How do the pros measure humidity?
The WMO report Instruments and Methods of Observation has a chapter on Measurement of humidity and a separate chapter on Measurement of upper-air pressure, temperature, humidity. The first deals with instruments used for surface measurements. The second deals with instruments used for measuring vertical profiles, which have to deal with a larger range of temperatures and pressures.

For the surface instruments they list:


Gravimetric: take an air sample, extract all the water and weigh it (typically  used for calibration);
Chilled mirror: dewpoint or frostpoint: used for observations and for calibration;
Salt-solution: matching vapour pressure of a salt solution to atmospheric vapour pressure: used in automated weather stations;
Psychrometric: comparing wet or ice bulb temperature with actual temperature - used in observations and calibration;
Sorption methods: using hair or an alternative which stretches when wet, or using a material with a electrical resistance or capacitance that changes when wet.


In other words, the professionals use a lot of different methods.