Instrumentation & Automation
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# Thermodynamic Properties of Water and Steam IAPWS-IF97

Calculation of the thermodynamic properties of steam and water is widely used in metrology. For calculating the primary devices for flow rate measurement, for example. At present, there are two main methods, two standards of the organization of the IAPWS (International Organization of Water and Steam Properties) - IAPWS-95 and IAPWS-97.

The IAPWS-95 has been released earlier. It is recommended for scientific calculations. In Russia, the analogue is GSSSD 187-99. The main disadvantage was the slow algorithm of computer calculations. Therefore, soon appeared a faster industrial standard of IAPWS-97, used in automated control systems. The difference in the calculations is small, but still exists.

In the Instrumentation & Automation application the IAPWS-IF97 stanard is used. The input and calculated parameters are listed in the table below:

 Calculated parameters Pairs of input parameters:p - absolute pressureT - temperatureh - specific enthalpys - specific entropy v specific volume p,T p,h p,s h,s ρ density + + + + s specific entropy + + + + h specific enthalpy + + + + u specific internal energy + + + + g specific Gibbs free energy + - - - Cp specific isobaric heat capacity + + + + Cv specific isochoric heat capacity + + + + Kt isothermal compressibility + + + + λ thermal conductivity + + + + k thermal diffusivity + + + + η dynamic viscosity + + + + ν kinematic viscosity + + + + w speed of sound + + + + αv isobaric cubic expansion coefficient + + + + Pr Prandtl number + + + + ε dielectric constant + + + +

When the module for calculating the thermodynamic properties of water and steam starts, a window appears (see pic. on the left), with initially valid input pressure and temperature of the calculated substance. At the same time, the output values shown in the table above are calculated.

Valid values are temperatures from 0 to 1000 °C and pressures from 10 Pa to 100 MPa.

When the input parameters change, the calculated values change too. If the input parameters are out of range, the output values are set to 0.

The program uses the function library IAPWS-IF97 of Hummeling Engineering.