The relative humidity of your hermit crab tank is probably the most important aspect of setting up an ideal hermit crab habitat. Temperature is also very important, but the crabs need the proper humidity in order to exchange oxygen through their modified gills. This is so important that I recommend a hygrometer for every owner, along with a thermometer, so this combination is ideal.
What I Like
I was looking around for a combined thermometer and hygrometer for my new hermit crab tank I found this great little accessory from ZooMed. It was very inexpensive, but what I really like about this combination is that it has ideal ranges for both temperature and relative humidity. The ideal range for both temperature and humidity is highlighted in yellow; too low is blue is and too high is red. It makes checking the parameters in your hermit crab tank easier than ever!
What I Dislike
On the downside, once the hygrometer has been calibrated, you may have to mentally shift the ideal range a bit (read more about the need to calibrate hygrometers here). Otherwise, I have been very pleased with this little gadget.