Vol.3 No.3 2010

Research paper : National electrical standards supporting international competition of Japanese manufacturing industries (Y. Nakamura et al.)−216−Synthesiology - English edition Vol.3 No.3 (2010) the standards is based on the Japan Calibration Service System (JCSS) of the Measurement Law. This builds the hierarchy of standard dissemination as shown in Fig. 1. The high-precision basic range national standard of AIST is expanded by the calibration labs of each tier, and is disseminated promptly to the site of production. Specifically, AIST develops and organizes the 10 pF, 100 pF and 1000 pF capacitance standards, and disseminates these to the upper-tier calibration labs. The upper-tier calibration labs may, for example, expand the calibration range to 1 F based on the 10 pF standard, and this is provided to the lower-tier calibration labs. The lower-tier calibration labs may further expand the calibration range to provide the capacitance standard to the sites of production. By building this system, the necessary range of capacitance standard can be disseminated to the manufacturers’ site of production when needed, while maintaining the link to the national standard. This means that the metrological traceability system of the measurement device or the capacitor at the site of production to the national standard can be established efficiently.In building this standard dissemination system, the role of calibration labs at each tier, particularly the role of uppermost-tier calibration labs, is extremely important. Therefore, AIST must not only develop and disseminate the national standard, but also provide support to improve the technical skills of the calibration labs. Also, a standard (reference standard for skill examination) will be necessary to evaluate and judge the technical skills. This is because if the calibration lab expands, for example, to 1F or 10 F based on the 10 pF national standard, it is necessary to check whether the expanded result is correct or wrong. Therefore, as shown in Fig. 1, the range of the capacitance standard can be expanded to some extent at AIST (i.e. expansion to 1 F or 10 F based on 10 pF, 100 pF and 1000 pF), and these are used as reference standards to check the techniques of the calibration labs. To plan and organize all the standards that must be developed, the standard organization plan is created for each fiscal year as shown in table 1. The resource allotment is planned according to this plan to develop and realize the standards. Since the understanding and cooperation of the Japanese calibration labs are necessary to achieve this system, we set out to build consensus by actively exchanging opinions with industry at committees and research presentations for standards.3 Development of the capacitance standard3.1 Selection of the methodTwo methods have been recognized in the world as ways to realize the capacitance standard. One is the method of using the specially shaped capacitor called the cross capacitor. As shown in Fig. 2, according to A.M. Thompson and D.G. Lampard, if the four electrode rods arranged parallel to each other, and the value per unit length of the capacitance (cross capacitance) between the two sets of opposing electrodes are set as C12 and C34, the average values of C12 and C34 can be expressed in the following equation[7]:(C12+C34)/ 2 =( 0ln2)/ (1)As seen from the above equation, the average value of the cross capacitance per unit length is dependent only on the permittivity 0 between the electrodes. If the entire cross capacitor is placed in a vacuum, the cross capacitance per unit length will be 1.953549043… pF, and this is not dependent on the shape of the electrode. This means that if the length of the electrode rod is determined accurately, the capacitance can be determined by the length standard of the cross capacitor. However, the condition that makes equation (1) valid assumes that the four electrode rods are infinitely long. Therefore, the capacitance for unit length of electrode rod of infinite length is expressed by equation (1). Therefore, to actually realize the cross capacitor, it is necessary to insert a separate guard electrode between the four electrodes. The area where the guard electrode is inserted will have capacitance zero. When the guard electrode is moved in this state, the capacitance increases or decreases in accordance to the distance transferred. If the cross capacitance for the transferred distance of the guard electrode is calculated, it will follow equation (1). Many NMIs have established the capacitance standard using this method[2]-[5]. However to fabricate the actual cross capacitor, the precise machining of the electrode rods is extremely important. The surface roughness and the degree of parallel-ness of the electrode rods will directly affect the uncertainty Fig. 2 Cross capacitorTable 1 Organization plan for the capacitance standard20012002200320042005200620072008200920101000 µF100 µF10 µF1 µF0.1 µF0.01 µF1000 pF100 pF10 pFCapacitance tobe disseminatedGuard electrode1 234


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