Product review (5 stars): A product that works!
The hygrometer readings were correct within 1% relative humidity right out of the box.  No need to calibrate or adjust (there is no way to adjust this instrument anyway).  I placed it next to a calibrated electronic display, and they were within 1% of each other.  One suggestion is to lightly tap the cover glass before making a reading.  This allows the temperature and humidity needles to slightly jiggle to their correct readings.