The Planet Waves Two Way Humidity Control System is the only 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 Two Way Humidity Control System provides true _two-wayÓ 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 Planet Waves Two Way Humidity Control System comes with 3 Humidity Control packets and a soft pouch that will not harm your instrument's finish. Planet Waves Two Way Humidity Control Systems last between 2 and 6 months and should always be kept in a sealed environment like a hard-shell case. Once the humidity packets become firm, discard and replace using the 3 refills that come in the Planet Waves Two Way Humidity Control System Replacement Pack (PW-HPRP-03).