RDM Humitron® Acoustic Guitar Humidifier. Humitrons are made of the most advanced material available. The tubes were custom extruded of the finest non-vinyl and non-allergenic resin, soft enough not to exert pressure around the F-holes / sound-holes and sufficiently sturdy to minimize contact with the inside parts of an instrument. The water-suction stems within the Humitrons absorb water ten times of their weight and then slowly release it through the holes.