Combining an Humidity Indicator and Relief Valve have the advantage of only require one hole to be fitted to equipment or containers where space is limited. They are designed for mounting in moisture sensitive electronic or electro-optic equipment, or any other humidity controlled enclosure that will be subjected to air transportation. 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.
Most of the units can be fitted with a desiccant cartridge which will adsorb any moisture entering the equipment during its use. These additional units are known as static desiccators.