The D’Addario Acoustic Guitar Humidifier Pro protects your acoustic guitar even in the driest conditions. Using a specialized hydrophilic sponge that holds 12 times its dry weight in water (much more than an ordinary sponge), the Humidifier Pro allows for longer periods between re-fills and increases protection in drier environments. Preserving your instruments ultimate playability by guarding against cracks, warped necks, and shrinkage is now easier and less work. Suspended by the strings, the humidifier releases moisture evenly and slowly inside your guitar, yet never touches the guitar’s body. Additionally, the Humidifier Pro's stopper allows you to check the sponge status without removal of the unit and provides a tight seal when in place. Lastly, maintaining this D'Addario humidifier is as easy as moistening the sponge when it becomes dry.