According to their website, the Oasis Guitar Humidifier is made from specially-designed fabric that allows water vapor, but not water, to pass through.
As the vapor transmission takes place, a vacuum is created and the Oasis shrinks to compensate for the loss of water. By watching it shrink, you can determine when it is ready for refill.
When refilling, you don’t have to guess whether or not it’s full: just unscrew the black cap, fill with water to the top of the container, and replace the cap. Once sealed, the water-tight container is your first line of defense against leaks, and super absorbent polymer crystals inside the Oasis and are a second. The humidifier holds up to 500 times its weight in water!
We also sell hygrometers, which measure the humidity level – in a Connecticut winter our humidity is often at 20-30%, and when guitars or violins spend time at that level of humidity, they dry out and the tops shrink or crack. The Oasis releases moisture as needed and can keep an instrument properly humidified.
There are various opinions on what a proper humidity level should be, but based on my experience with solid-top instruments, I have found 40-50% to be a desirable range; if someone is controlling their humidity, I would recommend 45%.