Product review (3 stars): save your money, wish i had!
the one i bought is 10-11% off the correct humidity. i don't have to use a "salt" test to calibrate as i checked it against a Fluke 971 humidity meter (i am an HVAC contractor). the fluke meter also jives with two other hygrometers. the Hydra only gives you a little adjustment to their so called factory calibration. in my case it isn't enough to get it back to a correct number. i am increasing the set point to account for the lack of calibration. this is a pricey fan unit that sits atop some cheap floral sponge material. the concept is cool. the goods are cheap.
hope this helps