Bought a Madelaine Hygrometer. The salesman said it was new for them and their other products are good. It was about $25. Thought I'd give it a try.

Here's a picture of the wet salt in a shot glass and the hygrometer before the test:


After about 24 hours, the result (taken through the double ZipLoc bags) :


Have to admit, that the next day, it read 73%, but it seems like a good unit. Will put it in my new Capri humi that I am setting up.