Another kind of chemical contamination is Ionic contamination. Ionic contamination can be identified through a signature RH error profile, where at room temperature sensor RH appears normal at low ambient RH levels but as ambient RH increases the sensor RH error begins to drop precipitously resulting in very large errors, often exceeding -10%RH, at high RH levels (>80%RH). Small, high charge density ions like sodium can impact the local polarity of water molecules causing losses or less change in the dielectric constant of the sensor polymer. At higher RH, there is a lot more conductive ions dissolved paired with a larger amount of moisture being absorbed, which causes more leakage. Because of this effect, the RH output is significantly lower at higher humidity levels causing a negative gain in accuracy.

Figure 2-4 Ionic Contamintion Resulting in Significant Shift in RH Accuracy at High Ambient RH.

Ionic contamination can occur by using tap water for cleaning or due to the condensation of water molecules containing salts. Saliva and sweat contamination into the cavity shows similar signature shifts as ionic contamination as these human fluids also contain conductive ions from salt. At times, dust accumulation interacting with moisture under high RH conditions can also activate insulating dust into a conductive nature. Figure 2-4 shows the ionic contamination on the surface layer, and not penetrating into the polymer. This is because in almost all cases, ionic contamination exist in nature as liquids or solids that sit on the surface of the polymer.