How is humidity typically measured? I was under the assumption a hygrometer uses both dry and wet bulb measurements.
Not very practical to include in your electronics.
I don't know exact method used but I can assure you, there is no water reservoir in any of my gadgets:)
My experience is that they tend to get out of cal quite often which can lead to bad scenarios, like if using enthalpy setback for air-handling
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.
"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. "
https://en.wikipedia.org/wiki/Hygrometer
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.