In the desert, relative humidity levels may be as low as 5%. In the tropics, 100% is possible. Anything over 40% is generally considered humid. Humidity causes metal to corrode and mold to form. Lower humidity is a problem too, as drops in temperature cause condensation. If that happens inside a transit case or shipping container, the contents can be damaged or completely ruined.
There are two types of humidity indicators. One type shows the current humidity. The other shows the peak humidity experienced since the indicator was put to use. Both work by showing a change in color at a set level of humidity. That can be anywhere from 10 to 90%.
An indicator of the current level of humidity is considered reversible. The color changes from blue to pink as the humidity rises past a threshold. The color changes back when the humidity falls. These respond quickly and are accurate to +/-5% at 75°F.
In a peak or maximum humidity indicator, the color change is not reversible. It permanently shows if the humidity exceeded a given threshold for some duration at any point since the indicator was put into service.
Maximum humidity indicators are typically used in shipping containers. They show the peak humidity experienced by the container’s contents. Installation must be performed in very dry conditions. Otherwise, they may be activated before actually entering service.
Reversible humidity indicators are used to confirm that a desiccant (drying material) is working. As desiccants absorb only a finite amount of moisture – typically 25 to 40% of their own mass – there are circumstances where they cease to be effective. A reversible indicator will thus show when the moisture in a container has become excessive.
Silica gel desiccant can be purchased with a coating that indicates humidity. Alternatively, a humidity indicator card may be placed inside the packaging or container.
The problem with silica gel and indicator cards are that they require the case or enclosure to be opened to determine the effectiveness of the desiccant. An alternative is to use a window-mounted indicator. Fitted into the wall of a case, these display the internal humidity without having to open the case.
If you think your application needs a humidity indicator, talk to Sierra Cases. We fit window-type humidity indicators that monitor desiccant performance and show you when moisture becomes a problem.
(This is the second of two parts. Read Part 1, An Introduction to Humidity Indicators.)