Vol.2 No.3 2009

Research paper : A marked improvement in the reliability of the measurement of trace moisture in gases (H. Abe)−207−Synthesiology - English edition Vol.2 No.3 (2009) excellent capability for measuring trace moisture. Because of these three reasons, namely, the national standard for trace moisture was established, the calibration service system traceable to the SI was organized, and a high-performance measuring instrument became available commercially, the reliability of trace moisture measurement has very recently increased markedly. Simultaneously, the problems concerning the methods conventionally used for measuring trace moisture have been revealed. This paper describes the scenario adopted by AIST for increasing the reliability of trace moisture measurement, research activities toward achieving this objective, and the recent changes in the reliability of trace moisture measurement brought about by the results of this research.2 Scenario and objective of increasing the reliability of trace moisture measurementTo maintain the high reliability of trace moisture measurement in the production sites and scientific experiments, the following are necessary: (1) establishment of a national standard for trace moisture as the basis of the scale of the trace moisture analyzer, (2) organization of a calibration service for measuring instruments, and (3) a high-performance measuring instrument that can be used for the trace moisture measurement in the production sites or scientific experiments. The relationships among these three factors are shown in Fig. 3. The issues that must be resolved to realize (1)-(3) are shown in the second column from the left, and are connected by arrows to (1)-(3). These issues must be solved by national metrology institutes, calibration service providers, and measuring instrument manufacturers. In the figure, the arrows connect the issues that each party must tackle. In the relationships among (1)-(3), it is important to organize the calibration service system traceable to the SI, and it is extremely effective to demonstrate the measurement capability of the instrument on the basis of an experiment in which the traceability to the national standard is maintained. Therefore, we considered (1) as the priority among the three. Because (1) was an issue that must be addressed by national metrology institutes, AIST initiated research to establish the trace moisture standard in a gas (nitrogen) in 2001. The goal for the lower limit of trace moisture was set as 14 nmol/mol (ppb), corresponding to a frost point of -100 ºC at atmospheric pressure, and the upper limit was set as 1 µmol/mol (ppm), corresponding to -75 ºC, considering the lower limit of the frost point achievable using the current frost point generator at AIST. The measurement uncertainty was set as a relative standard uncertainty of 11 %, corresponding to 0.5 ºC around the frost point of -100 ºC. This figure surpasses the standard uncertainty of 0.5 ºC at a frost point of -95 ºC that was set as a goal by the National Physical Laboratory (NPL) of UK at that time[4]. For (2), we planned, for some time immediately after the establishment of the standard, that the calibration service would be provided directly by AIST to individual users upon request using the national standard, and that calibration service providers would be recruited at the same time. For (3), we planned, after the establishment of the national standard, that the evaluation of the measuring instrument would be performed using the national standard to find instruments demonstrating high performance. Research activities performed in line with this scenario are described below.3 Establishment of the national standard for trace moistureThe national standard for trace moisture can be realized with a generator that generates gas containing a certain amount of moisture with a known standard value (trace moisture generator), and then performing the uncertainty evaluation of the standard value. When establishing a national standard, Fig. 1 Current state of calibration service for moisture in gas (humidity) in Japan.The calibration service for the frost point range between -75 ºC and -70 ºC is currently in preparation.Fig. 3 Scenario for elemental technologies and integration to increase the reliability of trace moisture measurement. Fig. 2 Traceability system for trace moisture measurement.46 %2.7 %0.3 %1 ppm14 ppbAmount-of-substancefraction(101.325 kPa)Dew point (℃)High humidityNormal humidityLow humidityTrace moisture+95+23-10-75-100 National standards (overseas)Measuring instrumentsReference standardsNational standard (Japan)The International System of UnitsAISTCalibration laboratoriesUsersInternational comparison(in progress) Measuring instrument manufacturersAIST: National metrology instituteCalibration laboratoriesOrganization of reference standard(3)High-performance trace moisture analyzers(2)Organization of calibration service system(1)Establishment of trace moisture standardIncreased reliability of tracemoisture measurement at site of production, etc.Establishment of calibration technologyDevelopment of national standardUncertainty evaluationPerformance evaluation of measuring instrumentDevelopment of trace moisture analyzers


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