do they make a sensor that can register difference in signal from a sending unit to a receiving unit to measure the difference in across a 100% humidity filled chamber and a 75% humidity filled chamber? maybe IR or RF? This way you would be able to mount them on the outside of the chamber with a small whole on each side to send the signal through that could be sealed with the face of the sensor and silicone.
Saturation of air is usually considered a humidity of 100% However the saturation level is temperature dependent. That is why we have relative humidity readings and dry bulb temperature readings.
While It might be possible to control the cyclic spraying of a 50 micro droplet system pretty well with a relative humidity controller type thermostat arrangement, the problem is with a large droplet size the droplets if contacting the controller sensor would always saturation. With really small micro drops the controller would seldom ever consider the chamber 100% saturated. A 50 micron can be run 100% of the time as long as the chamber relative humidity is not up to saturation (100%). This could nean setting upa multiple of misters seperately controlled so that you could run as many as you need to run 100% of the time to maintain say 95% relative humidity (the max allowed by most controllers). But your talking very small nozzles unless you have quite large chambers. For a chamber like you mentioned earlier your talking a gallon or so per day. That would mean ideally using nozzles that spray only 750 or 100 ml per hour, and running constantly from 2 to 4 at a time depending on night or day, size of plants, plant room hunmidity, temperature, lighting, CO2 etc.
I know humidistat circuit boards are usually epoxy coated and you could coat your electrical connections. I am not sure I would trust the factory coating though. Fertilizer and circuit boards are not very compatable.