Thick accumulated layers of dust, smoke debris, paint and conformal coatings can physically block the diffusion of moisture into the polymer. In such cases, it is typical to see no change in sensor RH output even though ambient RH is changing or a negative RH gain with a negative RH offset. Depending on the amount of blocked sensor area with the solid debris, this negative gain can be drastic. While the illustration in Figure 2-5 shows solid contaminations distributed across the polymer surface, solid contaminations must be in the inner circle of the sensing area (where the electrode sensor mechanism is present) to impact RH accuracy. When the contaminant (in any state of matter) gets onto the polymer, it diffuses down into the polymer. If that diffusing contaminant is not over the inner sensor area, then it will not interact with the electric field and not impact the sensor's capacitance. For gases, this is irrelevant because those evenly distribute over the cavity space. But solid contamination can be localized. If solid contamination is observed but is not located on or adjacent to the inner sensor area, then it is unlikely to impact the RH accuracy.

Figure 2-5 Surface Contamination Impact on RH Accuracy.