Planet Waves PW-GH-HTS - humidity sensor with air moisturiser. Suitable for steel and nylon string acoustic guitars. Flat sensor for use inside a guitar case, can be clipped inside the guitar (fixed by the strings).

"Taylor Guitar Humidifier, fits into the soundhole, ensures the perfect humidity for your guitar, the Oasis unit contains Humigel water-absorbing crystals, which capture up to 500 times their weight in water and trap..."

Planet Waves Humidpack ABSORB, PW-HPRP-03, replacement packet, 3 pcs, life period after opening app. 2-4 month (depending on temperature and air humidity (see instructions)

Grover 7960 Humidifier - air moisturiser for guitar, fits into the soundhole, protects the guitar from cracks and other damage caused by dryness. Made in the USA.