Highest 2% RH accuracy with fast 30 seconds Response Time. The Extech RH390 Precision Psychrometer measures humidity and temperature simultaneously. A dual-illuminated readout can be alternated to display wet bulb and humidity, or dew point and humidity, or temperature and humidity. This handheld device registers temperatures from -22 degrees to 199 degrees F (-30 degrees to 100 degrees C) and measures the full spectrum of relative humidity with ±2% accuracy between 20 and 90% RH.