An immersible relative saturation moisture sensor with application to transformer oil

Two polyimide-based capacitive humidity sensors were calibrated in Shell Diala A transformer oil and 10 cSt silicone oil at 35, 50, and 70°C. The moisture content of the oils was found to be a linear function of the output voltage with the slope increasing with temperature. Thus, the sensitivity of the sensors to changes in the absolute moisture content of the oils was found to decrease with increasing temperature. For transformer oil the sensitivity changed from 30 mV/p.p.m. at 35°C to 9 mV/p.p.m. at 70°C; for silicone oil the sensitivity changed from 10 mV/p.p.m. at 35°C to 4 mV/p.p.m. at 70°C. Normalization of the absolute moisture content to the saturation level of moisture in the oil for all three calibration temperatures and both oils demonstrated that the sensors respond to the relative saturation of moisture in the oil. The saturation level of moisture in the oils was measured for the three calibration temperatures and could be expressed in Arrhenius form with an activation energy of 0.317 eV for transformer oil and 0.231 eV for silicone oil