If the humidity sensor housing (located above the rear label) is wet, the Kestrel cannot take an accurate humidity reading. If the sensor does become wet, shake the unit vigorously to remove the water, and then let it dry completely before use. If the sensor comes in contact with salt water, rinse it thoroughly with clean water, then shake it and allow it to dry.
The humidity sensor has been factory calibrated to be accurate to within ±3%. If your Kestrel appears to no longer meet this specification, it may need to be recalibrated. NK now offers an RH Calibration Kit for your home use.