Planet Waves PW-HPK-01 Humidifier Pack for Guitars.
The two-way humidification system is the only maintenance-free bidirectional humidity control system for guitars.
It automatically maintains the optimum relative humidity level of 45-50% within the instrument case, eliminating the guesswork and potential problems associated with refilling a humidifier. Unlike refillable humidifiers, this system provides true “two-way” humidity control by adding and removing humidity on demand. No more manual adjustments based on seasonal changes, geographic locations, temperature or other factors.
The two-way humidification system comes with 3 humidipaks and a soft mesh bag that will not damage the finish of your instrument. These packs last 2 to 6 months and should always be kept in a sealed environment such as a hard case. Once the packs become firm, discard and replace them.