the NPK is a very important issue and the instability factors should be taken into consideration for every measurement. From the previous data, the change in mass values are estimated 218 mg from 1955 to 1985 (7.3 mg/year), 36 mg from 1985 to 1992 (5.1 mg/year) and 24 mg from 1992 to 2002 (2.4 mg/year) between after cleaning and washing of previous calibration to before cleaning and washing of next calibration. Now, the recalibration period of NPK is maximum 10 years. In this paper, the BIPM method, NPL, UK formula and NMIJ, Japan formula are used to evaluate the change in mass values of NPK No. 57.