AFAICT, this is the last word in hygrometers ( yes, those crazy Germans again!):
https://www.abbeon.com/store/item.cfm?code=1562
12th Fret - our local great acoustic store, uses them.
That being said, I brought my digital hygrometer into their store, left it in a controlled room for a day, then simply noted the discrepancy from Abbeon in order to make my own readings (in my case, my digital hygrometer was right on).
Caveat: Doesn't account for possible variation across the entire range, tho.