This dehumidifier from Hydrosorbent comes with an inbuilt recharge indicator. The indicator is at first orange in shade, but it really turns by itself crystal clear when it requires a reactivation. Both of those have a digital humidistat to manage and manage the humidity all the way down to 30% https://dstdehumidifiers.co.uk/case-studies/desiccant-dehumidifiers-in-cold-stores/