REPLACEMENT FOR STRING INSTRUMENT Planet Waves humidifier pack for acoustic instruments. Planet Waves humidifier pack for acoustic instruments. The two-way humidification system (formerly Humidipak) is the only maintenance-free bi-directional humidity control system for guitar. It automatically maintains the optimum 45-50% relative humidity level inside the instrument case, taking the guesswork and potential problems out of refilling a humidifier. Unlike rechargeable 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 pouch that will not harm the finish of your instrument. These packages last 2-6 months and should always be kept in a sealed environment such as a hard case. Once the packages become firm, throw them away and replace them.