How do I know if it is humid enough in the tank?

Question
I recently purchased 2 land hermit crabs. I was reading about the humidity levels and I'm not sure if it is humid enough in the tank for the crabs. I covered one side of the tank top with plastic and put in 2 humidity pillows. The temp stays around 72 or so. I was hoping that this would get them movin a little more. The one crab has been really active - eating, pooping, and moving around. The other moves around a little but I noticed that he hasn't been pooping in the Hermie hut, which is pretty much where he stays. I also have about 2/3 of the tank covered with calcium-carbonate sand and the other 1/3 gravel (where the water shell and food shell are). I haven't yet purchased a container for them to bath in and I'm not sure whether it should be salt water or fresh water bath or if I should switch it up every few days. The sponge in the shell has fresh water but they can't really bath in it just sit on top! Help me please ... I'm a new owner and sort of confused. There are so many opinions out there and I just want the best for the crabs! Thank you!

Answer
Dear Courtney,
thank you for your question.
You need to get a hygrometer that measures relative humidity, I recommend a digital one because they are more exact that the analog ones. To see if the hygrometer is showing the correct humidity, wrap it in a moist towel for a few hours, it should show close to 100% humidity then.

Newly bought crabs need to destress and I wouldn't worry too much if they don't move that much. They might bury themselves and stay under for weeks, but don't go looking for them. They will be destressing or even molting, which can take a long time.

You need two water bowls, one fresh water and one salt. Both should be big enough that the crabs can submerge themselves (you don't need to bathe them, they will do it themselves when they feel like it). You don't really need a sponge. It makes a good snack for the crabs (it's an excellent source of chitin), but they cannot get water from it and it doesn't help that much with humidity that it's absolutely necessary.

I recommend changing the substrate to either coconut fibre (Eco Earth or Forest Bedding) or sand. You can also use a combinations or a mixture of both. Coconut fibre substrate is great for keeping moist and helping with humidity. Calci sand and gravel don't absorb moisture and the water tends to pool at the bottom of the tank, it's also not a good substrate for the crabs to dig and molt in. Cuttlebone should be offered as a calcium source, either crushed or in big chunks.

Here are some websites with excellent info on crab care:
www.thecrabbagepatch.com
www.hermitcrabassociation.com
www.landhermitcrabs.com
www.crabstreetjournal.com

I hope I was of some help to you
Jennifer