Humidity Indicator are designed for mounting in moisture sensitive electronic or electro-optic equipment or any other humidity controlled enclosure. They readily detect changes in the humidity of the space being monitored by using a colour change indicator paper, which turns from blue to pink as the humidity increases and turns back to blue when the humidity decreases. The Humidity indicator solution used is water soluble and therefore humidity indicator cards should not be placed in direct contact with water or steam or exposed to extremely high humidity's for protracted periods of time. Also strong solvents such as ammonia fumes will cause the bleaching of the colour change dye and lose colour reversibility.
Most of these humidity indicators can be fitted with a desiccant cartridge, which will adsorb any moisture entering the equipment during its use. These are known as static desiccators.