Interesting.
I just go by look and feel and a hygrometer for the cabinet / box. The one that I found online was only good between 60 and 70 RH, which isn't great.
The patent was for a much broader use, including cereals and other uses where RH is critical (
https://patents.google.com/patent/US4287470). Since it was awarded in 1981, the patent has expired (20 years in the US) and anyone can make the device now. Well, not me because I can't even open a box of cigars without help.