Figure 1-2 shows the images of a sensor area with visible contamination such as solid or liquid and gaseous contamination. Solid contaminants such as dust particles and smoke can accumulate onto the sensor over a period. Liquid contaminants such as water with ions, saliva, sweat, cleaning flux as well as gaseous contaminants, such as volatile organic compounds, can affect the sensor polymer and hence the RH output. Volatile organic compounds (shortened to VOCs) are organic chemicals, often human-made, that have a high tendency to vaporize into the air, otherwise known as off-gassing. These interactions cause shifts in RH accuracy and depending on the level of exposure can cause permanent RH accuracy errors as well.

Chemical contamination can be in the form of any state of matter. Liquids, gases, and solids can cause RH accuracy errors. Table 1-1 shows a non-exhaustive list of potential sources of chemical contamination that can be in different states of matter.

While knowing the state of matter is helpful for understanding that chemical contamination can occur from a wide variety of sources, to understand the potential impact on RH sensor performance, sort chemical contaminations into two categories: Bulk contamination, and Surface contamination. Surface contamination refers to solids and liquids, where there is a visible material deposited onto the surface of the sensing polymer. Bulk contamination can be defined as gaseous contamination where the molecules embed into the body of the polymer and are not able to be seen with a microscope looking at the surface of the sensor cavity. While much of the impact on RH performance is determined by the chemistry of the contamination, whether the contamination is surface or bulk also matters. Surface contaminations can often be cleaned off with distilled water or wiping (See section 4.6.4 of How to Debug RH Accuracy Issues in RH Sensors for additional details), but bulk contaminations cannot be externally cleaned. Resolving a bulk contamination requires heat to accelerate the diffusion of the contaminant out of the polymer structure.

Figure 1-2 illustrates multiple different examples of what surface contamination can look like. In images A & B, the surface contamination is solid material deposited on the polymer in different amounts. Image C shows dried rings from liquid contamination, potentially leaving solid contamination behind after the liquid has evaporated.

In Figure 1-2, the larger green circle shows a view of the entire center cavity zoomed in to show the polymer. The smaller yellow circle is the inner sensor area, where the capacitive sensing electrodes are located. The capacitive electrodes do not cover the whole sensor die or the entire polymer surface. The inner sensor circle is much more sensitive to chemical contaminations than the outer circle area. Figure 2-1, Figure 2-2, Figure 2-3, Figure 2-4, and Figure 2-5 show this inner circle sensor area in a simplified manner and from a cross-sectional view plane, omitting the outer polymer area.