The aim of the guideline will therefore be to reproduce as accurately as possible the thermodynamic behaviour of the process gases within the entire region of interest when taking into consideration the technological developments to be expected; to demonstrate the influences of dissociation, of real-gas behaviour and of condensation which limit the accuracy of the simple models, and, where necessary, to include such influences; to optimize the formulation of the equations with regard to accuracy and calculating speed; by means of comparisons to demonstrate the shortcomings of popular physical property models so as to make users aware of the necessity for a common database of sufficient accuracy. Purpose of the guideline is not the estimation of the composition of gas mixtures.