Treatment of table uncertainties

 

The table uncertainties for water/steam are determined in the algorithm by interpolation in the tables from the “IAPWS Release on the Skeleton Tables 1985 for the Thermodynamic Properties of Ordinary Water Substance“ stored in the code, using the raw measured values.

When using the table uncertainties in VDI 2048 Sheet 2, a functional correlation in the form  (equation 91) is assumed when calculating enthalpies. Here a is a pseudo-measured value with an estimated value 0 and a confidence interval that arises from the table uncertainty to pressure p and temperature t according to the stored skeleton tables.

Accordingly, in the two-phase range is determined. Here the boiling temperature ts is calculated from the pressure and is afflicted with a table uncertainty r. Thus altogether, enthalpies result for it in the two-phase range with the table uncertainties

As the skeleton tables specify the saturation pressure as a function of the temperature, therein the table uncertainty r of the saturation temperature of the pressure r_P is determined via the conversion

  _.

_{The procedure for the table uncertainties of the flue gas is analogous. However, in VDI 4670 the table uncertainties of the flue gas are not specified as absolute, but as relative quantities ε, related to the function value. Thus equation 91 on Sheet 2 for the flue gas enthalpies becomes .}