People whose opinion I respect have recommended this hand held hygrometer (I have not used it). I use Accurites myself, but only as a backup for my own eyes (since hygrometers are prone mechanical failure and user error).
Not knowing anything about crab pathogens, I'd personally consider a bleach soak for reusing materials. One part regular Chlorox bleach in 19 parts tap water, soak for ten minutes, then rinse under tap water and allow to dry for 24 hours.
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