The Oasis OH-18 Ukulele Humidifier has two unique features: The mounting system allows the humidifier to hang horizontally inside the ukulele rather than vertical (like the guitar humidifier).Secondly, the humidity output is about 50% of the Royal Blue Oasis guitar humidifier, to accommodate the smaller instrument size. This also allows the humidifier to last twice as long between refills.The body of the OH-18 Ukulele Humidifier is made from a specially designed fabric that allows water vapor (but not water) to pass through it. As the vapor transmission takes place, a vacuum is created and Oasis shrinks to compensate for the loss of water. By watching the Oasis humidifier shrink, you can determine when it is ready for refill. When refilling, you don"t have to guess when it is full. Simply unscrew the black cap, fill with water to the top of the container and replace the cap. Once sealed, the watertight container is your first line of defense against leaks. Humigel, the super absorbent polymer crystals inside the Oasis Humidifier, are your second line of defense. Humigel captures up to 500 times its weight in water in an absorbent gel matrix.Like other sound hole humidifiers, Oasis fits between the strings of your instrument. The materials that touch your strings are relatively soft and will not scratch your strings. A blue polyurethane stabilizing bar fits over the neck of the ukulele humidifier. Once positioned in the sound hole, the stabilizing bar lies across the guitar strings to ensure even weight distribution. Oasis never touches the body of your ukulele.