I never trust digital hygrometers they never seem to hold any amount of accuracy for long. (sensitive to dust dirt and rough treatment they are very delicate all in all) I use ones like this
http://www.amazon.com/Analog-Hygrome...log+hygrometer
They can easily be checked and calibrated as they have adjustments for this. Salt tests work very well with them and are easy and cheap to do. This type is very tough I have one that spent a night in a water bowl and still is correct even a year after its swim.
There are a few very cheap analogue ones out there if the unit has a calibration screw and a metal face card it might be worth trying. It is just hard to beat the western instrument brass analogue one it is certified and accurate out of the box (I have 14 all were within 3%RH of correct out of the box, better than any digital I have ever owned.)
Just remember that you don't want to make a 'sick building syndrome' situation inside the enclosure it is hard to know what is enough but too little can carry a heavy price for the wallet and animal.
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