Precision Hygro-Thermometer Psychrometer VS Aspirated Psychrometer

There is a clear difference between the Extech RH390 and RH490 precision Hygro-Thermometer Psychrometer versus an Aspirated Psychrometer

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.

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