Thanks Jim,
The thing came with a calibration kit. Tear open a salt bag (?) and put the hydrometer in it for 12-24 hours. When you take it out it should read 75% (if memory serves me correctly) If adjustment is needed, there's a tiny screw on the back. Mine was right on. Both of them (I bought two). But I don't know if it is reading relative humidity, absolute humidity, or specific humidity. I've always assumed it was RH.