The Oasis HH Combo OH-30 is the only humidifier/hygrometer designed to fit in the soundhole of a steel string acoustic guitar. When using a humidifier to boost the humidity inside a musical instrument it is important to monitor humidity levels. Otherwise, maintaining correct humidity is just guesswork. The HH Combo has two parts. Above the strings, the upper portion of the unit contains a clearly visible and accurate hygrometer, which can be calibrated by the user if necessary. The humidifier clips securely into the bottom of the hygrometer. Suspended by the strings, the humidifier releases moisture evenly and slowly inside the guitar yet never touches the instrument body. A specialized, drip-resistant sponge holds 10 times its dry weight in water, which allows for longer periods between refills. There are four openings on each side of the humidifier for the water vapor to escape. This is sufficient for most dry areas. In dryer climates like Arizona, there are four additional openings which can be punched out to provide more humidity. The sponge is easily recharged by moistening the sponge with distilled water. Using the HH Combo during dry months will protect your instrument safe from cracks and warping.