Humidity causes mold and corrosion. Add in temperature changes, condensation can be a problem, too. A desiccant placed in an enclosure adsorbs moisture for a while, but eventually it loses effectiveness and needs replacing. Knowing the level of humidity in a transit case or shipping container, action can be taken before damage occurs.
Cobalt chloride is blue when dry but turns pink in the presence of water. It is an excellent way to measure humidity. Chemists engineered this color change to take place at various humidity levels. Indicators are available in 10% increments – from 10% to 90% Relative Humidity.
Accuracy is generally quoted as +/- 5% at 75°F. At temperatures above 75°F, the indicator will show a slightly lower humidity than is actually in the case. At lower temperatures, it will read slightly higher. There is some concern over the possible toxicity of cobalt chloride. Its use is regulated in Europe. Cobalt-free indicators are available. So, ask for advice if this might be an issue.
In most applications, it’s enough to use an indicator that shows the current level of humidity. But, there are situations where an irreversible or permanent indicator is better. An irreversible indicator shows the maximum humidity experienced inside a container. That might be useful for long term storage and shipping. Sometimes termed “color stain” elements, they turn orange or blue when exposed to excess humidity. Once the color changes, a maximum humidity indicator cannot be reactivated. It must be replaced.
Silica gel desiccant can be cobalt-chloride coated. It turns pink when humidity reaches a certain level. This is known as an Indicating Desiccant.
An alternative is an indicator card. The size of a playing card, it incorporates small circles that change color at different humidities. Thus, a card might show 30, 40 and 50% humidity. Cards are available for military, electronics, and general commercial applications.
The third type of indicator, the one used by Sierra Cases, is an observation window. It features a color-change disc. The disc is a form of indicator card meeting SAE AS26860 and MIL-I-8835 standards. It can be viewed from outside the case. This makes it possible to see if the desiccant should be replaced – without opening the container. A disc indicating current relative humidity or maximum humidity may be installed.
If humidity is a concern, ask Sierra Cases if a humidity indicator would be appropriate in your application.
(This is the first of two parts. Read Part 2, Choosing the Right Humidity Indicator.)