Vol.2 No.3 2009
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Research paper : A marked improvement in the reliability of the measurement of trace moisture in gases (H. Abe)−212−Synthesiology - English edition Vol.2 No.3 (2009) of the water molecule and measurement data obtained using the CRDS trace moisture analyzer, trace moisture can be measured without a calibration curve. Because this characteristic was considered an advantage over VUV or APIMS, we immediately started to gather information on the instrument performance and introduced it in early 2003. According to the calibration certificate issued by the instrument manufacturer, the accuracy of the indications of the CRDS trace moisture analyzer was confirmed in the range of 0 nmol/mol (ppb) to 1000 nmol/mol (ppb) using a reference standard traceable to NIST of the United States. To check the accuracy of the indications independently, a trace moisture gas with a known value (standard value) generated by the AIST trace moisture generator was measured using the CRDS trace moisture analyzer (indication), and the indications and standard values were compared[5]. Figure 8 shows the differences between the indications and standard values as relative values. In addition, absorption spectra were measured using a built-in function of the CRDS trace moisture analyzer, and xw was calculated from the analysis of the absorption line (analyzed value). Figure 9 shows a measured spectrum. The absorption lines in Fig. 9 are assigned to the vibration-rotation transitions of the water molecule, and the strongest absorption line shown in the figure (202 303 transition of v1 + v3 band) was used for the analysis. The indications of the CRDS trace moisture analyzer were determined on the basis of the peak intensity of this absorption line. The three xw values obtained independently agreed within 11 % in the range of 20 nmol/mol (ppb) to 600 nmol/mol (ppb), and it was found that the CRDS trace moisture analyzer can be used for the measurement of Nb and xb. Using the CRDS trace moisture analyzer, we found methods of achieving xb < 0.15 nmol/mol (ppb) and Nb < 6 nmol/h and of establishing a technology to fulfill the conditions xw >> xb and N >> Nb. In the process of this research, it was also demonstrated that the CRDS trace moisture analyzer was a high-performance measuring instrument.3.2.3 Measurement of dry gas flow rateThe flow rate F of the dry gas (nitrogen) is controlled and measured using a thermal mass flow meter. We initially considered calibrating the flow meter to obtain the traceability to the SI using the Japan Calibration Service System (JCSS)[18], introduced in Japan by the Measurement Act. In late 2004, we asked the technical staff of the calibration laboratory accredited by JCSS who was capable of calibrating flow meters with the smallest uncertainty, but they replied that they had not yet had any experience of calibrating the thermal flow meter, and that they were not yet confident in performing the calibration. At that time, the accuracies of thermal flow meters stated by the manufacturers were generally 1 % of the full scale, and the uncertainty increased markedly in the small flow range relative to the full scale of the flow meter. This meant that it was necessary to set up several flow meters and calibrate all of them in order to cover the entire flow range with the small uncertainty needed in this research. Moreover, we considered that the differences in the indications and standard values observed in Fig. 8 originated mainly from the uncertainty of the flow rate measurement. For the above reasons, from about the first half of 2005, we started to consider using a flow meter with a high reliability based on a different principle.For the measurement of a small gas flow, the most reliable flow meter is considered to be a critical-flow Venturi nozzle (sonic nozzle) mass flow meter[19][20]. The critical-flow Venturi nozzle flow meter was developed through joint research between the National Research Laboratory of Metrology and a flow meter manufacturer[21] and it became commercially available in around 2000. We introduced this type of flow meter in 2005, and the traceability to the SI was established using the JCSS. The flow rate was calculated independently using the measurements of the pressure and temperature of the gas and the information on the size of nozzle, and the operating principle was checked through the comparison between the calculated values and the indications of the flow meters. From experiments using these flow meters and by adding some improvements in controlling the flow rate, it Fig. 10 Difference between indication of CRDS trace moisture analyzer and standard value after introduction of critical flow Venturi nozzle flow meter (relative value).The dots represent [(indication – standard value) ÷ standard value] × 100.Fig. 11 Change of generation chamber temperature against change of room temperature.Relative value of difference / %xw / (nmol·mol-1)0105-10-5020050100150250 Time / hTemperature / ℃010030020024282630RoomGenerator

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