Humidity sensors are widely used in various fields to measure the amount of water vapor present in the air of a particular environment. Humidity sensors are configured as capacitive sensor devices that use capacitance to measure humidity. Capacitive humidity sensors include a dielectric layer interposed between a pair of electrodes. The dielectric layer is formed of a material, such as polymer, that is configured to absorb and retain water molecules at concentrations that are proportional to the ambient humidity. The water molecules alter the capacitance between the two electrodes in a manner that depends on concentration. Humidity can therefore be determined by measuring the capacitance between the two electrodes and correlating the measured capacitance to a corresponding humidity value.
While effective in measuring humidity, capacitive humidity sensors rely on bulk effects to alter capacitance and indicate changes in humidity. As a result, capacitive humidity sensors typically have a relatively slow response time to changes in ambient humidity. This is because it takes time for the water molecules to diffuse into and out of the dielectric layer of the sensor in response to changes in humidity. To avoid time-lag errors, capacitive humidity sensors require a long time constant to allow the water concentration in the dielectric layer to reach equilibrium before capacitance measurements are performed.
Capacitive humidity sensors are also susceptible to drift and damage as a result of contamination and/or aging. For example, as water molecules are absorbed into and released from the dielectric layer, non-water molecules can be absorbed into the dielectric material absorbed into the dielectric material along with the water molecules. In some cases, the non-water molecules can become trapped in the dielectric material. Over time, the buildup of non-water molecules, or contamination, in the dielectric can alter the capacitance response of the sensor and/or reduce the ability of the dielectric material to absorb water molecules.
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