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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.

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In order to improve the accuracy of gas flowrate measurement by means of the differential pressure flowmeters, it is necessary to apply equations for determination of coefficients of gas flow equation, which would provide the lowest total relative expanded uncertainty of calculation, and to increase the accuracy of input values measurement in real time. One of these values is the arithmetic mean deviation of roughness profile of pipe internal surface. The equation for calculating the arithmetic mean deviation of roughness profile of pipe internal surface in real time was obtained. The equation for calculating the total relative expanded uncertainty of measurement result of arithmetic mean deviation of roughness profile of pipe internal surface in real time and components of this uncertainty were obtained.

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