Product review (1 stars): AVOID- totally inaccurate and unable to calibrate.
The specifications indicate that the unit measures down to 10%, but the unit I've received never goes below 27%. The RH in the environment that the meter is in is 0.6% (electronics assembly glovebox), as measured by a very expensive professional dewpoint meter. I left it in the glovebox for 72 hours and it's still reading 27%RH.

Calibration has no measurable effect. This unit is total, absolute garbage. Avoid entirely. I will be sending it back to All-Spec where it was purchased. I am reviewing here to save as many people from this unit as I can.