There is a clear difference between the Extech RH390 and RH490 precision Hygro-Thermometer Psychrometer versus an Aspirated Psychrometer
These units measure relative humidity using 2 thermometers, one dry and one wet (dew point). A reservoir of water is in the unit and a wick is attached to the wet bulb (dew point) thermometer. A small fan blows on both thermometers. The difference in temperatures determines the Relative Humidity either electronically or through a manual look-up chart using both bulb temperatures.
An aspirated Psychrometer measures Air Temperature and Dew point (wet bulb) temperature and computes Relative Humidity. These units are typically +/- 2% accurate.
Extech RH390 and RH490
The Extech RH390 and RH490 Measure Air Temperature and Relative humidity.The humidity sensor uses a capacitive sensor. The capacitive sensor accuracy is listed. The humidity sensor can be calibrated using the RH300-CAL calibrating salts.
These units measure Air temperature and humidity and computes the Dew point (wet bulb) temperature.