Vol.4 No.4 2012

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Research paper : Improvement of reliability in pressure measurements and international mutual recognition (T. Kobata et al.)−218−Synthesiology - English edition Vol.4 No.4 (2012) relative order of 10−6 or less. Currently, the calibration and maintenance of numerous pressure balances as well as the national standard devices maintained by AIST are done using the digital pressure gauge, and the process has been advanced greatly and can be conducted efficiently.The first calibration method is the method of directly measuring the generated pressure difference of the two pressure balances using a high-resolution digital differential pressure gauge. In this method, small weights are used to adjust the pressure difference of the two pressure balances measured with the differential pressure gauge until the difference reaches zero.The second calibration method is the comparative method where the digital pressure gauge is used as the comparator[17][18]. As shown in Fig. 5, the generated pressures of the two pressure balances are measured alternately with the digital pressure gauge by switching the constant volume valve. The advantage is that even if there is some degree of difference between the generated pressures of the two pressure balances, the difference can be obtained accurately and corrected. Moreover, this method can be applied simultaneously to three or more pressure balances. In the second calibration method, the short-term stability was utilized in addition to the high resolution and consecutive measurement functions of the digital pressure gauge. 6.2 International comparison of the national standardsNormally, in the international comparison of the national standards of pressure, the transfer device, which is a measuring device that can be transported, is circulated among the countries, and the participating national institutes obtain the calibration value for the transfer device based on their own national standards. By mutually comparing the calibration values of each country, the differences of the values of the national standard for pressure of each country can be seen. For the international comparison where the measurement of highest precision is necessary, the pressure balance that has equivalent performance as the standard was used traditionally as the transfer device.In such a situation, AIST focused on a digital pressure gauge that was small, lightweight, and of low cost, as well as easy to handle, and engaged in the world-leading development of a transfer device for international comparison using the high-precision industrial digital pressure gauge. Using the developed transfer device, several international comparisons were actually done[19]-[23]. Multiple digital pressure gauges were used simultaneously as the transfer devices, to increase the reliability by redundancy. Through the execution period of the international comparison that normally takes over one year, the details of the long-term property changes of the digital pressure gauge was studied and the results were used to correct the calibration value. By these corrections, the lack of long-term stability of the transfer device was compensated, and sufficient comparative precision could be maintained[19]-[22][24].As an example of use of the digital pressure gauge in international comparison, we describe the international comparison conducted from 2002 to 2004 to check the international equivalence of the national standard for hydraulic gauge pressure with pressure range from 10 MPa to 100 MPa (APMP.M.P-K7[19]). Figure 6 shows the transfer device used in that international comparison. The transfer device was composed of a set of two digital pressure gauges with full scale of 100 MPa, resolution of 0.1 kPa, and used the crystal oscillator as the pressure sensor. With the use of the digital pressure gauge, the total weight of the transfer device during transportation could be kept at 50 kg or less, and this was dramatic weight reduction compared to using the pressure balance (which usually weighs over 200 kg). Also, three sets of transfer devices with the same specifications were prepared and sent simultaneously to the participating institutes by Fig. 6 Photographs of the transfer device for international comparison for hydraulic pressure using the digital pressure gauge (two pressure gauges on each transfer device, total three sets)Fig. 5 Calibration by comparative method using the digital pressure gauge as the comparator Standard deviceDevice under calibrationDigital pressure gaugeConstant volume valvePressure generatorConstant volume valve

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