Vol.1 No.3 2009

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Research paper : A new density standard replaced from water (K. Fujii)−192 Synthesiology - English edition Vol.1 No.3 (2009) (38)−3.3.1 Density calibration of solid material by hydrostatic weighingFigure 2 shows the structure of hydrostatic weighing apparatus that was developed to calibrate the density of solid materials using silicon spheres[32]. Tridecane, (n-C13H28) that has low surface tension and stable density, is used as a working liquid. Density difference is measured with a relative standard uncertainty of 4×10-8 by alternately weighing the silicon spheres and the solid sample in the liquid. To correct the effect of density gradient due to temperature distribution and gravity in the liquid, the solid sample is placed between the two silicon spheres placed in vertical direction. This apparatus is now used to calibrate the density of arbitrary solid materials such as stainless steel weight, glass, semiconductor crystal, and precious metals. The density of silicon single-crystals used as secondary standard in JCSS is also calibrated with this apparatus. Figure 3 shows a photograph of the apparatus and density-calibrated silicon single-crystals.3.3.2 Hydrometer calibration by the weighing methodFigure 4 shows the principle of scale calibration for hydrometers by hydrostatic weighing. By measuring the buoyancy force acting on the hydrometer in a working liquid using electronic balance, the scale on the stem of hydrometer can be calibrated[33]. In the conventional weighing method where water was used as the density standard, it was difficult to calibrate the hydrometer scale with a small uncertainty since the surface tension of water is high and sensitive to surface contamination. Currently, tridecane is used as the working liquid instead of water. Therefore, the method involves calibrating the density of tridecane by hydrostatic weighing using a ring-shaped silicon single-crystals with calibrated density (National Secondary Standard). It measures the density of the working liquid near the body of the hydrometer. Standard Instrument Testing according to the Measurement Law and hydrometer scale calibration in JCSS are conducted using this method.3.3.3 Calibration of density standard liquid using magnetic suspension densitometerVibrating tube densimeters[34] are used in various fields including petroleum, alcohol, brewing, food industries, as well as for medical testing as a high-sensitivity density measuring device. Normally, since calibration is done using only water and air as the density reference materials, the uncertainty of the density measurement is relatively large when used in a range that departs from the density of the reference materials. Therefore, reliability and traceability of the vibrating tube densimeters can be maintained by supplying several density standard liquids in a range of about 0.5 g/cm3 to 2.0 g/cm3.Figure 5 shows the magnetic suspension densimeter developed at the AIST to calibrate the density standard liquids[35]. This densitometer measures density of fluid according to the same principle as the hydrostatic weighing shown in Figure 2. Since it uses non-contacting mechanism To precision electronic balanceTridecaneHydrometerSilicon single-crystalTemperature-controlled bathWeight exchange mechanismFig. 2 Structure of hydrostatic weighing apparatus.Fig. 3 Hydrostatic weighing apparatus (left) and silicon single-crystals calibrated for density as National Secondary Standard (right). Other than sphere, silicon single-crystals of various forms such as cylinder and ring are used as Secondary Standard.Fig. 4 Ring shaped silicon single-crystal used for calibrating hydrometers.

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