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I'm seeing rather new reviews. I don't know what other people are doing, but here's what I'm doing. I've enclosed an RH humidity gauge (electronic) in the case with my guitar and this humidity system. It's a Gretsch hard case that seems to seal up pretty nicely. Obviously temperature changes will allow it to breath. The storage location has a fairly steady temperature. However, in the winter, humidity gets as low as 30%. I've measured this. This is too dry for the wood. So the plan is to check the humidity in the case at random intervals over time to see what the humidity inside the case actually is. Obviously I don't want it too high such that it can corrode metal parts. I also do not want the terminal black mold to start growing on things. My basement has that problem. You can't kill the black mold without fire. On the dry side, I don't want the wood to crack or split.
The three stars is just a midline rating for now. If I edit this review, I will see how the product stands up in relation to advertised humidity range (40%-50% RH). I'll also throw in efficacy as I don't know what the ideal temperature should be. In the storage location, that can be as low as 60ºF to 75ºF. Rapid swings (less than a few hours) are not expected.