While it may seem logical that one could ascertain the accuracy of relative humidity (RH) readings on your HOBO logger simply by comparing it with a reading from a reliable hygrometer, the RH sensor in the logger requires further analysis if it is to be of any use to you. It would not be impossible to you to carry out the analysis, but the time and cost of doing so make that highly unlikely, at best. Looking at the Onset website white paper on the subject will give you an idea of what is involved, and, to my mind, it provides a convincing argument against trying it yourself. In the end, it is entirely possible that the sensor cannot be made to function properly, thus requiring replacement. You can contact Onset, the manufacturer, directly for further details. I have always found their staff to be very helpful.
A.B. MacLeish
Curator Emeritus, Newport Restoration Foundation