A two-way humidity pack holds a salt solution behind a membrane that passes water vapor but not liquid. Because a salt solution of a given strength holds the air around it at a fixed humidity, the pack gives off moisture when the air is drier than its rating and takes moisture in when the air is wetter. Packs are sold in fixed levels, are not refillable, and are replaced when they turn hard. Boveda is the best-known brand. Don't mix levels in one humidor or combine packs with another type of humidifier.
Wiki · Terms · Two-way humidity pack
Also called: humidity pack, Boveda pack