About the D'Addario PW-HPK-01 Auto Two-Way Humidity Control System:
 
D'Addario’s Humidipak is the only maintenance-free, two-way humidity control system for guitar. It keeps your case at the ideal 45–50% humidity—no refills, no guesswork, and no mess. No need to worry about changes in weather, seasons, or location. Just place it in your case and let it do the work. Note: Humidipak is made for maintaining proper humidity. If your guitar is very dry or hasn’t been humidified before, use Humidipak Restore first to safely bring it back to the right level. Using Maintain packets on a dry guitar will wear them out faster.