The issues with (cheap) digital hygrometers is they measure the micro changes in conductivity caused by the presence of water vapour. The changes are very very small. Many other things can change the reading too, dirt/dust can impede the reading. Even a slight manufacturing error can grossly alter the reading (why they have such a range for accuracy some are 5% +/- some are up to 20% +/- ) Poor solder joints crushed wires can effect the readings. The point is they are very fragile and frail if they are to hold the orginal accuracy.

Good digital hygrometers are too expensive and far to fragile to be used around a snake. (IMO) this leaves good analogue units. They are robust (I have one that ended up in a water bowl over night and is still reading correctly a year later)

What to look for,

1. a METAL face card not a paper one paper swells and warps with water and can prevent the indicator from moving. (all the reptile analogue ones I have seen all are paper cards)

2. a metal spring system (not hair... hair is more accurate, and fragile they do not stand up well)

3. a calibration screw this allows a simple salt test to be done and the unit corrected to accurate again if it is suspect. (google salt test hygrometer)

I have quite a number of ones that fit this criteria, I test them (because I am OCD) every 6 months most are with in 2% or better and very rarely beyond 5%RH off. Cheap and reliable and robust the perfect solution.

something like this one...
http://www.amazon.com/Cheaphumidors-.../dp/B000H69QXE

The down side is they have a magnetic attachment that sucks. I toss the 'magnet' they come with re-glue the steel disk on the back and use a good magnet and have few issues again.