When creating a liquid/vapor or vapor only fluid table, one of the items you may have to specify is the acentric factor.

The acentric factor is only specified when you are entering a fluid table with vapor properties.  It is not required for liquid only tables.  The acentric factor is used in the Peng-Robinson and Soave equations of state.  These equations of state are used to calculate a compressiblity factor (Z), which is applied to the vapor density calculated using the ideal gas law:

density (real gas) = density (ideal gas) ÷ Z

   where,
        Z = compressiblity factor

density (ideal gas) = (P * MW) ÷ (R * T)

   where,
        P = fluid pressure
        MW = molecular weight
        R = universal gas constant
        T = fluid temperature

Note that if your fluid table specifies the ideal gas law or the Redlich-Kwong equation of state to calculate the density, the acentric factor is not used.  Also, some gases (e.g. noble gases such as Neon) do not have an acentric factor.  For these gases, you can just leave the acentric factor field blank.