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Research paper : Improvement of reliability in pressure measurements and international mutual recognition (T. Kobata et al.)−219−Synthesiology - English edition Vol.4 No.4 (2012) different circulation routes to greatly reduce the measurement period required for the international comparison.During the measurement period, a total of six digital pressure gauges that comprised the transfer device were regularly returned to AIST that acted as the pilot institute. The stability of the transfer device was evaluated by recalibrating these digital pressures gauges using the Japanese national standard. Figure 7 shows the change per day of the calibration values of each digital pressure gauge, as the evaluation of the function of measured values. As seen in the graph, the change at the maximum measured pressure of 100 MPa was over 50 Pa per day for some pressure gauges. Since there were about 400 days for the entire measurement period of the international comparison, the measured pressure value at the maximum pressure was estimated to change 20 kPa. This corresponds to relative change of 2 × 10−4 against the measured value. On the other hand, when the pressure balance that was traditionally used as the transfer device in the international comparison was used in this pressure range, the stability was expected to be about relative 1 × 10−5. Therefore, when the above digital pressure gauge was used without correcting the calibration value, the stability necessary for the international comparison could not be obtained. However, it was found in the evaluation that the change in each pressure was the linear function of the number of lapsed days. By correcting this and by using the average value of the calibration values obtained from the two digital pressure gauges that comprised the transfer device, ultimately, 5 × 10−6 or less for the stability of each transfer device was obtained throughout the entire measurement period[19]. We were able to obtain sufficient comparative precision that could be used in the international comparison, by understanding the details of the property of the digital pressure gauge as the transfer device and conducting appropriate corrections.Figure 8 shows the results of the calibration values at 100 MPa of the participating institutes in the two international comparisons (CCM.P-K7[25] and APMP.M.P-K7[19]), including the aforementioned international comparisons. The horizontal axis shows the acronyms of the participating institutes and the country names, and the vertical axis shows the relative deviation from the international comparison reference value of the calibration values of each institute. The reference value was obtained by calculation of the average value of the calibration values of the participating institutes. The length of the bar for the data for each country is the uncertainty (confidence level about 95 %) of the calibration value claimed by the participating institutes. The results obtained in the same international comparison are indicated by the same markers (black or white circles). The equivalence of the national standards for pressure of the countries can be seen from Fig. 8. The Japanese national standard has almost zero deviation from the reference value, and the uncertainty is similar to the major countries. Therefore, it was confirmed that the Japanese national standard for pressure established, maintained, and supplied for the target pressure range has excellent international equivalence.The use of digital pressure gauge as the transfer device is being done in other international comparisons. Excellent comparative precision was obtained by using the digital pressure gauge with silicon resonant sensor in the international comparison of the gas differential pressure[20]. In this manner, the efficacy of the digital pressure gauge as the transfer device was confirmed in the international comparison in various pressure ranges. 6.3 Standard provision through remote calibrationIn the calibration of pressure gauges used in the industrial sites, the “carry-in calibration” is the common practice where Fig. 7 Changes per day of the calibration value of the six digital pressure gauges used for the international comparison of hydraulic pressureFig. 8 Results of the two international comparisons at pressure 100 MPa(:CCM.P-K7, :APMP.M.P-K7) Pressure/MPa100806040200-100102030405060654321Change in calibration value/(Pa/day)PTB (Germany)IIMGC-CNR (Italy)BNM-LNE (France)NPL (UK)CENAM (Mexico)NIST (USA)INMS/NRC (Canada)NMIJ/AIST (Japan)NPLI (India)CSIR-NML (South Africa)NIS (Egypt)KRISS (Korea)SCL (Hong Kong)SPRING (Singapore)NMIA (Australia)VMI (Vietnam)NMLSIRIM (Malaysia)KIM-LIPI (Indonesia)NSCL (Syria)NIMT (Thailand)CMS/ITRI (Taiwan)NIM (China)Relative deviation from the reference value for international comparison/10-6-100-80-60-40-20020406080100

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