Humidity Indicators
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 indicator solution used is water soluble and therefore humidity indicator papers 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 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.
Humidity indicators in combination with the drying agents serve as a guide to the climate and moisture level inside the packaging. Humidity indicators consist of a special absorbent paper. Several areas of the paper are printed with cobalt (II) chloride solution varying in concentration. The numbers inside the moisture sensitive spots indicate the relative humidity with a visible colour change from blue to pink. A drop in humidity is shown through the reverse process of the colour change.
Application
The humidity indicators are placed either inside the barrier foil or form a part of the vision panels or viewing plugs. Properly packaged exported goods with sufficient desiccants experience a 20% RH drop at normal temperature within a very short period of time. The colour of the humidity indicator used should therefore be blue in the 30% spot. If this should not be the case after 2 to 3 days have passed, the packaging ought to be inspected for any leaks or tears, too low amounts of drying agents or moist packaged goods. The colour reaction of the humidity indicator card, however, can be delayed through low temperature levels during winter time, a lack of air circulation between humidity indicator and drying agent or foils not suitable for the purpose.
Accuracy
The humidity indicator cards are small-sized, easy to handle and inexpensive. When it comes to precision they should not be directly compared with expensive and sophisticated electronic measuring equipment. An inspection temperature of 20°C (+-2°C) allows for a tolerance level of + -5% RH. The humidity indicators thus comply with the military technical specifications TL 6685-0003 version 4 . Note that the colour change in regions with extreme climatic conditions maybe delayed or decreased, depending on whether the temperature is above or below 20°C. The deviation levels are approx. 2.5% RH for each 5°C above, resp. below 20°C.