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Research paper : Improvement of reliability in pressure measurements and international mutual recognition (T. Kobata et al.)−217−Synthesiology - English edition Vol.4 No.4 (2012) scenario. What follows are the contents and results of the R&D that was conducted according to the scenario. 5 Evaluation of the properties of the digital pressure gaugeTo build the practical and efficient standard system of pressure while maintaining reliability, the effective use of digital pressure gauge is the key. Therefore, AIST engaged in the R&D for the property evaluation and calibration method of various digital pressure gauges using the pressure standard devices[15]-[17]. The following factors can be given as the main reason for the uncertainty of the measured value displayed by the digital pressure gauge:a)Uncertainty of the applied pressureb)Uncertainty due to the variation in measured valuec)Uncertainty due to historical effectd)Uncertainty due to resolution or short-term stabilitye)Uncertainty arising from changes in surrounding temperature (temperature property)f)Uncertainty arising from attitude of installment (attitude property)g)Uncertainty due to changes in voltage of power supplyh)Uncertainty due to the linearity of input-output relation (linearity)i)Uncertainty due to changes in line pressure (in case of differential pressure gauge)j)Uncertainty arising from long-term stability (change over time)k)Uncertainty arising from changes in surrounding environment (relative humidity, atmospheric pressure, vibration, impact, etc.)In case the digital pressure gauge is used as the standard, the above property evaluation must be conducted thoroughly, and consideration is necessary on the uncertainty evaluation and correction of the calibration value according to the situation in which the device is used. Consideration must also be made in transporting the pressure gauge, since the property will be affected by the changes in surrounding environment during transportation.Currently, various digital pressure gauges with excellent performance are available in wide pressure ranges. The two gauges, for which the evaluation of the property was mainly done in our research, were a differential pressure gauge that uses a silicon resonant sensor as the pressure sensor manufactured by a Japanese measuring instrument company, and a pressure gauge that uses the crystal oscillator as the pressure sensor manufactured by a foreign company. The stability was evaluated by conducting appropriate corrections after performing detailed property evaluation, and then regularly repeating the calibration of the digital pressure gauges using the fixed calibration method. As a result, it was found that reliability equal to the conventional pressure standard device could be obtained for the digital pressure gauge by devising the usage method. The evaluation data for the stability of each pressure gauge is illustrated in the case studies of the incorporation of the digital pressure gauge into the standard system, as will be explained later. 6 Incorporation of the digital pressure gauge into the standard systemThe three case studies of the incorporation of the digital pressure gauge into the standard system for pressure are described in this chapter.6.1 Establishment and maintenance of the national standardThe liquid column manometer and the pressure balance are positioned as the national standard of Japan or the primary standard of pressure. To use them as the primary standard, it is necessary to determine and manage the various property values. The property values of the pressure standard used at AIST are accurately measured and managed, so they are traceable to the national standards of mass, length, temperature, and others. At AIST, several pressure balances and pistons and cylinders are managed for each respective pressure range. By managing large numbers of standard devices in groups, the national standard is maintained for a wide pressure range. For the maintenance of these standard devices, comparative measurements between the standard devices are conducted regularly to check the mutual conformity. Also, from the results of the comparative measurement for various combinations, the long-term stability of the generated pressure of each standard device is evaluated. Although it depends on the pressure range, in general, the long-term stability is on the relative order of 10−6 per year.Here, the case, where the digital pressure gauge was used to calibrate the pressure balance of a private calibration lab with AIST’s national standard pressure balance, is described. When the pressure balance is calibrated at AIST, two pressure balances are connected directly to compare the generated pressure. In traditional calibration, the two pressure balances are operated simultaneously, the changes in the fall rate of the pistons are observed, and fine weights are added to either balance until the equilibrium is attained. However, this method involves complex maneuver, and the calibration result is highly dependent on the system configuration and the technical prowess of the calibration operator. Therefore, AIST selected and advanced the two calibration methods using the high resolution and consecutive measurement functions of the digital pressure gauge. As a result, in both methods, it was found that if the performances of the digital pressure gauge used and the pressure balance are good, the equilibrium state could be attained at the

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