Top positive review
Very good product
June 3, 2016
I have a number of these units and they work very well. I'm in the L.A. beach area and things get very humid here. I have two Liberty "Ammunition Cans" which are about 12 cu ft each. One of these in each safe keeps the humidity at about 45%, down from about 65%. Note that these secure cabinets are not airtight so I wouldn't expect a silica gel desiccant-based dehumidifier to do much more. Silica gel will absorb moisture down to 40%RH, which is certainly adequate to prevent mildew and keep ammunition "fresh". If you need lower than 40% you'll need to go with a more expensive desiccant like aluminosilicate molecular sieves, which can operate down to 1% RH, or a powered dehumidifier.
Using one of these in each ammo safe requires a recharge about once every four to five weeks, I've recently added a second one in each safe and, as expected, the time between recharges seems to have nearly doubled. I get at least two months between recharges.
I also keep one of these in my gun safe, which is about 45 cu ft. The safe is equipped an electric (heater type) dehumidifier rod. The E500 brings the humidity down about 5% lower than just the heater rod alone, keeping the safe at about 40% RH. It may not be absolutely necessary, but it also gives me a backup in case the safe heater loses power. If you've ever had a safe without some sort of dehumidifier, it can be a disaster. A safe WILL become quite humid inside without one. Years ago, when I was young and dumb and not aware of this, I had a bunch of currency in a safe. Within a few months, the currency and other paperwork in the safe began to smell badly of mildew. In time it would have been destroyed.
Note that these units (which, oddly, the Amazon description implies are not electric) require a minimum of 12 hours connected to power to fully recharge. Unless you keep spares around, that leaves you unprotected for a time. Using two at a time eliminates this problem, of course. Personally, I've purchased two extras. I leave them on a power strip for two days, then seal them in a zip-lock bag. When one of the ones in use needs to be replenished, I grab a "ready" one out of a bag and cycle the spent one through the same process.
You should also be aware the these things do get quite hot during the recharging process. The manufacturers of silica gel desiccant specify a recharge temperature of 240F to 310F for 12 hours. Since these units contain silica gel, they're going to need to get at least 240F internally during the recharge cycle. There are some reviewers on Amazon who are upset by this, but physics is physics. They're going to get hot.
One last point. There are two versions of the Eva-Dry E500. One has indicating silica gel that ranges from blue (dry) to pink (wet). The other ranges from orange (dry) to green (wet). I have some of each. I don't know which of them is "newer", if either is, and there doesn't seem to be a difference in part numbers between the two. They both seem to work equally well, though I've not done any experimentation to prove this. At the very least, this may cause some confusion when reading reviews here, since the color ranges are quite different.
[Update 6/19/2016: The "New" version of the Eva-Dry E500 ranges from orange (dry) to green (wet). The original "blue-to-pink" desiccant used cobalt chloride as a moisture indicator. The EU now classifies cobalt chloride as a carcinogen if inhaled which, I'm sure, is why the Eva-Dry folks changed the indicator.]
Complaint: Only a minor one. As others have pointed out, the units cannot easily plug in to a wall outlet because they're too heavy to stay plugged in. That's not a huge problem. Power strips are cheap these days. But the way the fold-out plug works, the plug prongs are at 90 degrees to the length of the device. This means that one E500 takes up an entire power strip. There is no way to plug more than one of them (or anything else) in on the same typical size strip. If the plug was fitted at 90 degrees to the way it is, you could stack at least two per power strip. I did find larger commercial power strips with the outlets at 90 degrees which allows a number of these to plug in at once. But, as I say, that's a minor quibble. Overall these work well and I'm on Amazon today to buy two more.