Boveda is pure water and natural salts sealed inside a leak-proof membrane. Boveda adds and absorbs water vapor slowly and precisely to achieve and maintain an RH level between 40% - 60% in your instrument's closed case.
Use one pack in the case for oboe, violin, viola, ukulele, clarinet, electric guitar, electric bass or mandolin. Use 2 packs in a guitar case. For cello, double bass, pop 3 bags into the case.