There are a lot of threads on this subject, here are a couple:
http://www.thatquailplace.com/smf/index.php?topic=3822.0
http://www.thatquailplace.com/smf/index.php?topic=1322.0If I understand your question correctly, I have never used anything but the digital hygrometers/thermometers when incubating, in conjunction with mechanical t-stats. I use at least 2 or 3 in order to be able to verify their accuracy. I have the wet bulb stuff, but have never used it.
Do a search ("hygrometer" or "humidity") on this forum and you will find a pile of threads on it.