Chemical hygrometer

From Glossary of Meteorology

chemical hygrometer

A group of hygrometers based on chemical effects that depend on humidity.

These hygrometers are not used in practical meteorology. Examples include pictures or figures, covered with cobalt chloride, that appear blue at low and pink at high relative humidity. In industrial processes electrolytic methods are used, which are especially suited for extremely low humidity.
See also absorption hygrometer.