The D'Addario Humidipak is the world's leading maintenance free, two-way humidity control system for guitar. It automatically maintains the optimal 45-50% relative humidity level within your instrument case, eliminating the guesswork and potential mess related to refilling a humidifier. Unlike refillable humidifiers, the Humidipak system provides true “two-way” purified humidity control by adding and removing moisture on demand. No more manual adjustments based on seasonal changes, geographic locations, temperature, or other factors. The Humidipak Kit comes with 3 Humidipaks and a soft mesh pouch that will not harm your instrument's finish. Humidipaks last between 2 and 6 months and should always be kept in a sealed environment like a hard-shell case. Once the Humidipaks become firm, discard and replace using the 3 refills that come in the Humidipak Replacement Pack (PW-HPRP-03).