I presume you mean the relative humidity defined as the ratio of the partial pressure of water vapor in the mixture to that of saturated vapor pressure of water at a prescribed temperature, knowing operating temperature and water saturation pressure and water content/fugacity in the mixture (I use tools as Prode and REFPROP for these calc's) it is easy to get the results in Excel, I don't know your simulator but if there are no direct methods you can follow the same procedure...