Carbon-film hygrometer element

From Glossary of Meteorology

carbon-film hygrometer element

An electrical hygrometer element constructed of a plastic or glass strip coated with a film of carbon black dispersed in a hygroscopic binder.

Changes in the ambient relative humidity lead to dimensional changes in the hygroscopic film such that the resistance increases proportionally with humidity. The successful implementation of this sensor type requires a well-controlled manufacturing process and very careful handling. The sensor is used in some radiosondes and named carbon hygristor.