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)−218−Synthesiology - English edition Vol.2 No.3 (2009) that is currently in progress. This is because, as mentioned above, Sonntag’s equation is used when converting the results of the frost point method to the amount-of-substance fraction for comparison. I say “to some degree” here because the verification by this alone is insufficient; the uncertainty of the international comparison (combining the uncertainties of the standards of the two national metrology institutes and the uncertainty caused by the comparison) is larger than the uncertainty of Sonntag’s equation in the trace moisture range that requires verification. In this international comparison, we question the validity of Sonntag’s equation as a possible factor only when the equivalence of the AIST standard and the NPL or NIST standard cannot be confirmed within the uncertainty.The derivation of the vapor pressure equation for ice using the diffusion tube method is possible by measuring the frost point of gas generated by the diffusion tube method. However, considering the accuracy of the frost point measurement and the current uncertainty of the diffusion tube method, it is difficult to obtain significant results. I think there is a better possibility of deriving the equation by some other method such as simultaneous measurements of ice temperature and vapor pressure.2 Calibration for matrix gases other than nitrogenQuestion and comment (Akira Ono)In this research, you established the trace moisture standard by the diffusion tube method using nitrogen as a matrix gas, but what other gases can be used as the matrix gas? For other matrix gases, will it be necessary to individually establish the trace moisture standard using diffusion tube methods? If the standard can be established for one matrix gas (such as nitrogen), can the standard be set easily by relative measurement against other types of matrix gas?Answer (Hisashi Abe)In the field of semiconductor manufacturing, matrix gases other than nitrogen that require the trace moisture standards include hydrogen, argon, helium, oxygen, ammonia, and so forth. Although I think it is possible to establish their trace moisture standards and provide a calibration service using the diffusion tube method (although a method must be devised for a gas that is highly soluble in water such as ammonia), it seems not to be highly realistic considering the development cost and user’s convenience. Rather, I think it is more realistic to perform a relative measurement to obtain a conversion factor as you mentioned in your question. Please also refer to the second half of the answer to the next question.3 Use of cavity ring-down spectroscopy for various matrix gasesQuestion and comment (Akira Ono)It appears that the absolute values determined using cavity ring-down spectroscopy are consistent with the standard values obtained using the diffusion tube method. Does this mean that the absolute value of the absorption cross section of the water molecule has already been accurately obtained at the wavelength of the laser?It is mentioned that the absorption cross section is considerably temperature-dependent, but what do you think of the degree of change caused by different matrix gases? If the dependence of the absorption cross section on the matrix gases is small, then can you use the CRDS trace moisture analyzer for measuring trace moisture in other matrix gases if the analyzer was calibrated against a gas generated using the diffusion tube method?Answer (Hisashi Abe)A number of studies have reported the line strength of the absorption line monitored in this research used for determining AuthorHisashi AbeHe received his Ph.D. degree in physics in 1996 from Kanazawa University. After working at the National Institute for Advanced Interdisciplinary Research and the National Institute for Resources and Environment as a postdoctoral fellow, he joined AIST in 2001 and was assigned to the Humidity Standards Section, Temperature and Humidity Division, National Metrology Institute of Japan, AIST, where he has been working on the trace moisture standard.Discussion with Reviewers1 Reliability of the vapor pressure equation of iceQuestion and comment (Akira Ono, Vice President, AIST)In many national metrology institutes in countries other than Japan, the national standard for trace moisture is created using the Sonntag’s vapor pressure equation for ice. When deriving the Sonntag’s equation in the first place, absolute measurements of the trace moisture must have been performed in a manner traceable to the SI. What was the method? Is there an uncertainty attached to the Sonntag’s equation?In this research using a diffusion tube method, the absolute measurement of trace moisture was performed in a manner traceable to the SI. What are the possibilities of investigating the adequacy of Sonntag’s equation, or deriving a new vapor pressure equation for ice with an uncertainty using this method?Answer (Hisashi Abe)Many of the national metrology institutes other than that in Japan generate a humid gas so that a frost point becomes a set value and constant using a frost point method, and therefore, the traceability to the SI is through temperature. The Sonntag’s vapor pressure equation for ice is used when expressing the amount of moisture in a gas as an amount-of-substance fraction and for the calculations when changing the frost point by varying the gas pressure.The Sonntag’s vapor pressure equation for ice was published in Paper [8] in 1990, and this was a recalculation of the existing Wexler’s vapor pressure equation for ice (J. Res. Nat. Bur. Stands. 1977, vol. 81A, pages 5-20) in accordance with ITS-90, which is the current international temperature scale. Wexler’s equation was calculated on the basis of IPTS-68, which was the international practical temperature scale in 1977. Both the Sonntag’s and Wexler’s equations are obtained by integrating the Clausius-Clapeyron equation. The gas constant and compressibility factor for saturated water vapor over ice needed for the calculation were taken from the values recommended by the Committee on Data for Science and Technology (CODATA) and the values reported in the literature. For details of the calculation, please refer to Wexler’s paper.The relative standard uncertainty of the Sonntag’s equation is reported to be less than 0.5 % in the -100 ºC to +0.01 ºC range[8]. Comparisons with experimental data have been performed[9], but relatively large deviations are observed in the trace moisture range (frost point of about -75 ºC or less).The verification of Sonntag’s equation is possible to some degree by comparing the experimental results of AIST’s diffusion tube method and the experimental results of the frost point method obtained by NPL and NIST through the international comparison

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