The Dampit Guitar Humidifier
I purchased the Guitar Humidifier for one of my first guitars. Not only are they very simple, they work very well too. I don't bother with the provided plastic "hole cover" – I just locate the humidifier inside the guitar while it's in it's case – that way it humidifies the whole instrument. After filling the unit with water, be sure to shake out the excess water to preclude the water from staining the unfinished surfaces on the guitar interior. The procedure for doing this is clearly stated in the instructions that come with the unit.
If you are looking for a simple, no hassle, and economical way to humidify a wooden instrument pick up a few of these units. Other systems may be more elaborate and expensive, but this one is difficult to beat.
Dislikes: The supplied humidity measurement system is not very accurate. Since it tends to me somewhat inaccurate, you get a false feeling of security. About the best way to do this is with a good hydrometer that is placed in your guitar case such as the Oasis OS-2.
Planet Waves Guitar Humidifier
All you do to fill this compact guitar humidifier is to fill-up the included plastic syringe and drip it into the sponge material. You have to exercise caution not to overfill the reservoir as this would cause the unit to drip within the interior of the guitar. The good thing is that the humidity will reach the neck since it is not enclosed in the body like other humidifiers.
Dislikes: I became irritated with the amount of time it takes to load the sponge just a few drops at a time. Also by cramming the humidifier between the strings can throw your guitar slightly out of tune. No humidification checking is provided and you ought to invest in a good in-case hydrometer so you can closely monitor the humidity.
The Herco Guitar Humidifier
When looking for an instrument humidification system for my violins and mandolins, I came across this handy little humidifier. Considering the compact size of this little humidifier, and being relatively leak-free, you can locate it nearly anywhere in your case. It just uses a simple principle of water absorbent clay within a plastic container. For extra protection you can position 2 of them in each case, since they are so compact. This device comes without directions so it might be useful to know that the correct way to use this is to drop it in a cup of water for at least five minutes. Next just wipe the excess water from the unit and pop it into your case. This needs to be repeated every few days. My experience has shown that his little humidifier will not leak and it will keep your wooden instruments humidified.
Dislikes: There is not a humidity monitoring system and you should purchase a good hydrometer such as the Oasis OH-2
The Complete System Oasis OH2 Hydrometer and OH-6 Humidifier
I personally use the Oasis OH-6 and OH-2 guitar humidifier system as the ideal one-two punch to keep humidity in check for my most valuable and fragile pre-war Martin and Gibson acoustics. This combination of guitar humidifier and hydrometer will easily deliver a consistent level of humidity to your guitar case to keep your instruments between the ideal 30% and 40% relative humidity.
This can additionally be monitored with the OH-2 hydrometer so adjustments can be made when necessary. The system comes with a clipping system that will allow simple positioning in various locations in your guitar case. Couple that with the excellent OH-2 Hydrometer and you have a one-two punch that is tough to beat. I absolutely love this system and haven't had a problem with it or any of my very valuable instruments since using it.
Dislike: I have no dislikes of this system.