The sensor type that dominates the transportable data logger and hand-held instrument market is, however, the capacitive sensor. The capacitive RH sensor consists of a thin layer of water absorbent, polymeric or inorganic material, that is coated onto a conductive base. The layer is then covered with a porous conductive layer. As the relative humidity increases the water content of the polymer increases.
Water has a high dielectric constant.
This means that the combination of the two electrodes with the water between can store more electric charge. 7 x 4 x 0.5 mm thick. The principle is simple enough, but there is a long history of development of practical sensors that are resistant to air pollutants, or immersion in liquid water. This means that the electronic processing has to be completed close to the sensor. This is an irritating extra expense, because some data loggers have all the necessary processing power built into them.