I have a meter outside but near the guitar cases. In another room I have another hygrometer and that's how I know the humidity in the guitar's room is borderline.

While I have not measured humidity in the individual cases I'm confident that the moisture from the small sponge(s) (~ 3/4 X 1 1/2 X 1/2") is sufficient for the guitars' needs. The sponges dry out and require rewetting weekly as their moisture is 'lost' to the inside of the cases and then humidifies the guitars as any other inside-the-case system.

It's important to note that this small system is suitable for a small humidity issue. Probably not so much with larger humidity extremes or frequent fluctuations. Also, different guitars of different sizes and materials will have different requirements.

Check out Stephen White's sub-forum, "Ask a Guitar Tech" for threads about guitar humidification and go to YouTube for vids on both manufactured and home made systems.