Well, wet bulb thermometer works specifically by evaporating water and measuring temperature.
Psychometer is completely different idea.
Yes, measuring humidity reliably is a difficult problem as basically any other measurement that requires direct exposure of eroding/corroding components and fragile electronics to elements.
I believe a psychrometer operates on the same principle.
"A psychrometer, or a wet and dry-bulb thermometer, consists of two calibrated thermometers, one that is dry and one that is kept moist with distilled water on a sock or wick. At temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, such that the wet-bulb thermometer will be at a lower temperature than that of the dry-bulb thermometer. "
The psychrometer just compares the wet and dry bulb temperatures to arrive at a relative-humidity value. I.e., it still operates on the principle of a wet-bulb thermometer.
Psychometer is completely different idea.
Yes, measuring humidity reliably is a difficult problem as basically any other measurement that requires direct exposure of eroding/corroding components and fragile electronics to elements.