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A humidity indicator is a device which measures the amount of water vapor in the air. These devices range in complexity and functionality from simple cards impregnated with moisture sensitive chemicals to complex sensor and microprocessor systems. Most are used to establish the humidity levels in areas or environments which are sensitive to excessive airborne moisture or significant fluctuations in moisture levels. These include shipping containers, packaging for perishable or sensitive goods and electronic equipment bays. There are three common presentations of humidity indicators: chemical, electronic, or dial.
In varying degrees, humidity is a constant environmental factor even in the driest and coldest climates. Generally the moisture present in the air poses no serious threats, but in certain environments too much or even significant changes in the amount of airborne water can be problematic. When keeping track of humidity levels becomes an issue, a humidity indicator can return useful information regarding the amount of water vapor suspended in the air. There are many different examples of humidity indicators in general use all of which, however, fall into three basic categories. As noted, these are chemical indicators, electronic sensors, and dial type hygrometers.
The simplest and most commonly used type of humidity indicator is the chemical moisture indicator. These make use of specific chemicals tendency to change color when exposed to moisture. Cobalt chloride and copper chloride change color as they are exposed to increasing amounts of airborne moisture; cobalt chloride changes from blue to pink and the copper variety from yellow to blue. Although these types of humidity indicator are not as sophisticated or precise as other types, they can give a good indication of general humidity levels.
Chemical humidity indicators are often presented in the form of paper strips with several circular areas impregnated with moisture sensitive chemicals. Each chemical “spot” is formulated to change color at a specific moisture level; a single card can give a fairly good indication of a range of humidity levels. These chemical indicators are often incorporated into small inserts which feature a plastic viewing window. When installed in the side of a shipping container, electronic equipment enclosure, or any enclosed package they allow a quick visual display of the moisture level inside the container. The same type of chemical indicators are also fitted to compressed air lines in applications where controlling water content in the air flow is critical.
Electronic and dial type humidity indicator types are capable of returning more accurate humidity readings in applications where exact moisture content control is required. Electronic types typically consist of a remote sensor and a digital readout station and are often employed as part of extensive, centralized environmental control systems. Dial variants usually feature two scales which indicate both humidity levels and temperature.