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Research paper : Improvement of reliability in pressure measurements and international mutual recognition (T. Kobata et al.)−214−Synthesiology - English edition Vol.4 No.4 (2012) comparisons, the international equivalence of the Japanese national standard for pressure has been demonstrated in the aforementioned Key Comparison Database. AIST is also responsible for maintaining the national interest by displaying the capabilities in calibration and measurement that are recognized by other countries.With this background, we have been involved in advancing the national standards and in demonstrating the equivalence of the national standards with other countries, and we decided to utilize the industrial digital pressure gauge that has shown significant increase in performance. 2.3 Technological issuesThe national standard for pressure[4][5] realized by AIST is for stationary fluid where hydrostatic pressure mediated by gas or liquid is established. In general, gas is used as the medium of pressure standard for low pressure, and liquid for high pressure. AIST also establishes the vacuum standard for pressure with small absolute pressure[6].At the time of the establishment of AIST in 2001, the national standard was established for the pressure range from 5 kPa to 500 MPa using multiple pressure standard devices, and the calibration service was provided. With the advancement of science and industrial technology, various improvements such as the increased types of pressure standards, the expansion of pressure range, and the decrease of uncertainty were requested by the users at the industrial sites, according to the survey of demand for physical standards conducted in 2002[7]. Specifically, there were many requests for low-pressure standard and differential pressure standard for 10 kPa or less that were necessary for air conditioning, pharmaceuticals, medicine, and semiconductors. There were also requests for high-pressure standard surpassing 500 MPa for the development of internal combustion engines, as well as in the fields of materials, manufacturing, and machine processing.AIST upgraded the national standards by conducting new technological development[8]-[10]. Currently, the pressure range of 1 Pa or 1/100,000 of atmospheric pressure to 1 GPa or about 10,000 times the atmospheric pressure are covered, using various pressure standard devices. To check the conformity of the generated pressure of these national standard devices, the comparative measurement among the standard devices is necessary, and efficiency and advancement were issues in managing the multiple standard devices.At the field measurement of pressure, calibration and provision of standards that were effective yet with little burden on the user as much as possible were desired due to the increased number of devices that must be calibrated. As mentioned earlier, there have been recent new technological developments by companies in Japan and overseas, and the reliability of industrial digital pressure gauge using various pressure sensors increased. The high-precision pressure gauge with high resolution of six digits or more against full scale and with excellent long-term stability is now available. Therefore, AIST experimentally investigated whether such high-precision industrial digital pressure gauges could be used to increase the efficiency and performance of pressure standard maintenance, and whether it satisfied the requirement as standard devices.3 Various pressure gaugesFigure 2 shows the various pressure gauges that are used at the site of pressure measurement.The measuring device that is independently capable of conducting the absolute measurement of physical quantity without calibration by other measuring device is called the primary measuring device. Various devices are considered as primary measuring device for pressure (or primary pressure gauge), and the main ones are pressure balance[11] and liquid column manometer. Figure 3 illustrates the principles of the pressure balance and the liquid column manometer. On the other hand, the measuring device that is not independently capable of conducting the absolute measurement of physical quantity and requires calibration by primary measuring device is called the secondary measuring device. The secondary measuring device for pressure (secondary pressure gauge) includes digital pressure gauge[12] and mechanical pressure gauge (Bourdon tube pressure gauge[13]).Fig. 3 Principles of pressure balance (a) and liquid column manometer (b)Fig. 2 Examples of various pressure gaugesLiquid column manometerMechanical pressure gaugeDigital pressure gaugePressure balance(b)(a)Working MediumU-shaped tubeSurrounding pressureGravityMeasured pressureRotary motionSurrounding pressureMeasured pressureGravityWeightPistonCylinder

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