Evaluation of measurement uncertainty in volumetric operations: the tolerance-based approach and the actual performance-based approach

Although relatively small compared to other sources of uncertainty in an analytical procedure, uncertainty in volumetric operations needs to be properly evaluated. In this paper, the problem of volumetric uncertainty is addressed with the critical examination of the procedure for its evaluation recommended in the EURACHEM Guide Quantifying Uncertainty in Analytical Measurement. Some characteristic features of volumetric apparatus as a measuring device are considered in relation to accuracy specification usually expressed in the form of “capacity tolerances”. On the basis of the underlying metrology, written standards to which volumetric ware is manufactured, and evidences available from experimental studies, it is shown that the tolerance already includes a random error inherent in the use of volumetric apparatus. Therefore, no additional allowance, except for temperature effects, needs be made if the uncertainty is derived from the tolerance. A detailed analysis of relevant uncertainty sources is presented, with two different procedures for evaluating the uncertainty identified; one of them relies on the prescribed tolerance while the other is based on the experimental estimation of the actual performance in the user’s hand. The uncertainty budget for each of these two approaches is evaluated, analysed and illustrated with a numerical example.