Technique for Evaluating the Uncertainty of Enthalpy of Water and Steam for Thermal Energy Metering Systems

The paper presents an analysis of existing techniques for calculating the enthalpy of water (steam). Based on the analysis the techniques were defined that should be applied for computer-aided design of thermal energy metering systems. Equations for calculating relative expanded uncertainty of water (steam) enthalpy are developed taking into account methodical uncertainty of enthalpy calculation and uncertainties of metering temperature and pressure of water (steam). Simplified dependencies are developed for calculating the relative sensitivity coefficients of the enthalpy uncertainty to the uncertainties of water pressure and temperature. These dependencies make it possible to calculate the values of the sensitivity coefficients for a range of pressure from 0 to 5 MPa and temperature from 300 to 550 K. The relative deviations of the sensitivity coefficients obtained by the simplified dependencies from the values obtained by the equations of International Association for the Properties of Water and Steam (IAPWS) are 0.48% (for the sensitivity coefficient of uncertainty of enthalpy to uncertainty of pressure) and 0.56% (for the sensitivity coefficient of uncertainty of enthalpy to uncertainty of temperature) for the above pressure and temperature ranges. The developed simplified technique for evaluating uncertainty of water (steam) enthalpy can be applied to determine the metrological characteristic of thermal energy metering systems and also for their computer-aided design.