Psychrometer sensor with wet&dry bulbs and forced ventilation.
Wet bulb is closed inside a wet cotton sock from a distilled water tank. This sensor has a very good stability because it is based on a direct measurement of temperature using 1/3 DIN element and it is not sensitive to the time drift of every hygrometer element. LSI-Lastem data loggers can calculate Relative Humidity starting from wet&dry temperatures.