The metal housing (calibratable) hygrometers are quite accurate (better than cheap digitals by a long shot) Accurite I believe is 10% up or down? The calibrated spring type are 3-6%.
Do not get a 'hair' spring literally a horse hair type. They are fragile, more than the digital units.
Look for a metal face card and calibration screw in the back. many use a magnetic attachment, the disk is loosely stuck on the back of the units so personally I remove them and use hot melt glue to properly affix them and replace the magnet with a strong one, they move around and pull off less. Mine have stood up to a night in the water bowl (100% humidity lol) and still worked fine after 12 months (it might still be I lost track of which one it was.)
I like the western instruments brand
(http://www.ebay.com/itm/Western-Inst...-/201048398492)
but the others seem fine as well I suspect the guts are the same for all.
Georg Jensen and robert white make fantastic ones. they are likely too costly and large to place inside the enclosure but in an incubator or room... I love them!
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