Vol.1 No.3 2009

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Research paper : A new density standard replaced from water (K. Fujii)−188 Synthesiology - English edition Vol.1 No.3 (2009) these measurements are converted to ITS-90, the current temperature scale, the density at 20.000 °C and 101.325 kPa is 13 545.854(3) kg/m3 [12]. Although the relative combined standard uncertainty of the measurement is reported to be 0.2×10-6, due to differences in isotropic compositions, there are relative deviations of about 1.7×10-6 in the density of mercury samples from different places of origin. Therefore, the relative combined standard uncertainty of the density of non-calibrated mercury is thought to be larger than 1×10-6 [13]. The uncertainty of the density of mercury not only affects the pressure standard[14], but also remains as the major uncertainty source in the absolute measurement of the universal gas constant R determined by the spherical resonator[15] and absolute measurement of Josephson constant KJ = 2e/h (here, e is the elementary charge and h the Planck constant) determined by the mercury electrometer[16].Since conventionally used liquid density standards have problems caused by inconsistencies in the measurements and uncertainty in the isotropic compositions, it is extremely difficult to construct a density standard with a relative uncertainty smaller than 1×10-6 from these data. On the other hand, the demands to accelerate the global Mutual Recognition Arrangement (MRA) and to clarify traceability of measurement have increased in the field of measurement standard. Particularly, there is increasing necessity for measuring the density in a way traceable to the definition of SI. In industry, density sensors with high-sensitivity such as vibrating tube densimeters are already used widely, but it has become difficult to provide sufficiently accurate density standard from conventional testing and certification for hydrometer calibration conducted under Measurement Law. Particularly in the brewing industry, alcohol concentration of alcoholic beverages are determined by density measurement using the alcohol table, and introduction of vibrating tube densimeters allowing automatic and high precision measurement had been considered. Therefore, there was increasing demand for building a traceability system in the Japan Calibration Service System (JCSS) established in the Measurement Law.Recently, high precision density standard has been required from science and technology such as the redefinition of SI and the determination of fundamental physical constants. Particularly, experimental research to redefine the kilogram, which is the only SI base unit still defined by material artifact, is being conducted by NMIJ and other national metrology institutes[17]. In the x-ray crystal density (XRCD) method to determine the Avogadro constant from absolute measurements of the density, lattice constant, and molar mass of silicon single-crystals, an absolute measurement of the density of isotopically enriched silicon single-crystals at a relative standard uncertainty of 1×10-8 is required[18].Considering these circumstances, AIST decided in 2001 to build a new density standard with silicon single-crystals as the top of traceability, and started providing reference standards for solid density that could not be handled by conventional measurement methods. To date, we have provided a new density standard traceable to the definition of SI to industries and users by developing new calibration technologies for hydrometer, density standard liquid, vibrating tube densimeter, solid material, thin film, and PVT properties.2 Need for new density standard2.1 Social demandIn petroleum, alcohol, brewing, and food industries, density of liquids is measured during manufacturing process and for quality control. Particularly, accurate measurement of alcohol concentration is essential for quality control of alcoholic beverages, ingredient labeling, distribution, and fair assessment of alcohol tax. In the Measurement Law, Specific Measuring Instruments are set for important measurements in certain economic activities and services, and Type Approval is conducted for the structure and specification of the measuring instruments. For the density measurement of liquids, standards are provided for density, specific gravity, and alcohol hydrometers. Hydrometer, also called the “float,” is a measuring instrument for density, and the scale of the alcohol hydrometer is calibrated using the alcohol table[19] that shows the relationship between the alcohol concentration and the density. Hydrometer scale was conventionally calibrated using the density of water. However, due to its structure, it is easily influenced by the surface tension of the liquid sample, so the relative standard uncertainty of density measurement by hydrometer is limited to be about 0.01 %. The relative standard uncertainty of alcohol concentration measurement based on this measurement is therefore about 0.1 %. Although this method is inexpensive, its scale must be read visually by the operator, and cannot be readily automated.The measurement method most widely used by the National Tax Agency (NTA) for assessing alcohol tax was the alcohol concentration measurement using alcohol hydrometers. Alcohol hydrometers certified according to the Measurement Law must be used in the Official Analysis Method of the NTA. This is because the data used for tax assessment must be fair, and the alcohol hydrometers certified by the Measurement Law was the only available alcohol measuring instrument with third-party verification.On the other hand, the demand for more precise density measuring instrument had increased in the brewing industry to improve quality control and to introduce automation. The industrial demand was to measure alcohol concentration at an uncertainty of 0.05 % for fine control of manufacturing process, and it was therefore necessary to supply a new density standard traceable to the national standard with an uncertainty of 0.005 %.(34)−

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