When the relative humidity drops below 40%, musical instruments are at risk of cracking. The Oasis Digital Hygrometer accurately measures relative humidity and can help you decide when to use your Oasis Guitar Humidifier. Because relative humidity varies greatly, even inside your home, we recommend that you keep your hygrometer close to your guitar, or any other stringed musical instrument made of wood. With the convenient case clip, it is now possible to measure the relative humidity inside your guitar case so you see when it's necessary to humidify the instrument. The clip attaches to the back of the OH-2 hygrometer and is mounted on the flap of the supply box inside your guitar case.