R600a_MH
The thermodynamic properties of refrigerant isobutane (here called R600a_MH) are provided by Martin-Hou equation of state (A.I.Ch.E. Journal, 1:142, 1955) based on thermodynamic information from McLinden et al., Measurement and Formulation of the Thermodynamic Properties of R134a and 123, ASHRAE Trans., Vol. 95, No.2, 1989.
R600a_MH is NOT built into EES. The property data were read in from file R600a.MHE in the USERLIB\mhe Property Files folder. A much more accurate formulation for both thermodynamic and transport properties is available with fluid ISOBUTANE. Use ISOBUTANE or R600a unless computational speed is an issue.