Measurement Uncertainty of Gas Properties

The calculation of gas properties from the gas composition is an activity that occurs frequently in gas analysis. Such calculations play an important role in the transmission and distribution of energy gases, carbon dioxide and other commodities. They also occur in monitoring air quality. Applications include the calculation of compressibility factors of natural gas to convert metered gas volumes from actual to reference conditions, the conversion of amount fractions into mass concentrations and calculations in process design and optimisation. Evaluating measurement uncertainty is important, as usually there are legislative, regulatory and commercial requirements to be met. Assessing and demonstrating compliance with such requirements requires knowledge about the uncertainty of the measurement result. It is shown how the well-known law of propagation of uncertainty can be used with models from which it is not evident how to calculate partial derivatives, such as equations of state, which are used to calculate, e.g., compressibility factors, densities and energies. Furthermore, it is shown how to calculate time averages from measurement data in grids and networks.