The article analyses the main problems associated with evaluation the combined standard uncertainty of the water pH measurement by the type A and B methods. It is shown that, for a small number n of the tested water samples, the type A standard uncertainty determined by the conventional method is underestimated. Therefore, the correct expression to calculate this component of uncertainty is presented. The authors also highlighted that since in the practical measurement the influencing quantities and sensitivity coefficients are not known absolutely precisely, therefore their uncertainties often have to be taken into account when estimating the combined uncertainty. For this purpose the authors have propose approach to correctly determine the type B components of combined standard uncertainty caused by not only the values of influencing quantities and sensitivity coefficients, but also their uncertainties. The proposed approaches are illustrated by estimating the uncertainty in the measurement of drinking water pH, presenting the corresponding components of measurement uncertainty budget.



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