iw2fvo, It is not possible to have this sensor or that equation to give you acurate results below freezing. Please read this artical: http://www.rhsystems.net/papers/RH_WMO.pdf. I have used a chilled mirror in the past for this type of measurement sampling air from a test chanber at - 20 F. However the type of sensor you are using is limited as far as humidity measurements and that is why the formula is not acurate below 0C.

Dave Purola,
N8NTA