The GERG-2004 project (Groupe Européen de Recherches Gazières) created a mathematical model for natural gas and similar mixtures, in which the main components are represented by Jacobsen–Stewart (JS) or Wagner–Span (WSsh) equations. Here these equations have been implemented as special cases of the Wagner–Setzmann (WS) equation of state. For mixtures an empirical excess function is added.
An extended version is known as GERG-2008.
adjustable parameters: 0
 O. Kunz, R. Klimeck, W. Wagner, and M. Jaeschke, The GERG-2004 Wide-Range Reference Equation of State for Natural Gases and Other Mixtures, GERG Technical Monograph 15, European Gas Research Group (GERG), VDI-Verlag, Düsseldorf 2007.
 O. Kunz and W. Wagner, J. Chem. Eng. Data 57 (2012) 3032–3091.
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