A Practical Approach of Measurement Uncertainty Evaluation for Gravimetrically Prepared Binary Component Calibration Gas Mixture

摘要

Measurement uncertainty is the prime requirement of any calibration standard which is indented to use for the calibration of any instrument. In this study, uncertainty evaluation for the gravimetric prepared binary component calibration gas mixtures (CGMs) of propane in nitrogen has been performed in a practical way. CGMs are the key for accurate and consistent results in gas analysis. Three CGMs of propane in nitrogen in the range of 5000-11,000 μmol/mol were prepared using gravimetric method according to ISO 6142. Uncertainty estimation of these prepared gas mixture standards is done following the bottom up approach of ISO Guide to the Expression of Uncertainty in Measurement" (ISO/GUM) in which individual contribution of every step of preparation process contributes to the overall uncertainty estimation. According to ISO/GUM guidelines, four main stages are involved for the estimation of uncertainty in any method such as specification of measurand, identification of sources of uncertainty, quantification of these sources of uncertainty and then combining all uncertainty estimates. In this paper, magnitude of all probable uncertainty components is evaluated for the preparation of CGMs. The uncertainty component of the gravimetrically prepared CGMs of propane is found to be 0.34% relative (k = 2) at 95% confidence interval. The purity of components gas and weighing process are found to be the major sources in uncertainty contribution of preparation of calibration gas mixture. The composition of gravimetrically prepared calibration gas mixtures is verified by gas chromatography technique following ISO 6143 standard. The final estimated uncertainty associated with the certified value of CGM is found to be 0.68% relative, which includes both gravimetric and analytical uncertainty components out of which the gravimetric component of measurement uncertainly is estimated in detail in this paper.