I find that elaborate humidity chambers and controls are overkill. I just tell my customers to coat their paper with distilled water or 10% oxalic solution and let it sit until it's about 90% dry before putting emulsion down. If you're trying to get water into the fibers of the paper, I don't see any reason to jump through hoops by controlling the ambient humidity. You're not trying to trick the paper into having water in it, you don't have to sneak up on it and surprise it. Just put the water on the paper and forget about the circus tricks.