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.)−215−Synthesiology - English edition Vol.4 No.4 (2012) The pressure balance is a measuring device that is capable of highly precise generation of pressure. The reliability of measurement is high, and it is used as the national standard for pressure in many countries. It is also used widely as the standard device at the industrial sites. The basic components of the pressure balance are piston, cylinder, and weights with known mass. The exterior surface of the piston and the interior surface of the cylinder are precisely machined to maintain good circularity and cylindricity. Normally, when generating pressure with the pressure balance, the piston and the weights are rotated after floating the piston, which is under downward gravity by the piston and weights, to appropriate position by upward pressure. By doing so, the mechanical contact resistance between the piston and the cylinder is reduced, and the gravity on the piston and the weights is converted accurately into pressure.The liquid column manometer is a measuring device where the pressure is obtained from the height and density of the liquid column in the U-shaped tube that balances the measured pressure. Normally, the mercury or water with accurately known density is used as the working medium in the U-shaped tube. The foundation of the Japanese national standard for pressure is the primary interferometric standard manometer, which is a liquid column manometer using mercury as the working medium[14]. Similar pressure gauges are used as the national standard devices for pressure in the atmospheric pressure region at the national metrology institutes around the world.The digital pressure gauge is a device that has a pressure sensor and a mechanism to output the result on a display or as digital signals[12]. There are several measurement principles of the pressure sensor, and the typical one includes the detection of change in resistance such as the strain gauge, change in capacitance of the capacitance sensor, or change in frequency of the crystal or silicon oscillator. Many are capable of consecutive external output of the measured results as analog or digital signal. With the recent rapid advances in technology, digital pressure gauges with excellent resolution and stability are becoming available.The mechanical pressure gauge is a measuring device that measures pressure by mechanically expanding the deformation of the elastic element under pressure. The indicator with flat cross-section and bent tube with closed off ends as the elastic elements is called the Bourdon tube pressure gauge. It is used widely in society and industry due to its relatively low cost[13].Table 1 is a summary of the characteristics and properties of the four types of pressure gauges. The pressure balance and the liquid column manometer, which are primary pressure gauges, have small uncertainly of measured pressure, excellent long-term stability, and high reliability. However, these pressure gauges, in general, require complex maneuver, are heavy and expensive. Also, the acceleration of gravity of the place of installation is necessary to obtain accurate values for generated pressure. The pressure balance, mechanical pressure gauge, and digital pressure gauge can be used in wide pressure ranges, while the liquid column manometer has limited range compared to the other gauges. The digital pressure gauge and the mechanical pressure gauge that are secondary gauges are lower in reliability compared to the pressure balance and the liquid column manometer, but are easy to handle, light, and are low in cost. Therefore, redundant installation of multiple measuring devices is done to increase the reliability. Also, while the generated pressure of the aforementioned primary gauges changes according to the acceleration of gravity, the measured values of the digital and mechanical pressure gauges are not directly affected by the acceleration of gravity. In general, the digital pressure gauge, in which the continuous measurement and transmission of measured results are easily done, is suitable for automation and systemization, and is useful in increasing the operability and efficiency.We engaged in the development to utilize the advantages of the digital pressure gauge that are its small and lightweight portability, simple operability, and high applicability to automation and systematization. 4 R&D scenarioIn this research, we set the goals as the utilization of the digital pressure gauge to confirm the equivalence of the national standard necessary for international mutual recognition and to increase the reliability of pressure measurement at the industrial sites.Table 1. Characteristics of various pressure gaugesMechanical pressure gaugeDigital pressure gaugeLiquid column manometerPressure balanceAutomation, systemizationTypes of pressure gaugesDifficultSmall, light, easyOver 1 GPaSecondaryEasySmall, light, easyOver 1 GPaSecondaryDifficultLarge, heavy, difficult300 kPaPrimaryDifficultLarge, heavy, difficultOver 1 GPaPrimarySize,weight, operabilityMaximum pressureTypes of measuring instrument

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