Abstract

Moisture indicators are not the sensors. However, their concept of operation is based on the same physical and chemical principles that are used in colorimetric-based humidity sensors. Therefore, the description of moisture indicators is presented in this chapter. Sometimes it is desirable to detect only a change in the amount of moisture in the sample, whereas the exact concentration is of only of secondary importance. Research has shown that indicated problem can be solved by using certain salt crystals, which change color with the appearance of moisture. It has been found that the use of this effect is the most economical method for detecting changes in moisture level. Experiments have shown that cobalt chloride is the material most suitable for such applications. In reality, cobalt chloride has six states of hydration and exhibits progressive color changes with corresponding changes in hydration state. On the basis of this effect, a large number of different humidity indicators have been developed and presented on the market. Humidity indicators can be manufactured as strips, cards, and in the form of elements embedded in the gas main. These indicators are inexpensive, easily stored, not easily damaged, and may be used in many variants. One should note that in addition to cobalt salt indicators, other salts can be also applied in humidity indicators. Studies have shown that there is a sufficiently large set of materials that can also change color when exposed to moisture.