Humidity indicator cards are impregnated with a solution of cobalt chloride that has been carefully mixed to change color from blue through various shades of magenta - red, to a pink. These cards are not precision indicators. However, they are very good at telling you the maximum possible humidity in a container. This is very important for some products and household items because as long as the humidity is below 40%, basic corrosion (rust) will not occur. Neither will mildew or mold in clothing or paper products. Finally, all sophisticated electronics: video cameras, computers, SLR cameras, sophisticated instruments etc. enjoy a much longer and more reliable life if they are maintained at a very low relative humidity. **No returns 14 days after purchase**
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