Uncertainty evaluation of thermodynamic properties estimated with online data tools: vapor pressure as case study

Online databanks are a convenient source of information whenever thermodynamic properties are required for calculations, in both scientific and engineering environment. The data listed in these repositories is usually reported along with a range of validity and their sources. The values of the thermodynamic properties can usually be retrieved from databases and handbooks, in the form of tables, curves or correlations with regression parameters. Most of the times, little to no information is given on the uncertainty of the reported estimates, let alone regression parameters uncertainty. Evaluating the sources of uncertainty in any model is fundamental to verify the significance of the results for a specific application. Knowing the uncertainty, makes the difference between trusted values and random values. Depending on the model where the property is required and its application (the use of results), the contribution to the total uncertainty of thermodynamic properties and constants can surpass the contributions of other experimental input quantities. In this work, the methods described in the 'Guide for the expression of uncertainty in measurement' are used to evaluate the uncertainty of a thermodynamic property of a pure substance, calculated based on the constants and references reported in two important online data banks (NIST and Dortmund Databank). The vapor pressure is considered as case study and several definitions with their relevant sources of uncertainty are presented. The aim of the work is to highlight the limitations of properties data commonly used in engineering estimations, as well as the importance of accounting for their uncertainty. Following established metrological guidelines, a full procedure has been developed, with specific considerations aimed to account for the 'quality' of the realization of the definition of the measurand as a source of uncertainty of the property.