If you want to check your hygrometers, use the salt test. There are directions to do this on this site, or you can go to xikar.com and use their instructions written to check their hygrometer units:
http://xikar.com/files/salt_test_instructions.pdf
I've used this test for a number of my smaller units and it works well. But use the search function and check out what some of the other members have to say also.
Good Luck!