Upon studying this issue further, I have found the answer which I will include here for anyone else interested. It seems that the common practice for psychrometric charts is to choose an enthalpy zero reference point of 0-degrees. However, since 0 deg. F is not the same as 0 deg. C, is stands to reason that the values from one chart cannot be directly converted to the values on the other chart. The main use for psychrometric charts, though, is to calculate energy required to change the condition of air (by removing humidity, for example.) So, to calculate the energy required, you would look up the enthalpy at two different states and subtract. For this purpose, the reference point is not important - you just want to know the difference. And, the amount of the difference CAN be converted and shows good agreement between the two charts. I compare it to the potential energy of an object. You know it has energy, but how much it has depends on your reference point. So the 10-pound weight has zero energy with respect to the table it is sitting on, but it has some positive amount relative to the floor below.
