Vol.1 No.3 2009

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Research paper : A new density standard replaced from water (K. Fujii)−193 Synthesiology - English edition Vol.1 No.3 (2009) using magnetic suspension, it can measure the density of fluids under high-pressure and liquids with high vapor pressure without influence of surface tension at meniscus.By using silicon single-crystals as a sinker, and calibrating its density by hydrostatic weighing, traceable density standard liquids can be supplied in a wide temperature and pressure range. The relative standard uncertainty of the density standard liquids calibrated by this principle is 7×10-6. The measurement result at AIST is used as a reference value when conducting performance test to verify the correctness of measurements of registered calibration service providers of JCSS.4 Investigation for uncertainty and international equivalence As mentioned in Section 3.2, in Japan, the density of silicon spheres is determined by absolute measurements of their diameters and masses, and they are designated as the National Primary Standard of density by Measurement Law. The values of the National Primary Standard and uncertainty are shown in Table 2. This is based on the absolute measurements of the density conducted by AIST until 2005, and the relative combined standard uncertainty of the density is 1.2×10-7.To verify the correctness of the absolute values and uncertainty of Japanese solid density standard and to confirm the international equivalence are important issues in advancing MRA of measurement standard and to demonstrate the reliability of Japanese density measuring instruments. Therefore, the author distributed questionnaires to national metrology institutes of various countries and surveyed the density calibration method and the status of density reference in each country as key comparison study for the Working Group on Density of the CCM. As a result, it was found that although there were very few national metrology institutes that have independent standard for absolute value of density like AIST, many countries have already started to establish traceability based on solid density standard such as silicon single-crystals instead of water. In this key comparison, designated as CCM.D-K1, a silicon sphere of NMIJ was transported to participating countries from 2001 to 2002, to evaluate the international equivalence of the density standards. The participating countries measured the density of the silicon sphere by hydrostatic weighing and the values were compared.The measurements at national metrology institutes of eight countries including NMIJ are shown in Figure 6. The value of NMIJ showed smallest uncertainty, and the values of other participating countries fell in the range of uncertainty. The reference value calculated from the weighted mean of these values is most reliable as an absolute density value traceable to the definition of SI. The value of NMIJ agrees very well also with the reference value. This verifies the high reliability of the absolute and relative density measurements conducted at the NMIJ.Sphere S5Volume cm3 429.601242 429.615 387 0.119Mass g 1000.578 619 1000.612 019 0.016Density kg•m-3 2329.086 89 2329.087 95 0.120Relative combined standarduncertainty uc,r/10-6Physical quantityUnitSphere S46420-2-4-6NMIJPTBIMGCKRISSMETASNRCCEMCENAMReference ValueRelative density defference in 10-6(39)−Table 2. Absolute values of the volume, mass, and density of Primary Standards S4 and S5 at 20 °C and 101.325 kPa.Fig. 6 Measurement result of the key comparison CCM.D-K1 organized by the Working Group on Density of the CCM, CIPM. A 1 kg Silicon sphere was transported as a travelling standard to NMIs of participating countries, and the density of the travelling standard was measured using hydrostatic weighing apparatus using solid density standard of the participating institute. Error bar expresses the expanded uncertainty (k=2). Participating metrological institutes were NMIJ (Japan), PTB (Germany), IMGC (Italy, currently INRIM), KRISS (Korea), METAS (Switzerland), NRC (Canada), CEM (Spain), and CENAM (Mexico). Reference value is the weighted mean of measurements of all participants.To precision electronic balanceSample outletElectromagnePermanent magnetSilicon single-crystalWeight exchange mechanismDetector for position of suspensionSample inletFig. 5 Magnetic suspension densitometer.

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