Joe, as you may already suspect, hand-held digital hygrometers are nearly worthless. We've been having trouble incubating cockatoo eggs for... well, always. Recently we got an ASTM lab calibrated thermometer to make sure the incubation temperature was correct. It was. That only left inaccurate humidity as the main suspect. So, we ordered a wet/dry bulb hygrometer from some outfit in Canada for c. $98CDN. When we got it, we checked both thermometers against the ASTM thermometer to make sure they were accurate. They were. Then we checked the wet/dry against the toy hygrometers we'd been using. Although we expected some difference, we didn't expect a difference of 10% RH; certainly that kind of difference will kill a cockatoo egg.
The good thing about a wet/dry (if the thermometers are accurate), is that you know for sure what the RH is. No uncertainty and no guessing. The instrument we purchased came with wet/dry tables, but they're all over the 'net, too. If you're serious about RH measurement, get a wet/dry.
FWIW, I measured the RH in the GH @1430 after the exhaust fan had been running off and on for a few hours and came up with:
Main floor: @ 74° - 61%
Mezzanine : @ 82° - 51%
I'd watered lightly that morning and the floor was dry.
Main floor @ 1015, watered heavily 5 hours previously: @ 72° - 91%
I can live with that.
"I thought you were spoiled, but you're just grateful for having the world by the ass!" Colleen - 2005
The harder I work, the luckier I get. - Samuel Goldwyn