Vol.1 No.3 2009

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Research paper : A new density standard replaced from water (K. Fujii)−194 Synthesiology - English edition Vol.1 No.3 (2009) For the density standard liquids, key comparison CCM.D-K2 lead by the Working Group on Density of the CCM was conducted and completed in 2005. International equivalence for the calibrations at the NMIJ was thus verified also in this key comparison.5 Establishment of traceability system for density and contribution to societyAIST cooperated with the National Institute of Testing and Evaluation (NITE), which is the accreditation body of JCSS, since 2000, and started creating guideline for technological application necessary for implementing calibration service for density based on ISO/IEC 17025 standard. This guideline describes the methods for: maintaining traceability from silicon spheres designated as the National Primary Standard; calibration method of hydrometer, density standard liquid, and vibrating tube densimeter, and the frequency of calibration. Considerations and investigations were conducted as joint work with several candidate calibration service providers. In preparing the guideline, as much freedom as possible was given to the calibration system built by registered calibration service providers, while ensuring accurate evaluation of the uncertainty of the calibration, to allow evolution of density calibration service into various forms in the future. From 2001 when the first technological application guideline for the density calibration service was set up, AIST personnel cooperated as technical advisor in the certification process conducted by NITE.Figure 7 shows the traceability system constructed on the absolute and comparative measurements for density. The silicon spheres S4 and S5 for which the absolute measurement of density were conducted are set at the top of traceability (National Primary Standard), and silicon single-crystals calibrated by hydrostatic weighing (see Figures 2 and 3) are used as the Secondary Standard at the registered calibration service. In JCSS, registered calibration service provider that fulfilled the ISO/IEC 17025 standard may use the Secondary Standard to calibrate the users’ measurement instruments such as hydrometer, density standard liquid, and vibrating tube densimeter.The number of provision of density standard by JCSS increased drastically since 2001, and in 2007, about 6,000 calibration certificates were issued for a year for users’ density measuring instruments. Particularly, for alcohol concentration measurement of alcoholic beverages, “measurement of alcohol concentration using vibrating tube densimeter” was conventionally approved for use as “method that is deemed practical and accurate, although it is listed in the Official Analysis Method of the National Tax Agency,” and application to the National Tax Agency was required to use the vibrating tube densimeter. Also, for alcohol table to convert density to alcohol concentration, values were provided by the Official Analysis Method, and it was necessary for AIST to compare the values with the alcohol table used in Measurement Law before introducing the vibrating tube densimeters calibrated according to JCSS for use in assessing alcohol tax. Therefore, AIST collaborated with the National Tax Agency and disclosed the alcohol table used by AIST on the website, and the people in the brewing industry can readily refer to the AIST alcohol table. It was now possible to use the alcohol concentration measured by the vibrating tube densimeters calibrated according to JCSS as basis of tax assessment by Alcohol Tax Law. Hence, vibrating tube densimeters came into wide use, and in 2007, the National Tax Agency decided to deregulate, and “measurement of alcohol concentration by the vibrating tube densimeter” was included in the Official Analysis Method. As long as the vibrating tube densimeters are calibrated by the density standard liquids provided by the registered calibration service certified by JCSS, it may be used for measurement of alcohols concentration for tax assesment. The Japanese government thus recognized the reliability of the densimeters approved by JCSS, and the use in the brewing industry is increasing steadily.6 Development into new measurement evaluation technologyThe comparative measurement technology for density employed by JCSS is described in Section 3.3, and AIST is developing other new density comparison technologies for material science and energy conservation. Comparison of the densities between silicon single-crystals using the pressure-of-flotation method[36] is a technology originally developed to improve the accuracy of the Avogadro constant. It detects very small density distribution in the crystal, and now it is applied to density measurement of thin films[37], since the measurement sensitivity is extremely high and can detect a relative density difference of 10-7 to 10-8. The density of thin (1~2×10-4)(5~20×10-6)(1~50×10-6)(1~5×10-4)(1~5×10-5)Standard of basic quantityNational Primary StandardSecondary StandardMeasuring instrument to be calibrated by calibration serviceOptical frequencyPrototype KilogramLength standard (m) Mass standard(kg)Optical interferometryMass measurementSilicon single-crystals (0.3 to 10×10-6)Silicon sphere (0.2×10-6) Vibrating tube densitometer(Comparison method)Weight and solid materialDensity standard liquidHydrometer(Weighing method)(Hydrostatic weighing method)Hydrometer(Hydrostatic weighing method)(Hydrostatic weighing method)(40)−Fig. 7 Traceability system with density of silicon sphere at the top. Values in parentheses show the estimated relative expanded uncertainty at 95 % confidence level.

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