This 3-pack of Two-Way Humidification System Replacement Packets from Planet Waves is a maintenance-free, two-way humidity control system which maintains the optimal 45-50% relative humidity level within your instrument case. Eliminate the guesswork and the potential mess related to refillable humidifiers. Unlike refillable units, the two-way system automatically adds or removes moisture on demand. The replacement packets contain 3 Humidipaks, which last from 2 to 6 months.